WO2024110694A1 - Method to produce capsules consisting of aqueous solution droplets encapsulated with very low amounts of hydrophobic silica particles and a material consisting of salt particles or organic particles coated with precipitated silica microparticles - Google Patents

Abstract

The invention relates to a new method for encapsulation reducing the mass of silica particles needed for encapsulation significantly. In this method, the contact angle between the aqueous solution, hydrophobic silica particles and air is carefully controlled. When the solution and silica particles are then mixed, a continuous liquid-like mixture is created with the silica particles encapsulating air pockets inside the liquid. When this continuous liquid-like mixture is shaken in a manner which allows the sample to expand to a larger air volume, the continuous liquid-like mixture is converted into liquid-in-air powder i.e., encapsulation of the aqueous solution droplets with silica particles is achieved.

Description

Method to produce capsules consisting of aqueous solution droplets encapsulated with very low amounts of hydrophobic silica particles and a material consisting of salt particles or organic particles coated with precipitated silica microparticles

Field of invention

The present invention relates to a method of producing aqueous solution droplets coated with hydrophobic fumed silica nanoparticles or hydrophobic precipitated silica microparticles. Furthermore, the invention relates to a material comprising a salt particle or an organic particle coated with hydrophobic precipitated silica microparticles.

Background

Amorphous silica (SiC ) can be formed in different processes, resulting in materials with quite different properties [1] as is briefly described first. Fumed silica is manufactured in a flame from SiCU, H2 and O2. The first part of the process gives near spherical nanoparticles referred to as primary particles. These primary particles generally have average particle sizes ranging from 1 nm to 50 nm with exact values varying depending on the details of the manufacturing process.

In the second part of the process these primary particles collide and merge together to create chain-like, branched aggregates with mean aggregate sizes (see Fig. 1 ) often ranging from 100 nm to 500 nm, although smaller and larger sizes are possible. Note that the mean aggregate size of fumed silica aggregates is not easy to define due to aggregates’ branched structure that is illustrated in Fig. 1 . Furthermore, mean aggregate size is not the same as average particle size as it is defined in this patent application. Due to the vague definition, mean aggregate size is only used to illustrate the scale of the particle size. When the fused aggregates reach a cooler part of the flame, they can no longer merge and instead form weakly bonded agglomerates that are held together by weak interactions such as Van der Waals forces or hydrogen bonding. These agglomerates can often have a particle size much above 1 pm. However, the agglomerates can easily be broken by applying energy e.g., by strong mixing or stirring. Thus, agglomerate size is not generally used to characterize fumed silica particle size and fumed silica are considered nanoparticles.

Precipitated silica is created by reacting alkali silicates, most often sodium silicate, with sulfuric acid [1 , 2]. Silicas made with such wet processes are often ground or spray dried which can greatly alter the shape and size of the particles. Precipitated silicas do not have a similar nanostructured branched structure as fumed silica and they are not generally considered nanoparticles as the average particle size of most commercial precipitated silica products is above 1 pm. Thus, precipitated silicas are categorized as microparticles in this patent application.

Both untreated fumed silica and untreated precipitated silica are hydrophilic, i.e., they are easily wet by water [1], This is due to freely accessible silanol groups on their surface. To make the silica particles hydrophobic, a surface treatment is applied. The surