WO2009149449A1 - Dispersions and methods of producing dispersions having predetermined optical properties - Google Patents

Abstract

A method of producing an emulsion includes producing a first emulsion comprising a first plurality of droplets of a first liquid dispersed in a second liquid, the first plurality of droplets having a first ensemble average radius; and removing a plurality of droplets from the first emulsion that each have larger radii than the first ensemble average radius to obtain a second emulsion comprising a second plurality of droplets having a second ensemble average radius that is less than about 100 nm. The first liquid is at least partially immiscible with the second liquid and the second emulsion is more transparent to visible light than the first emulsion. An emulsion includes a first liquid, and a plurality of droplets of a second liquid dispersed in the first liquid. The second liquid is at least partially immiscible with the first liquid. The plurality of droplets have an ensemble average radius that is less than about 100 nm and a standard deviation about the ensemble average radius of that is less than about 25% such that the emulsion is substantially transparent to visible light.

Description

DISPERSIONS AND METHODS OF PRODUCING DISPERSIONS HAVING PREDETERMINED OPTICAL PROPERTIES

CROSS-REFERENCE OF RELATED APPLICATION

[0001] This application claims priority to U.S. Provisional Application No.

61/129,142, filed June 6, 2008, the entire contents of which are hereby incorporated by reference.

BACKGROUND

1. Field of Invention

[0002] The current invention relates to methods of producing dispersions, and more particularly to dispersions and methods of producing dispersions having preselected optical properties.

2. Discussion of Related Art

[0003] Nanoemulsions are dispersions of metastable droplets of one liquid in another immiscible liquid that have droplet radii a below 100 nm (Meleson, K.; Graves, S.; Mason, T. G. Soft Mater. 2004, 2, 109). They are kinetically inhibited against coalescence by a surfactant that provides a strong stabilizing repulsion between the droplet interfaces. Typically, extreme shear or extensional flow are necessary to create a nanoemulsion, since the viscous stresses, r_v, on the droplet's surfaces must overcome the Laplace pressure, EI_L = 2σ/a, where σis the interfacial tension, of spherical parent droplets (Mason, T. G. Curr. Opin. Colloid Interface Sci. 1999, 4, 231). As a result, very high strain rates ^approaching 10⁸ s^"1 are usually necessary to create water-based nanoemulsions (Meleson, K.; Graves, S.; Mason, T. G. Soft Mater. 2004, 2, 109). A strong surfactant and low solubility of the dispersed oil phase in the continuous phase are critical for producing long-lived nanoemulsions that do not coarsen through Ostwald ripening (Durian, D. J.; Weitz, D. A.; Pine, D. J. Science 1991, 252, 686; Gopal, A. D.; Durian, D. J. Phys. Rev. Lett. 2003, 91; Mason, T. G.; Krall, A. H.; Gang, H.; Bibette, J.; Weitz, D. A. Encyclopedia of Emulsion Technology; Marcel Dekker: New York, 1996; Vol. 4). (The terms oil phase and continuous phase used herein refer to two immiscible materials that can be used to produce an emulsion. In some embodiments, the continuous phase can be an aqueous material in which oil droplets are dispersed to form an oil in water emulsion. In other word, each of the two immiscible materials is sometimes referred to as a "phase" for conciseness.)

[0004] The size distribution of nanoemulsion droplets depends on the history of flow to which they have been subjected. Two emulsions with identical compositions can have very different droplet size distributions depending upon their flow histories (Meleson, K.; Graves, S.; Mason, T. G. Soft Mater. 2004, 2, 109; Mason, T. G.; Wilking, J. N.; Meleson, K.; Chang, C. B.; Graves, S. M. J. Phys.: Condens. Matter 2006, 18, R635). Taylor's scaling prediction (Taylor, G. I. Proc. R. Soc. 1934, A146) for emulsions subjected to applied viscous stress can be used to estimate the average radius of the resulting droplets: \a) ^∞ σ^/ τ_v ^« σ^/{η_ca⁾ _{^ w}jj_ere η_c j_{s me} viscosity of the continuous phase (Taylor, G. I. Proc. R. Soc. 1934, Al 46; Taylor, G. I. Proc. R. Soc. 1932, 138, 41). This scaling prediction is accurate for emulsions at dilute droplet volume fractions φ, although it neglects the viscosity 77_d of the dispersed phase and numerical factors that depend on the details of the flow history. Many refinements of this basic prediction have been made for a variety of cases beyond this simple scaling expression (Mason, T. G.; Bibette, J.; Weitz, D. A. J. Colloid Interface Sci. 1996, 179, 439). Nevertheless, there is a general lack of detailed understanding of the production of concentrated emulsions, including the role of coalescence, when the interfaces of neighboring droplets strongly interact (Mason, T. G. Curr. Opin. Colloid Interface Sci. 1999, 4, 231).

[0005] The optical transparency of any colloidal dispersion depends upon the refractive index difference, Δn = n_<i- n_c, between the dispersed objects, which have a refractive index n_&, and the continuous phase, which has a refractive index n_c. Because An ~ 0.1 for many kinds of oil droplets in water, most concentrated microscale emulsions appear white due to multiple scattering over a broad range of wavelengths λ in the visible spectrum. To make microscale emulsions transparent, an index matching material that is soluble in the continuous phase yet insoluble in the dispersed phase can be added so that Δn effectively vanishes, at least at a specific wavelength for a given temperature. For example, in silicone oil-in-water emulsions, glycerol can be added to the aqueous phase to match the refractive index at room temperature (Mason, T. G.; Krall, A. H.; Gang, H.; Bibette, J.; Weitz, D. A. Encyclopedia of Emulsion Technology; Marcel Dekker: New York, 1996; Vol. 4).

[0006] An interesting transformation can occur when repulsive droplets in a microscale emulsion are ruptured down to nanoscale sizes: the emulsion can change from a white appearance, resulting from multiple scattering, to a highly transparent nanoemulsion (Mason, T. G.; Wilking, J. N.; Meleson, K.; Chang, C. B.; Graves, S. M. J. Phys.: Condens. Matter 2006, 18, R635). Although index matching is one way of making emulsions transparent, another potential approach is to rupture droplets to such small sizes that light in the entire visible spectrum is not multiply scattered. Indeed, a simple inspection of oil-in- water nanoemulsions having <a> < 50 nm reveals that they possess very different optical properties compared to more common microscale emulsions. Dispersions of nanoscale droplets can be quite transparent, even at high φ and large An.

[0007] Rayleigh scattering describes the scattering of light from polarizable dielectric objects much smaller than λ. The Rayleigh scattering cross-section of molecules is well known to be inversely proportional to A⁴ (Mason, T. G.; Bibette, J.; Weitz, D. A. J. Colloid Interface ScL 1996, 179, 439). Rayleigh scattering explains why the sky is blue when looking away from the sun; shorter wavelengths are scattered much more intensely by polarizable molecules in the atmosphere. By contrast, while looking toward the sun as it is setting, most of the short- wavelength light is scattered away, and only reddish light passes through a more extended distance of the atmosphere, yielding red sunsets. Due to Rayleigh scattering, nanoemulsions have similar optical characteristics even at high φ (van de Hulst, H. C. Light scattering by small particles; Dover Publications: New York, 1981). When illuminated from the side by white light, a nanoemulsion appears to have a faint bluish tint because shorter wavelength light is scattered more strongly at higher angles. By contrast, when looking through a nanoemulsion directly towards a source of white light, it appears to have a slightly reddish tint.

[0008] The optical properties of multi-phase dispersions of spherical colloids, deformable droplets, and gas bubbles, have been previously investigated for many years. For instance, diffusing-wave spectroscopy (DWS) relies upon a model of diffusing photons to describe multiply scattered light throughout a colloidal suspension (Pine, D. J.; Weitz, D. A.; Chaikin, P. M.; Herbolzheimer, E. Phys. Rev. Lett. 1988, 60, 1134; Weitz, D. A.; Pine, D. J.; Pusey, P. N.; Tough, R. J. A. Phys. Rev. Lett. 1989, 63, 1747). In another example, dissymmetry and turbidity measurements with fixed incident visible wavelengths of λ = 436 run, 546 nm, and 578 nm have been performed on polydisperse spherical particles having a ~ 0.1 μm in an attempt to deduce the particle size distribution (Atherton, E.; Peters, R. H. Br. J. Appl. Phys. 1953, 4, 344). Multiple light scattering measurements from concentrated suspensions of interacting polymer spheres having a = 0.23 μm at λ = 514 nm compare well with calculations that include interference effects from proximate nearest neighbors through the structure factor, S (Fraden, S.; Maret, G. Phys. Rev. Lett. 1990, 65, 512). In addition, theoretical studies have been performed on the effect of particle shape, size, and refractive index on transparency (Johnsen, S.; Widder, E. A. J. Theor. Biol. 1999, 199, 181). Backscattering experiments have been used to determine the size of polystyrene latex spheres with diameters of 0.14 μm and 1.0 μm over a range of φ for single and multiple scattering (Clapper, M. F.; Collura, J. S.; Harrison, D.; Fisch, M. R. Phys. Rev. E 1999, 59, 3631). The transport mean free path, £ *, and strong wavelength dependence of the transmission intensity of light that is multiply scattered by sub- 100 nm charged polystyrene spheres with tunable interaction potentials has also been investigated for/ up to about 0.16 (Rojas-Ochoa, L. F.; Mendez-Alcaraz, J. M.; Saenz, J. J.; Schurtenberger, P.; Scheffold, F. Phys. Rev. Lett. 2004, 93). Multiple scattering from foams that coarsen through gas diffusion has led to a better understanding of the topological dynamics of neighboring bubbles (Durian, D. J.; Weitz, D. A.; Pine, D. J. Science 1991, 252, 686). Theoretical studies on the diffuse transport of light through two and three-dimensional structures simulating the transport of photons through foams based on the rules of ray optics has provided insight into the diffusion of photons within foams (Miri, M.; Madadi, E.; Stark, H. Physical Review E 2005, 72; Miri, M.; Stark, H. Physical Review E 2003, 68; Miri, M. F.; Stark, H. Europhysics Letters 2004, 65, 567). Although much is known about the optical properties of many complex fluids, the transparency of concentrated systems of deformable droplets having <a> < 100 nm for a variety of droplet interaction potentials over a wide range of φ remains obscure.

[0009] Controlling the degree of optical opacity of emulsions (also known as biliquid dispersions) through the visible and ultraviolet wavelength ranges can be important for influencing the appearance and sun-blocking capacity of these materials. It is well known that consumers are affected by the appearance of foods and also personal care products, such as lotions, sunscreens, cosmetics, and moisturizers. There thus remains a need for a systematic method for creating biliquid dispersions that have highly desirable and controllable optical properties.

SUMMARY

[0010] A method of producing an emulsion according to an embodiment of the current invention includes producing a first emulsion comprising a first plurality of droplets of a first liquid dispersed in a second liquid, the first plurality of droplets having a first ensemble average radius; and removing a plurality of droplets from the first emulsion that each have larger radii than the first ensemble average radius to obtain a second emulsion comprising a second plurality of droplets having a second ensemble average radius that is less than about 100 nm. The first liquid is at least partially immiscible with the second liquid and the second emulsion is more transparent to visible light than the first emulsion.

[0011] A method of producing a material having a preselected optical property according to some embodiments of the current invention includes providing a first emulsion comprising a plurality of droplets having an ensemble average radius less than about 100 nm, and blending an additive with the first emulsion to provide a second emulsion. The additive includes at least one of a plurality a droplets having an ensemble average radius greater than about 100 nm or a plurality of particles having a dimension greater than about 50 nm. The first emulsion is more transparent to visible light than the second emulsion. [0012] A method of producing a transparent material according to some embodiments of the current invention includes producing a nano-emulsion comprising a first volume fraction of nano-droplets having an ensemble average radius less than about 100 nm, the volume fraction being less than about 10% and increasing a density of the nano-droplets to a second volume fraction. The second volume fraction is greater than about 10% and the nano- emulsion is more transparent to visible light than the nano-emulsion having the first volume fraction.

[0013] According to further embodiments of the current invention, an emulsion includes a first liquid, and a plurality of droplets of a second liquid dispersed in the first liquid. The second liquid is at least partially immiscible with the first liquid. The plurality of droplets have an ensemble average radius that is less than about 100 nm and a standard deviation about the ensemble average radius of that is less than about 25% such that the emulsion is substantially transparent to visible light.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Further objectives and advantages will become apparent from a consideration of the description, drawings, and examples.

[0015] Figure IA shows transmitted intensity, Λ-_an_s (%) as a function of wavelength, λ, and volume fraction, φ, of a nanoemulsion with average radius <a> = 32 nm in a cuvette with pathlength L = 0.1 mm according to an embodiment of the current invention. Figure IB shows transmitted intensity, /^_ns (%) as a function of φ for λ = 250 nm, <a> = 32 nm, and Z = 0.1 mm according to an embodiment of the current invention. Lines guide the eye.

[0016] Figure 2 shows the extinction coefficient, /, as a function of wavelength, λ, for nanoemulsions with a volume fraction φ = 0.10 in a cuvette with pathlength, L = 0.1 mm (•), 0.5 mm (B), and 1.0 mm (A) according to an embodiment of the current invention. The solid line is fit to the equation γ= γ_\0 (λ / λ_\0)^~^ + γ_∞. Inset: Average nanoemulsion radii. To facilitate viewing, <a> = 89 nm data have been multiplied by a factor of 10 and <a> = 32 nm have been divided by a factor of 10. Data for <a> = 47 nm are unshifted and correspond to the left axis. Arrows mark the departure from single to multiple scattering toward lower λ.

[0017] Figures 3A-3F show the extinction coefficient, /, as a function of wavelength, λ, for nanoemulsions in a cuvette with path length L = 0.1 mm according to an embodiment of the current invention. Solid lines are fit to the equation γ = }\₀ (λ / λ_\0)^~^ + χ_∞. Average nanoemulsion radii: <a> = (Figure 3A),( Figure 3B) 32 nm, (Figure 3C),( Figure 3D) 47 nm, (Figure 3E),( Figure 3F) 89 nm. Droplet volume fractions: φ = 0.01, (#), 0.05 (T), 0.10 (β), 0.15 (HI), 0.20 (A), 0.25 (half-filled squares), 0.30 (♦),0.40 (k), 0.50 (J).

[0018] Figure 4A shows fitting parameter γ_\0 from equation γ = γ_\0 (λ / λ_\0)^~^ + χ_∞ used to fit the graph of extinction function, γ, as a function of wavelength, λ, (see Figure 3) for nanoemulsions with average radius <a> = 32 nm (•), <a> = 47 nm (H), <a> = 89 nm (A) in a cuvette with pathlength L = 0.1 mm according to an embodiment of the current invention. The solid lines are fits using the modified hard sphere structure factor (Percus - Yevick closure)

⁼ φ/^^eak- Figure 4B shows structure factor parameters ;H_o ^peak (*)and øf^eak (•) obtained from Figure 4 A. The horizontal dashed line represents the predicted value of the volume fraction associated with the peak expected for hard spheres: <f^e3k = 0.13. The solid lines are empirical spline curves through the data.

[0019] Figures 5A-5F show calculated (•) and experimental (O) extinction coefficient, γ, as a function of wavelength, λ, for a nanoemulsion with <ά> = 32 nm in a cuvette with pathlength, L = 0.1 mm, and volume fractions: φ = 0.01, (Figure 5A), 0.05 (Figure 5B), 0.10 (Figure 5C), 0.15 (Figure 5D), 0.25 (Figure 5E), 0.40 (Figure 5F) (shown in separate panels for clarity) according to an embodiment of the current invention. Solid lines guide the eye through the Mie scattering calculations.

[0020] Figure 6 provides a comparison of the transmitted intensity, /_trans5 as a function of wavelength, λ, for three different nanoemulsions according to embodiments of the current invention. The more polydisperse nanoemulsion has average radius <a> = 57 nm and a standard deviation in the radial size distribution of Sa = 40 nm (open circles). Two other nanoemulsions are shown for which larger droplets have been removed from their size distributions, resulting in a smaller polydispersity: <ά> = 47 nm with Sa = 11 nm (solid squares), and <a> = 89 nm with Sa = 18 nm (solid triangles).

DETAILED DESCRIPTION

[0021] Some embodiments of the current invention are discussed in detail below. In describing embodiments, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. A person skilled in the relevant art will recognize that other equivalent components can be employed and other methods developed without departing from the broad concepts of the current invention. All references cited herein are incorporated by reference as if each had been individually incorporated.

[0022] Nanoemulsions provide significant scattering in the ultraviolet wavelength range, while maintaining a high degree of transparency in the visible wavelength range. By contrast, uniform emulsions having mean droplet radii <α> greater than about 100 nm can scatter visible light significantly, leading to multiple scattering that gives rise to the white appearance typical of emulsions, such as mayonnaise.

[0023] By blending a nanoemulsion, which can be made using a variety of processes

(see, e.g., T.G. Mason, J.N. Wilking, K. Meleson, CB. Chang, and S.M. Graves, Nanoemulsions: Formation, Structure, and Physical Properties, J. Phys.: Condens. Matter 18 R635-R666 (2006)), that possesses a size distribution having <a> < 100 nm with at least one different emulsion that has <a> > 100 nm, we can continuously tune the optical scattering properties of the nanoemulsions according to some embodiments of the current invention to vary in appearance from transparent, to translucent, to very opaque in the ultraviolet and visible regions of the optical spectrum. Generally, we can maintain significant scattering in the ultraviolet part of the spectrum, while controlling the transparency in the visible part of the spectrum of electromagnetic radiation according to some embodiments of the current invention. [0024] Methods of manufacture according to some embodiments of the current invention include taking a volume V_n of a nanoemulsion of fixed volume fraction $, that is stable against coarsening and blending it together with a volume V_e of a larger emulsion of a fixed volume fraction φ_e so that the droplets of different sizes are homogeneously dispersed through a process such as mechanical mixing. A refractive index difference An between the dispersed droplet liquid phase material and the continuous non-droplet liquid phase material that is larger than 0.001 is suitable for some applications of the current invention. Furthermore, a refractive index difference An between the dispersed droplet liquid phase material and the continuous non-droplet liquid phase material that is greater than 0.01 is suitable for particular embodiments of the current invention. For most liquids used in emulsions and nanoemulsions, 0.01 < An < 1. The process of controlling the appearance of the biliquid dispersion according to some embodiments of the current invention results from scattering of light from the blend of nanoemulsion and emulsion. Although biliquids dispersions are referred to as examples throughout this specification, the general concepts of the current invention are intended to include multi-liquid dispersions. Additionally dyes, reflective particles, absorbing particles, refracting particles, molecules, colorants, pigments, and other additives can be blended into the dispersed and/or continuous phases to further alter the optical properties of materials produced according to some embodiments of the current invention.

[0025] This method can be implemented in a continuous flow production environment according to some embodiments wherein a material stream containing a nanoemulsion (e.g. directed by pipes, tubes, or microfluidic channels) is combined and blended with a different material stream containing an emulsion or nanoemulsion having a different size distribution using a mixing device. In this case, the volume flow rates of the two different streams, the size distributions in the two different streams, as well as the volume fractions in the different streams, would set the effective scattering properties and optical appearance of the final blended biliquid dispersion.

[0026] The optical properties of nanoemulsions can depend on whether the interactions between the droplets are attractive in a way that can lead to aggregation without significant coalescence. For instance, if there is a secondary minimum in the pair interaction potential as a function of separation between two droplets that is deeper than thermal energy, then the droplets can become aggregated without coalescing. For the example embodiments shown in the specification, there are no such deep secondary potential minima in the interaction potential between the droplets, and the interaction potential between the droplets is predominantly repulsive so that the droplets do not aggregate. Thus, the optical properties shown in the example embodiments are characteristic of nanoemulsions composed of predominantly repulsively interacting droplets that are non-aggregated and do not experience attractive interactions.

[0027] It can be reasonably anticipated that the extinction coefficients of nanoemulsions can be further tuned, beyond modifying the first and second moments of the droplet radial size distribution as we have demonstrated, by modifying the degree of repulsion and/or attraction between the interfaces of droplets in the nanoemulsion. One way of introducing an attractive interaction between droplets that can change optical properties such as the extinction coefficient is to add monovalent or multivalent salts, such as sodium chloride or magnesium chloride, that dissolve into the aqueous continuous phase of an oil-in- water nanoemulsion. To reduce the extinction coefficient in the visible region of the spectrum, it is typically not desirable to have even residual attractive interactions between the interfaces of droplets at any separation. In this regard, anionic, cationic, zwitterionic, and nonionic materials that are surface active are typically desirable in nanoemulsions for which a significant degree of transparency in the visible spectrum is desired, while preserving a larger degree of scattering in the UV portion of the spectrum.

[0028] In one of the embodiments of the present invention, the optical properties of a nanoemulsion are tuned by raising the volume fraction of the nanoemulsion. When the initial droplet volume fraction of the nanoemulsion is about 15 percent, and the final droplet volume fraction is above about 20 percent, then the nanoemulsion typically has a lower extinction coefficient in the visible spectrum due to the role of the structure factor in the scattering. At the same time, while concentrating the nanoemulsion, it is possible to simultaneously alter a different physical property, the shear elasticity of the nanoemulsion, which can become dominantly elastic. Thus, it is possible to produce a gel-like material that has a smaller extinction coefficient in the visible spectrum than in the ultraviolet spectrum, as well as an elastic property that confers a gel-like rigidity to the nanoemulsion (see also "Elastic Vitrification of Emulsions by Droplet Rupturing," PCT/2008/000800, the entire contents of which are incorporated herein by reference). This elastic gel-like rheological property in combination with the optical scattering properties, makes it possible to create a sunscreen or sunblock product that has a desirable rheological response that includes an elastic shear modulus (i.e. storage modulus) at 1 rad/s that is greater than about 1 Pascal as well as desirable optical properties for inhibiting transmission of UV rays to the skin while maintaining a clear visual appearance.

[0029] One way of modifying the size distribution is to use droplet size reduction protocols involving mixtures of oils having different molecular weights. These methods can also be used to produce extremely small nanoemulsions that have an average radius of the droplet size distribution of about 10 nm (see also "Process for Reducing Sizes of Emulsion Droplets," U.S. Provisional Application Serial No 61/129,294, the entire contents of which are incorporated herein by reference).

[0030] According to some further embodiments, solid particles (e.g. titanium dioxide nanoparticles or microparticles) could be blended in with the nanoemulsion to make the material appear more strongly optically scattering in the visible part of the spectrum, while increasing the amount of scattering leading to sun protection in the ultraviolet part of the spectrum. Just a very small fraction of these solid particles would need to be added to a nanoemulsion to give it a very white appearance. One could alternatively, or in addition, blend, mix or disperse other materials with the nanoemulsion so that the resulting composition has a preselected color. In other words, a nanoemulsion according to some embodiments of the current inventions can provide an ingredient for the production of a wide range of products that have preselected optical properties.

[0031] If desired, real-time monitoring of the optical properties of the blend can be accomplished by installing a computer controlled UV-visible spectrometer that is connected by a network or wireless connection to a central process control facility. [0032] Additional embodiments for controlling the optical appearance of the biliquid

(or multi-liquid) dispersion according to the current invention are essentially the reverse of the above noted embodiments; i.e., taking a polydisperse biliquid dispersion and fractionating the size distribution through a separation process. For instance, a polydisperse nanoemulsion that has a significant number of larger droplets in the size distribution with radii that approach and exceed 100 nm leads to more scattering in the visible region of the spectrum. By size-fractionating the polydisperse nanoemulsion to eliminate these larger droplets (e.g. through filtration, creaming, or sedimentation), it is possible to make the resulting nanoemulsion with a more uniform size distribution to look much more optically transparent. This can have uses in products such as cosmetics, sunscreens, lip balms, ointments, and purfumes, for example, where clarity of appearance (e.g. looking translucent or nearly transparent without a lot of scattering) is typically desirable.

[0033] Examples of biliquid dispersions are oil-in-water emulsions, water-in-oil emulsions, oil-in-water nanoemulsions, and water-in-oil nanoemulsions. Multi-liquid dispersions can include double dispersions such as water-in-oil-in-water dispersions or oil-in- water-in-oil dispersions, for example. All of these systems generally contain surfactant that stabilizes the droplets against coalescence. The surfactant molecules are generally much smaller than the droplets, so scattering from these molecules can typically be neglected over the range of wavelengths we consider for many applications. These molecules can alter the refractive index difference somewhat, but usually conditions of index matching where An ~ 0 are rare in most economical formulations of oil-in-water emulsions where there is as little surfactant used as possible. A wide variety of oils, including natural oils, edible oils, plant- derived oils, and animal-derived oils, and surfactants can be used in the biliquid dispersions according to embodiments of the current invention. In some embodiments of the current invention either the dispersed phase material and/or the continuous phase material of a biliquid (or multi-liquid) dispersion can also be a mix, blend, or dispersion of a plurality of materials.

[0034] A method of producing an emulsion according to an embodiment of the current invention includes producing a first emulsion comprising a first plurality of droplets of a first liquid dispersed in a second liquid, the first plurality of droplets having a first ensemble average radius; and removing a plurality of droplets from the first emulsion that each have larger radii than the first ensemble average radius to obtain a second emulsion comprising a second plurality of droplets having a second ensemble average radius that is less than about 100 nm. The first liquid is at least partially immiscible with the second liquid and the second emulsion is more transparent to visible light than the first emulsion. Removing some of the larger droplets from the first emulsions that have radii larger than the ensemble average radius of the first emulsion leads to a second emulsion that is less polydisperse than the first emulsion. Throughout this specification, we characterize droplets in emulsions by a radius. In some cases, the droplets will be essentially spherical. However, the invention is not limited to only emulsions that have spherical droplets. In cases in which the droplets are not spherical, the term "radius" should be considered as an effective radius that characterizes the sizes of the droplets.

[0035] The second ensemble average radius of said second emulsion can be greater than about 10 nm according to some embodiments of the current invention such that the second emulsion is more transparent to visible light than to ultraviolet light. The second plurality of droplets of said second emulsion have a standard deviation about the second ensemble average radius of the second emulsion that is less than about 25% of the second ensemble average radius according to some embodiments of the current invention. According to some embodiments of the current invention, the second plurality of droplets of the second emulsion have a standard deviation about the second ensemble average radius of the second emulsion that is less than about 15% of the second ensemble average radius. According to some embodiments of the current invention, the second plurality of droplets of the second emulsion have a standard deviation about the second ensemble average radius of the second emulsion that is less than about 20 nm.

[0036] The removing according to some embodiments of the current invention includes at least one of a filtering, a dialysis, a field flow fractionation, a creaming, a sedimentation, a coalescence, or a centrifugation process. The method of producing an emulsion according to some embodiments of the current invention further includes mixing an additive with at least one of the first liquid, the second liquid, the first emulsion or the second emulsion. According to some embodiments of the current invention the additive includes at least one of ultraviolet-light-blocking molecules, moisturizing molecules, exfoliant molecules, anti-microbial molecules, anti-fungal molecules, anti-acne molecules, anti- wrinkle molecules, anti-septic molecules, insect-repellent molecules, dyes, pigments, particulates, nanoparticulates, clays, lipids, proteins, lipoproteins, vitamins, polypeptides, block copolypeptides, biopolymers, fragrances, pH modifiers, or water repellency molecules.

[0037] The method of producing an emulsion according to some embodiments of the current invention also includes, after removing the plurality of droplets from the first emulsion, measuring an optical transparency of the second emulsion and determining whether to remove droplets from the second emulsion based on the measuring. This can permit a feedback production approach and/or real time quality control for example.

[0038] A method of producing a material having a preselected optical property according to some embodiments of the current invention includes providing a first emulsion comprising a plurality of droplets having an ensemble average radius less than about 100 nm, and blending an additive with the first emulsion to provide a second emulsion. The additive includes at least one of a plurality a droplets having an ensemble average radius greater than about 100 nm or a plurality of particles having a dimension greater than about 50 nm. In the case of this particular embodiment, the first emulsion is more transparent to visible light than the second emulsion.

[0039] A method of producing a transparent material according to some embodiments of the current invention includes producing a nano-emulsion comprising a first volume fraction of nano-droplets having an ensemble average radius less than about 100 nm, the volume fraction being less than about 10% and increasing a density of the nano-droplets to a second volume fraction. The second volume fraction is greater than about 15% and, at this second volume fraction, the nano-emulsion is more transparent to visible light than the nano- emulsion at the first volume fraction. Therefore, according to some embodiments of the current invention, an emulsion having a higher volume fraction of droplets than another emulsion can be more transparent to visible light. [0040] According to further embodiments of the current invention, an emulsion includes a first liquid, and a plurality of droplets of a second liquid dispersed in the first liquid. The second liquid is at least partially immiscible with the first liquid. The plurality of droplets have an ensemble average radius that is less than about 100 nm and a standard deviation about the ensemble average radius of that is less than about 25% such that the emulsion is substantially transparent to visible light. The standard deviation about the ensemble average radius is less than about 15% according to some embodiments of the current invention. The ensemble average radius is greater than about 15 nm such that the emulsion is more transparent to visible light than to ultraviolet light according to some embodiments of the current invention.

[0041] An emulsion according to some embodiments of the current invention also includes an additive mixed with the emulsion such that the additive causes at least a modification of an optical property of the emulsion. According to some embodiments of the current invention the first liquid is an aqueous liquid and the second liquid is an oil such that the first and second liquids have a difference in refractive index at a visible wavelength that is greater than about 0.01. According to other embodiments of the current invention, the second liquid is an aqueous liquid and the first liquid is an oil such that the first and second liquids have a difference in refractive index at a visible wavelength that is greater than about 0.01. According to further embodiments of the current invention at least some of the plurality of droplets comprises an internal droplet of a liquid that is immiscible with the second liquid such that the emulsion is a double emulsion.

[0042] According to some embodiments of the current invention, the ensemble average radius less than about 50 nm. According to some embodiments of the current invention, the ensemble average radius less than about 20 nm.

[0043] An emulsion according to some embodiments of the current invention includes an additive mixed with the emulsion. The additive can include at least one of ultraviolet-light-blocking molecules, moisturizing molecules, anti-microbial molecules, antifungal molecules, anti-acne molecules, anti-wrinkle molecules, antiseptic molecules, dyes, pigments, particulates, nanoparticulates, zinc oxide particulates, titanium dioxide particulates, clays, lipids, proteins, polypeptides, block copolypeptides, biopolymers, pH modifiers, fragrances, or water repellency molecules. The particulates can be microscale or nanoscale titanium dioxide or zinc oxide particles according to some embodiments of the current invention to enhance blocking of ultraviolet light, such as in sunscreens and sunblocks.

[0044] According to some embodiments of the current invention the emulsion has an extinction coefficient for transmitted light that is above about 1 mm^"1 for ultraviolet wavelengths of light below about 400 nm and an extinction coefficient below about 1 mm^"1 for visible wavelengths of light above about 400 nm. These conditions correspond to a numerical measure of the intrinsic ultraviolet sunblocking capacity of the nanoemulsion according to some embodiments of the current invention, while retaining a desirable clear visual appearance with less blocking of visible light. Since some people do not accept the appearance of white-looking layer of sunblock on their bodies and faces; a clear visual appearance can be desirable. Nevertheless, a good sunblock still has to block strongly in the ultraviolet. A higher extinction coefficient corresponds to more light-blocking power. Although many UV-visible spectrometers can measure extinction coefficients down to only about 250 nm wavelengths, it can be reasonably anticipated that, for nanoemulsions which offer significant transparency in the visible wavelength range, the extinction coefficient of such nanoemulsions will be even larger at deeper ultraviolet wavelengths below 250 nm, corresponding to even stronger UV-blocking efficacy.

Theory

[0045] Many aspects of the theory of scattering of electromagnetic radiation by colloidal dispersions are well known (Fraden, S.; Maret, G. Phys. Rev. Lett. 1990, 65, 512; Gate, L. F. J. Opt. Soc. Am. 1973, 63, 312; Kaplan, P. D.; Yodh, A. G.; Pine, D. J. Phys. Rev. Lett. 1992, 68, 393) and some features of this theory can be recalled to serve as a rough guide for comparison with measured optical properties of nanoemulsions. As a photon travels through a uniform dispersion of dielectric objects, it can either be transmitted, absorbed, or scattered away from a detector positioned directly opposite the source:

Λrans ^~*^~ -4bs + -<scatt ⁼ (1) where /_trans is the intensity of light that is transmitted through the sample, /_abs is the intensity of the light that is absorbed by the sample, /_scatt is the intensity of light that is scattered by the sample, and /o is the total intensity of light from the source. For non-absorbing oil droplets, /_abs can be neglected, and Eq. (1) becomes:

/trans + /scatt ⁼ /()• (2)

[0046] The intensity of transmitted light through a sample of thickness, or 'path- length', L, can be described by Beer's law (van de Hulst, H. C. Light scattering by small particles; Dover Publications: New York, 1981):

/trans = /_O e ^{" n}, (3)

where γ is the extinction coefficient that depends on the wavelength and the material properties of the sample.

[0047] Although Beer's law is typically used to describe an optical absorption process, it can also be used to describe the loss of photons by scattering if one assumes that all scattered photons exit the sample without being detected. In this limit, which effectively corresponds to a very small solid angle of acceptance by the detector along the direction of the incident light, the extinction coefficient is proportional to the natural logarithm of the ratio of the incident intensity to the transmitted intensity:

Jiφ,λ) = L^Λ In [/₀(;t)//trans(<2U)]- (4)

[0048] For highly dilute solutions, 'single-scattering' dominates. Each photon that enters the sample will be scattered at most once before exiting the sample at an angle θ relative to the incident beam over a range 0° < θ < 180°, from forward scattering to backscattering (Johnsen, S.; Widder, E. A. J. Theor. Biol. 1999, 199, 181). However, as φ is increased, double scattering can occur when a photon is scattered two times before it exits the sample. Multiple scattering occurs when photons are scattered many times before leaving the sample. Since γ is an intensive property, one way to test for multiple scattering is to examine extinction coefficients measured from spectroscopic transmission experiments for the same sample at different thicknesses; if the measured γ(L) overlap well, then there is little multiple scattering.

[0049] The scattering cross-section, C_scatt> is the effective scattering area of each isolated droplet, not including interference effects:

c_scatt(2) = ^ ρ_Scattμ) (5)

where A = m² is a droplet's geometrical cross-sectional area and Q_scM is the scattering efficiency, otherwise known as the dimensionless cross-section of each droplet (van de Hulst, H. C. Light scattering by small particles; Dover Publications: New York, 1981; Johnsen, S.; Widder, E. A. J. Theor. Biol. 1999, 199, 181). Calculations of £>_scatt for spheres having well defined radii can be performed over a wide and useful range of λ using a software program, MIETAB version 8.34, based on W. J. Lentz's Mie calculation routine. If C_scatt is known and the droplets are at dilute φ, then the extinction coefficient can be simply determined:

χ^) = N(ø c_scattμ), (6)

where N(φ) = 3φ/(4πa³) is the number density of spherical droplets. For non-absorbing droplets that always perfectly scatter light away from the detector, C_scatt = C_ext, where C_ext is the extinction cross-section of each droplet (Fraden, S.; Maret, G. Phys. Rev. Lett. 1990, 65, 512).

[0050] Mie scattering theory describes the distribution of scattered light intensity from an isotropic sphere in a homogenous medium (van de Hulst, H. C. Light scattering by small particles; Dover Publications: New York, 1981; Mie, G. Ann. Phys. 1908, 25, 377). Although Mie scattering predictions can be used directly to calculate γ for dispersions of spheres at low φ, for concentrated dispersions, interference effects from neighboring droplets can become important and this simple approach becomes inadequate. As a result, the extinction coefficient must include the effects of the structure factor, S (Rojas-Ochoa, L. F.; Mendez-Alcaraz, J. M.; Saenz, J. J.; Schurtenberger, P.; Scheffold, F. Phys. Rev. Lett. 2004, 93): γ{λ,φ) = ^ψ ^^k°c_scatt(q)S(φ,q)qdq (7)

Jt₀

where q = (47m_ef{/λ)sin(θ/2) is the scattering wavenumber, θ is the scattering angle relative to the direction of propagation of the incident photons, and ra_eff represents the effective refractive index of the dispersion, which is assumed to be the volume weighted average of the continuous and dispersed phases (Johnsen, S.; Widder, E. A. J. Theor. Biol. 1999, 199, 181). Here, the scale of the scattering wavenumber is ko = 2m_eff/λ, and the angle-dependent scattering cross section is c_scatt(<7) ⁼ (2π/ko²)F(q), where F(^) is the dimensionless form factor describing scattering from an isolated droplet. The integral covers the entire range 0 < q < 2ko from forward to backward scattering: 0 < q < π. For a given wavelength, the integral of ^_scattfø) over all accessible q gives the total scattering cross section:

Qca_tt ⁼ k_θ ^~ Jn ° ^c _scatt(^)^^- For emulsions, the separation of scattering contributions into a Mie form factor and a structure factor is valid as long as the droplets remain spherical and do not deform significantly. The extinction coefficient for single scattering in Eq. (7) is equivalent to the inverse scattering mean free path, Ml, described in multiple scattering approaches (Rojas-Ochoa, L. F.; Mendez-Alcaraz, J. M.; Saenz, J. J.; Schurtenberger, P.; Scheffold, F. Phys. Rev. Lett. 2004, 93).

[0051] The structure factor can have a profound effect on the optical transmission of concentrated dispersions, including nanoemulsions (Rojas-Ochoa, L. F.; Mendez-Alcaraz, J. M.; Saenz, J. J.; Schurtenberger, P.; Scheffold, F. Phys. Rev. Lett. 2004, 9; Graves, S.; Meleson, K.; Wilking, J.; Lin, M. Y.; Mason, T. G. J. Chem. Phys. 2005, 122). In the dilute limit, S(φ = 0, q = 0) ~ 1. However, as φ increases, the \ow-q scattering intensity, proportional to S{φ = 0, q = 0), decreases due to nearest neighbor interactions between droplets. Consequently, /also decreases towards larger φ. The interaction potential between the particles, U, plays an important role in determining the structure factor. For disordered glasses of hard spheres, S(q) has been calculated as the dispersion makes the transition to a non-ergodic system. This S for hard spheres can potentially provide a good starting point for beginning to understand concentration-related effects in the light intensity transmitted through concentrated nanoemulsion droplets that remain spherical. However, for nanoemulsion droplets that can deform, the separation of the extinction coefficient into a product of a form factor (i.e. c_scatt) and a structure factor may not be strictly valid, since the shape of the droplets can become non-spherical at very high φ where the nanoemulsion is elastic (Wilking, J. N.; Mason, T. G. Phys. Rev. E 2007, 75). In this limit, Eq. (7) is only an approximation.

Examples

[0052] We make uniform fractionated oil-in-water nanoemulsions using a multi-step emulsification process (Meleson, K.; Graves, S.; Mason, T. G. Soft Mater. 2004, 2, 109). We first create a pre-mixed emulsion of micron-sized droplets by slowly adding silicone oil (poly-dimethylsiloxane or PDMS: viscosity V_d = 10 cSt) to an aqueous solution of sodium dodecyl sulfate (SDS) at a concentration C_SDS = 116 mM while shearing over a period of five minutes using a high-speed dispersing wand (IKA). Typically, the droplet volume fraction is φ = 0.20. This pre-mixed emulsion is then subjected to extreme flow in a high-pressure, 'hard' microfluidic homogenizer (Microfluidics M-110S MICROFLUIDIZER® Processor). We subject the emulsion to the extreme flow by repeatedly passing it through the homogenizer; after roughly six passes, we find that <ά> does not change significantly because all droplets have experienced the region of the strongest flow. The resulting nanoemulsions typically have radial size polydispersities of around 30% (Meleson, K.; Graves, S.; Mason, T. G. Soft Mater. 2004, 2, 109).

[0053] We increase the uniformity of the droplet size distribution and remove very infrequent large droplets, which can cause significant light scattering, through repeated centrifugal fractionation (Mason, T. G.; Wilking, J. N.; Meleson, K.; Chang, C. B.; Graves, S. M. J. Phys.: Condens. Matter 2006, 18, R635). At the same time, we can also reset the SDS concentration to 10 mM by diluting with a stock solution rather than pure water. By diluting the nanoemulsion to φ = 0.10 and centrifuging for 10 hours at 25,000 RPM using swinging buckets, we obtain reduced size polydispersities that are typically around 15% after three fractionation steps. To control φ, we simply dilute a portion of the concentrated stock nanoemulsion with a 10 mM SDS solution. Using this approach, we have created three different fractionated nanoemulsions having <a> = 32 +7 nm, 47 + 11 nm, and 89 + 18 nm, as measured by dynamic light scattering. Each stock nanoemulsion is allowed to equilibrate for at least one week before measuring φ through evaporation of a small portion of the sample. At a temperature T = 22 ⁰C and at λ = 633 nm, the refractive index of the aqueous 10 mM SDS solution is n_c = 1.332, and that of the oil is »_d = 1-398.

[0054] Since nanoemulsions possess significant optical transparency (Mason, T. G.;

Wilking, J. N.; Meleson, K.; Chang, C. B.; Graves, S. M. J. Phys.: Condens. Matter 2006, 18, R635), we find it to be more convenient and direct to perform measurements of light transport in the single scattering limit, even when φ is far from the dilute regime. This single scattering approach, which involves using an optical cell with a small path length, L, is atypical of multiple scattering approaches that use thicker optical cells and require modeling of diffusive transport of the photons in the medium subject to particular boundary conditions. We show that this single scattering approach provides a practical means of measuring of the wavelength dependence of the extinction coefficient, γ, otherwise identified as the inverse scattering mean free path, l/£, in multiple scattering approaches.

[0055] To measure the transmission intensities over 250 nm < λ < 800 nm, we use an

Ocean Optics UV/VIS spectrophotometer (USB-2000) equipped with a direct-attach cuvette holder and a deuterium-tungsten light source. A narrow 25 μm x 1 mm detection slit is used to reduce contributions from multiple scattering into the detector. The entire spectrometer system is held at a constant T = 22°C inside a micro-incubator/refrigerator to minimize the fluctuations in

that would otherwise depend on external temperature changes. The lamp and CCD detector are turned on and allowed to equilibrate at least 90 minutes before acquiring data. To ensure that the lamp and electrical components are stabilized, we monitor the /_trans at six different A for 30 minutes. After this warm-up period, the deviations in the 4-_ans are less than 0.5%. To take each measurement, we load the samples in L = 0.1 mm, 0.5 mm, and 1.0 mm demountable quartz cuvettes that are first cleaned with isopropanol and methanol and then dried using pressurized house air. Results

[0056] The measured transmitted intensities as a function of λ and φ are shown in

Figure IA for a nanoemulsion with <a> = 32 nm and L = 0.1 mm. For all φ, we observe considerable scattering at low λ in the ultraviolet region as λ begins to approach <a>, and an increase in transmission up to /_trans ~ 100%, as λ increases above 450 nm towards 800 nm. As φ increases from the dilute limit, where scattering is minimal, towards φ = 0.15, the transmission decreases, most notably in the UV range at the lowest λ. This reflects the increasing number density of droplets that scatter more strongly in the nanoemulsion. However, for φ > 0.15, /trans increases and the nanoemulsion becomes more transparent again. We plot /_tran_s at /I = 250 nm as a function of φ for <a> = 32 nm and L = 0.1 mm (see Figure IB) to emphasize the observed minimum in /_trans(^) for several different φ. The overall trends in observed for <a> = 32 nm are also found for the larger droplet sizes, and the dips in the transmitted intensities at low λ are even more pronounced.

[0057] To verify that our measurements are within the limit of single scattering, we calculate the extinction coefficient using Eq. (4) for the three different nanoemulsions at φ = 0.10 for L = 0.1 mm, 0.5 mm, and 1.0 mm, as shown in Figure 2. We select this ^because it is near the minimum in /_trans, so multiple scattering will be easily seen if it is present at any φ. For the smallest droplets having <ά> = 32 nm, the excellent overlap of y{λ), regardless of/,, indicates that we are indeed measuring single scattering for all λ. The nanoemulsion having <α> = 47 nm exhibits good overlap of γ for all values of L at λ > 375 nm. However, the extinction coefficients for L = 0.5 mm and 1.0 mm do not overlap with data for L = 0.1 mm when λ < 375 nm. Thus, only Xl) for the thinnest sample cell, Z = 0.1 mm, represents true single scattering over the entire range of λ down to 250 nm. For the largest droplets <a> = 89 nm, there is an even larger range at low λ where / for the thicker L deviate, again indicating that only L = OA mm is a thin enough sample cell to ensure single scattering at all measured λ and over the entire range of droplet sizes we explore.

[0058] Since single-scattering transmission intensities from thinner path-length samples can be used to determine multiple scattering from thicker path-length samples, we focus on only single-scattering γ corresponding to L = 0.1 mm for the remainder of this example. Obviously, multiple scattering at lower wavelengths would further inhibit light transmission through thick samples. From Figure 2, a reduction in the droplet size by only about a factor of three can lead to more than two orders of magnitude in reduction in the extinction coefficient. Thus, relatively small changes in the radii of the nanoscale droplets can lead to a dramatically different visual appearance of nanoemulsions.

Analysis

[0059] The affect of the volume fraction of the droplets on the extinction coefficient of the nanoemulsions is explored and the experimentally determined values of γ are reported in Figures 3A-3F. For simplicity, the solid lines are fits to an equation that contains a Rayleigh-like power law plus a constant term:

where γ\₀ is the value of the extinction coefficient at the lowest wavelength λ_\0 = 250 nm measured, and γ_∞ accounts for residual scattering that may occur at high wavelengths. Such residual scattering may be present even when there is only a very small population of droplets in the size distribution that are significantly larger than <a>. For <a> = 89 nm and 0.10 < φ < 0.40, γ_∞ has been set equal to zero, since the value of γ_∞ is essentially indeterminate if allowed to vary and the assumption of γ_∞ is unnecessary due to the low residual scattering. The power law exponent, β, describes how rapidly γ decreases with λ over our limited measurement window. In the case of simple Rayleigh scattering, a value of β— 4 is expected due to the wavelength dependence of the scattering intensity.

[0060] The values of y\₀ from the power law fit for the three different namoemulsions are shown in Figure 4A. The increase and subsequent decrease of ]_\0, yielding a peak, is evident for all <a>. The maximum of γ_\0 occurs near φ = 0.10 for the smallest nanoemulsion sample, and near φ = 0.20 for the two larger sizes. The general shape of γ(φ) and the location of this peak for each <a> is in reasonable agreement with the peak found from structural studies using SANS for the low-g intensity Iι{φ) for nanoemulsions having a similar radius (Graves, S.; Meleson, K.; Wilking, J.; Lin, M. Y.; Mason, T. G. J. Chem. Phys. 2005, 122; Mason, T. G.; Graves, S. M.; Wilking, J. N.; Lin, M. Y. J. Phys. Chem. B 2006, 110, 22097). The values of β for <a> = 89 nm, 47 nm, and 32 ran are: β = 3.72 ± 0.55, 4.20 ± 0.27, and 5.66 ± 0.56, respectively. Due to the very limited dynamic range of wavelengths over which we can fit γ, all of the values of /?have relatively large uncertainties. However, the average β from the fits is consistent with the prediction of Raleigh scattering for nanoemulsions having <a> = 47 nm and 89 nm. For <a> = 32 ran, the average β from the fits is significantly larger than 4. This unexpectedly high value can be mostly attributed to the sensitivity of the power law exponent on the high-A fitting parameter, γ_∞, and the extremely limited range of λ over which the power law dominates. To more accurately determine β for <a> = 32 nm, we would need to measure transmissions to lower λ beyond the range of our spectrometer and further reduce the amount of residual scattering due to polydispersity. For all nanoemulsion samples, γ_∞ is on the order of 10^"2, indicating small residual scattering at higher wavelengths. For nanoemulsions that are not fractionated and have greater polydispersity, γ_∞ can be significantly larger. Even after fractionation, as φ increases, γ_∞ rises slightly, reflecting the increase in the number density of droplets still present in the upper tail of the size distribution.

[0061] Although Ti₀(^) qualitatively exhibit trends of peaks expected for monodisperse hard spheres (Ashcroft, N. W.; Lekner, J. Phys. Rev. 1966, 145, 83), the positions of the peaks actually depend on the droplet sizes, even when the Debye screening length is essentially the same for all samples (i.e. C_SDS is fixed at 10 mM). The solid lines in Figure 4A represent fits to a modified hard sphere structure factor corresponding to the Percus-Yevick closure relation (mod-PY-HS) and allow the peak position of ^₀ to shift by re- scaling the volume fraction by a multiplicative factor that can vary with <a> (Ashcroft, N. W.; Lekner, J. Phys. Rev. 1966, 145, 83; Baxter, R. J. Aust. J. Phys. 1968, 21, 563). The mod-PY-HS fits describe the data surprisingly well; especially since the theory is based on the assumption of hard interactions, not Debye-screened repulsions. Charge-stabilized nanoemulsions can behave differently than hard spheres, especially at small <a> where the Debye screening length (Z_D - 3.5 nm for 10 mM SDS) begins to significantly affect the effective volume fraction, (zW, which governs droplet jamming and deformation and influences the spatial correlation between droplets. In fact, nanoemulsions comprised of repulsive droplets can exhibit shear elasticity for φ < 0.4 (Wilking, J. N.; Mason, T. G. Phys. Rev. E 2007, 75), well below the maximally random jammed volume fraction Φ_MRJ ~ 0.64 (Torquato, S.; Truskett, T. M.; Debenedetti, P. G. Phys. Rev. Lett. 2000, 84, 2064), yet still have φ_eff ≥ Φ_MRJ- Lacking a theoretical structure factor for concentrated dispersions of deformable charged objects, we modify the hard sphere formalism in an ad-hoc manner, recognizing that this approach only provides a crude interpretation of the substantial richness of the interactions and structure present in concentrated nanoemulsions.

[0062] To characterize the trends in γ_\0 for nanoemulsions having different <a> in the ultraviolet, we show the two fit parameters obtained from the mod-PY-HS fits (Figure 4B): 71_o ^peak, the maximum value at the peak of the fit (Figure 4B) and ^^eak, the value of φ corresponding to the peak of the fit. Overall, ^_o ^peak increases with <a> because larger droplets scatter more light. Regular PY-HS theory yields a prediction /^*eak - 0.13 (Mason, T. G.; Lin, M. Y. Phys. Rev. E 2003, 67, 050401(R)/l). Droplets with <a> = 32 nm have ^^eak = 0.11 ± 0.01, slightly less than the predicted value for hard spheres. This lower value is likely to be due to the Debye-screened repulsion that causes considerable repulsive jamming at φ significantly less than Φ_MRJ- Rojas-Ochoa et al. measured a maximum in the \ow-q scattering intensity to be <fP^eΑk = 0.074 for charged polystyrene particles, also lower than ^^eak expected for hard spheres. Therefore, the charge repulsion between the surfaces of our droplets likely reduces /^eak. In contrast, the f^eΑk values for the <a> = 47 nm and <a> = 89 nm are closer to 0.2 and are larger than the hard sphere prediction of 0.13. Although this larger value of <^^e&k has also been seen in the \ow-q small angle neutron scattering (SANS) intensity from highly monodisperse fractionated nanoemulsions having similar size (<α> ~ 75 nm) (Mason, T. G.; Graves, S. M.; Wilking, J. N.; Lin, M. Y. J. Phys. Chem. B 2006, 110, 22097), the mechanism that gives rise to <f^eΑk ~ 0.2 for larger nanoemulsions observed using SANS or here using UV/Vis spectroscopy has not yet been identified theoretically. Residual attractions could give rise to larger values of <jP^e&k, but it is likely that Debye-screened repulsions dominate the droplet interactions at all <a> and φ we explore. The surfactant concentration is not large enough for any residual depletion attractions due to micelles to play a major role.

[0063] Next, we compare the experimental γ values with those calculated using the

MIETAB software and an approximate simplification of Eq. (7), where only the zero-g limit of the structure factor is considered: y{φ,X) ~ N(φ) C_scatt(2) S(φ,q=O). Because c_scztt(q) decreases rapidly towards higher q beyond roughly the inverse droplet size, the complete integral of Eq. (7) is most strongly determined by the behavior of the product c_scan(q)S{q) near and below the inverse droplet size. At higher φ, this product is governed mostly by S(q) at low-<7, so we make an approximation by simply replacing S(q) with its low-# limit, S(q=0) effectively taking S out of the integral. The remaining integral of c_SCΑtι(q) simply reduces to Q_catt, yielding the approximation: y{φ,X) ~N(φ) C_scM(λ) S(φ,q=O).

[0064] To see if this approximation at least can provide reasonable estimates of the measured extinction coefficients, we show results for <a> = 32 ran at six φ values from 0.01 to 0.40 in Figures 5A-5F. Overall, the agreement between our experimental data and this calculated estimate is surprisingly good; especially since there are no adjustable parameters in the calculations and since deformable charged nanodroplets that interact through a Debye- screened repulsion are not hard spheres. The maximum overlap between the experimental and calculated values occurs at intermediate values of λ. The largest deviation between the experimental and calculated values occurs at low wavelengths where λ begins to approach <a>, and towards larger λ when /_trans approaches 100%. The deviation at high λ is potentially due to poor instrument sensitivity and dynamic range when the intensity approaches the limit of 100% transmission. Higher measured values of /differ from the ideal predictions toward high λ are also due to residual droplet polydispersity that gives rise to γ_∞. Thus, the experimentally determined γ values based on Mie scattering theory and the hard sphere structure factor provide a method for estimating the transmission through concentrated nanoemulsions. To obtain a more accurate, albeit empirical prediction for a nanoemulsion having a known size and volume fraction, it is possible to deduce ^i_o ^peak and tf^eak from the plots in Figures 4A and 4B, and to use these along with Eqs. (3) and (8) to predict the transmitted intensity. Discussion

[0065] An interesting and potentially useful characteristic of nanoemulsions is their transparency across a wide range of the visible spectrum. It is possible to precisely control the degree of scattering, and hence γ, by fabricating and controlling <a> and φ of a specific nanoemulsion, even when the refractive index difference between the continuous and dispersed phases is significant. We have shown that droplets having <a> ~ 32 nm, when fractionated to remove larger droplets in the upper tail of the size distribution, results in a nearly transparent sample across all φ. Even for such small nanoemulsions, scattering in the ultraviolet is still significant. Based on the measurements and favorable comparison with a scattering model that accounts for structure factor effects is likely that for droplets having <ά> < 30 nm, even the ultraviolet scattering could be significantly reduced at either low or high φ. Indeed, the peak that we find in γ_\0(φ) can be well described by a modified PY-HS model (Ashcroft, N. W.; Lekner, J. Phys. Rev. 1966, 145, 83; Baxter, R. J. Aust. J. Phys. 1968, 21, 563; Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal dispersions; Cambridge University Press: Cambridge ; New York, 1989), even if further theoretical work is needed to explain the measured dependencies of the parameters. We note that the decrease in /j_o and the overall measured scattering from the nanoemulsion also correlates with the onset of elasticity at surprisingly low φ when <a> decreases and begins to approach λo (Wilking, J. N.; Mason, T. G. Phys. Rev. E 2007, 75). Overall, scattering from nanoemulsions can be finely tuned by simply varying <a> and φ. Blending nanoemulsions having different <ά> and φ also provides a way to tailor their optical properties.

[0066] Because the fitting parameters γ\₀, ;n_o ^pea\ and ^^peak are distinctly different for each nanoemulsion' s average radius, in principle, it is possible to use this information from an unknown sample of uniform repulsive nanoemulsion droplets to deduce <a> non- destructively. For instance, by measuring the transmission curve /_trans(A φ) in a quick set of experiments that neither destroys nor requires large amounts of sample, one can determine specific values for y\₀(Φ), from which ^i_o ^peak and <jP^S3k can be obtained. With these fitting parameters, one can conveniently deduce <a> for the nanoemulsion non-destructively by interpolating between the data that we provide here in Figures 4A and 4B. This is useful if one wishes to preserve the nanoemulsion, rather than destroy it through methods using evaporation, and if dynamic light scattering and SANS are not available. Of course, this approach is limited to uniform nanoemulsions predominantly comprised of repulsive droplets; if significant droplet attractions or size polydispersity are present, the transmission intensities could be dramatically different than those we have presented herein.

[0067] The degree of optical transparency of a nanoemulsion over time can also provide a qualitative quick check on the stability of the droplets. A nanoemulsion sample having <a> below 100 nm that is initially transparent will appear cloudy or opaque due to the presence of larger droplets if it undergoes Ostwald ripening or coalescence. Therefore, a simple visual check allows one to verify the stability of the emulsion.

[0068] Theories, or at least better models, of the structure factor and scattering from nanoemulsions over a wide range of φ are still needed to understand how parameters such as 7j_o ^peak and <jP^e3k depend on the degree of repulsion and attraction between droplet interfaces that can deform. Moreover, developing a theory that can predict the scattering properties from concentrated nanoemulsions that have significant size polydispersity would have important practical consequences for nanoemulsions that are not fractionated.

[0069] Although gaseous foams can be similar to nanoemulsions in terms of interfacial structure, due to the large refractive index difference between the gas and the liquid and the large sizes of the bubbles compared to the wavelength, multiple scattering of visible light is usually unavoidable (Durian, D. J.; Weitz, D. A.; Pine, D. J. Science 1991, 252, 686; Vera, M. U.; Saint-Jalmes, A.; Durian, D. J. Applied Optics 2001, 40, 4210). Therefore, for foams, it is sensible to examine and measure diffusive optical transport properties, such as the transport mean free path. However, for nanoemulsions, which have droplet sizes that are often orders of magnitude smaller than gas bubbles in foams, and that also have smaller refractive index differences than in foams, it is more direct and straightforward to determine single-scattering optical properties, such as the extinction coefficient which is essentially the scattering mean free path. As we have shown, simple UV- Visible spectrometry can provide sensitive measures of the scattering mean free path of concentrated nanoemulsions. These results could potentially be connected to the transport mean free path and then used to model the optical properties of wet foams consisting of much larger spherical gas bubbles below the limit of polyhedral foam at very high φ.

[0070] In conclusion, monodisperse nanoemulsions having <a> significantly less than 100 nm appear remarkably transparent due to the absence of scattering in the visible spectrum, even at large φ and without the use of index-matching modifiers. This physical characteristic distinguishes nanoemulsions from typical emulsions having micron and even sub-micron droplet sizes. For a reasonably uniform emulsion, a simple set of transmission measurements can yield information about the average droplet radius of the sample. One can also examining how the interaction potential between the droplets can affect their optical properties. Overall, the significant scattering at ultraviolet wavelengths, the relative transparency in the visible spectrum, and the ability to control the scattering through the droplet size distribution and φ, make long-lived nanoemulsions comprised of repulsive droplets useful for pharmaceutical and personal care products according to some embodiments of the current invention.

Effect of the Droplet Radial Size Distribution on the Optical Transparency of Nanoemulsions

[0071] Figure 6 shows the transmitted intensity, /_trans, of several different PDMS silicone oil-in- water nanoemulsions having ensemble average radius <a> < 100 nm, as seen through a 0.5 mm path length cell using UV-Visible spectroscopy according to an embodiment of the current invention. For all three samples, the SDS concentration in the aqueous phase is 10 mM, and the droplet volume fraction is φ = 0.05.

[0072] An example of a nanoemulsion that has <a> = 57 nm but a large standard deviation in radius of & = 40 nm, including a significant percentage of droplets larger than the mean radius is shown in Figure 6 (open circles). Since the droplet radial size distribution associated with this nanoemulsion has a longer tail that extends out to larger droplet sizes, the transmitted intensity at longer visible wavelengths is only about 25% at a wavelength of 700 nm. This nanoemulsion having <ά> = 57 nm and a large polydispersity (i.e. where Sa approaches <α>) could be typical of a nanoemulsion produced by a device that ruptures droplets inhomogeneously.

[0073] By contrast for the two other nanoemulsions shown in Figure 6, the larger droplets have been selectively removed and the droplet radial polydispersity is significantly less. Overall, compared to the nanoemulsion that has <a> = 57 nm and a larger polydispersity, the transmitted intensity is significantly higher for the nanoemulsion having <a> = 47 nm and a smaller standard deviation in radius Sa = 11 nm (solid squares). For the nanoemulsion having <a> = 47 nm, the transmitted intensity is above 75% at a wavelength of 700 nm. Thus, this nanoemulsion having <a> = 47 nm and a low polydispersity appears significantly more transparent than the nanoemulsion having <a> = 57 nm and a much larger polydispersity. Likewise, for the nanoemulsion having <a> = 89 nm with a standard deviation in radius δa = 18 nm (solid triangles), the transmitted intensity is still higher than the nanoemulsion having <a> = 57 nm over the range of wavelengths above about 500 nm. This shows that even a relatively small number of larger droplets in a nanoemulsion's radial size distribution can cause its transparency to be significantly reduced in the visible spectrum, even when compared to a nanoemulsion that has a larger ensemble average radius but a smaller polydispersity and therefore relatively fewer larger droplets.

[0074] In describing embodiments of the invention, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. The above-described embodiments of the invention may be modified or varied, without departing from the invention, as appreciated by those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the claims and their equivalents, the invention may be practiced otherwise than as specifically described.

Claims

WE CLAIM:

1. A method of producing an emulsion, comprising: producing a first emulsion comprising a first plurality of droplets of a first liquid dispersed in a second liquid, said first plurality of droplets having a first ensemble average radius; and removing a plurality of droplets from said first emulsion that each have larger radii than said first ensemble average radius to obtain a second emulsion comprising a second plurality of droplets having a second ensemble average radius that is less than about 100 nm, wherein said first liquid is at least partially immiscible with said second liquid, and wherein said second emulsion is more transparent to visible light than said first emulsion.

2. A method of producing an emulsion according to claim 1, wherein said second ensemble average radius of said second emulsion is greater than about 10 nm such that said second emulsion is more transparent to visible light than to ultraviolet light.

3. A method of producing an emulsion according to claim 1, wherein said second plurality of droplets of said second emulsion have a standard deviation about said second ensemble average radius of said second emulsion that is less than about 25% of said second ensemble average radius.

4. A method of producing an emulsion according to claim 1, wherein said second plurality of droplets of said second emulsion have a standard deviation about said second ensemble average radius of said second emulsion that is less than about 15% of said second ensemble average radius.

5. A method of producing an emulsion according to claim 1, wherein said second plurality of droplets of said second emulsion have a standard deviation about said second ensemble average radius of said second emulsion that is less than about 20 nm.

6. A method of producing an emulsion according to claim 1, wherein said removing comprises at least one of a filtering, a dialysis, a flow field fractionation, a coalescence, a creaming, a sedimentation, or a centrifugation process.

7. A method of producing an emulsion according to claim 1 , further comprising mixing an additive with at least one of said first liquid, said second liquid, said first emulsion or said second emulsion, wherein said additive comprises at least one of ultraviolet-light-blocking molecules, moisturizing molecules, exfoliant molecules, anti-microbial molecules, antifungal molecules, anti-acne molecules, anti-wrinkle molecules, anti-septic molecules, insect- repellent molecules, dyes, pigments, particulates, nanoparticulates, clays, lipids, proteins, lipoproteins, vitamins, polypeptides, block copolypeptides, biopolymers, fragrances, pH modifiers, or water repellency molecules.

8. A method of producing an emulsion according to claim 1, further comprising, after removing said plurality of droplets from said first emulsion: measuring an optical transparency of said second emulsion; and determining whether to remove droplets from said second emulsion based on said measuring.

9. A method of producing a material having a preselected optical property, comprising: providing a first emulsion comprising a plurality of droplets having an ensemble average radius less than about 100 urn; blending an additive with said first emulsion to provide a second emulsion, said additive comprising at least one of a plurality a droplets having an ensemble average radius greater than about 100 nm or a plurality of particles having a dimension greater than about 50 nm, wherein said first emulsion is more transparent to visible light than said second emulsion.

10. A method of producing a material having a preselected optical property according to claim 9, wherein said ensemble average radius of said plurality of droplets of said first emulsion is greater than about 10 nm such that said first emulsion is more transparent to visible light than to ultraviolet light.

11. A method of producing a material having a preselected optical property according to claim 9, wherein said plurality of droplets of said first emulsion have a standard deviation about said ensemble average radius that is less than about 25% of said ensemble average radius.

12. A method of producing a material having a preselected optical property according to claim 11, wherein said plurality of droplets of said first emulsion have a standard deviation about said ensemble average radius that is less than about 15% of said ensemble average radius.

13. A method of producing a material having a preselected optical property according to claim 9, wherein said plurality of droplets of said first emulsion have a standard deviation about said ensemble average radius that is less than about 20 nm.

14. A method of producing a material having a preselected optical property according to claim 9, wherein said providing a first emulsion comprises: producing an emulsion comprising a plurality of droplets having an ensemble average radius less than about 100 nm and a plurality of droplets each having a radius greater than about 100 nm; and removing at least a portion of said plurality of droplets having said radius greater than about 100 nm from said emulsion to obtain said first emulsion.

15. A method of producing a material having a preselected optical property according to claim 14, wherein said removing comprises at least one of a filtering, a dialysis, a field flow fractionation, a creaming, a sedimentation, a coalescence, or a centrifugation process.

16. A method of producing a material having a preselected optical property according to claim 9, wherein said additive comprises an emulsion comprising a plurality of droplets that have at least one of an ensemble average radius or a standard deviation about said ensemble average radius that is preselected to provide at least a contribution to said preselected optical property of said material produced.

17. A method of producing a material having a preselected optical property according to claim 9, wherein said additive comprises an emulsion comprising a plurality of droplets that have an ensemble average radius greater than about 100 nm.

18. A method of producing a material having a preselected optical property according to claim 9, wherein said additive comprises at least one of ultraviolet-light-blocking molecules, moisturizing molecules, exfoliant molecules, anti-microbial molecules, anti-fungal molecules, anti-acne molecules, anti-wrinkle molecules, anti-septic molecules, insect- repellent molecules, dyes, pigments, particulates, nanoparticulates, clays, lipids, proteins, lipoproteins, vitamins, polypeptides, block copolypeptides, biopolymers, fragrances, pH modifiers, or water repellency molecules.

19. A method of producing a material having a preselected optical property according to claim 9, wherein said additive comprises at least one of a biologically active agent, a therapeutic agent, a diagnostic agent, a nutritious agent, a cosmetic agent, scent molecules or flavor compounds.

20. A method of producing a transparent material, comprising: producing a nano-emulsion comprising a first volume fraction of nano-droplets having an ensemble average radius less than about 100 run, said volume fraction being less than about 10%; and increasing a density of said nano-droplets to a second volume fraction, wherein said second volume fraction is greater than about 10%, and wherein said nano-emulsion having said second volume fraction of nano-droplets is more transparent to visible light than said nano-emulsion having said first volume fraction.

21. A material produced according to the method of any one of claims 1 -20.

22. An emulsion, comprising: a first liquid; and a plurality of droplets of a second liquid dispersed in said first liquid, said second liquid being at least partially immiscible with said first liquid, wherein said plurality of droplets have an ensemble average radius less than about 100 nm and a standard deviation about said ensemble average radius of less than about 25% such that said emulsion is substantially transparent to visible light.

23. An emulsion according to claim 22, wherein said standard deviation about said ensemble average radius is less than about 15%.

24. An emulsion according to claim 22, wherein said ensemble average radius is greater than about 10 run such that said emulsion is more transparent to visible light than to ultraviolet light.

25. An emulsion according to claim 22, further comprising an additive mixed with said emulsion, said additive causing at least a modification of an optical property of said emulsion.

26. An emulsion according to claim 22, wherein said first liquid is an aqueous liquid and said second liquid is an oil, said first and second liquids having a difference in refractive index at a visible wavelength that is greater than about 0.01.

27. An emulsion according to claim 22, wherein said second liquid is an aqueous liquid and said first liquid is an oil, said first and second liquids having a difference in refractive index at a visible wavelength that is greater than about 0.01.

28. An emulsion according to claim 22, wherein at least some of said plurality of droplets comprises an internal droplet of a liquid that is immiscible with said second liquid such that said emulsion is a double emulsion.

29. An emulsion according to claim 22, wherein said ensemble average radius is less than about 50 nm.

30. An emulsion according to claim 22, wherein said ensemble average radius is less than about 20 nm.

31. An emulsion according to claim 22, further comprising an additive mixed with said emulsion, said additive comprising at least one of ultraviolet-light-blocking molecules, moisturizing molecules, exfoliant molecules, anti-microbial molecules, anti-fungal molecules, anti-acne molecules, anti-wrinkle molecules, anti-septic molecules, insect- repellent molecules, dyes, pigments, particulates, nanoparticulates, clays, lipids, proteins, lipoproteins, vitamins, polypeptides, block copolypeptides, biopolymers, fragrances, pH modifiers, or water repellency molecules.

32. An emulsion according to claim 31 , wherein said particulates are titanium dioxide particles that enhance blocking of ultraviolet light.

33. An emulsion according to claim 22, wherein said emulsion has an extinction coefficient for transmitted light that is above about 1 mm^'1 for ultraviolet wavelengths of light below about 400 nm and an extinction coefficient below about 1 mm^"1 for visible wavelengths of light above about 400 nm.

34. An emulsion according to claim 33, wherein said extinction coefficient for visible wavelengths of light above about 400 nm increases by less than 10% over a period of two months storage time at 25⁰C.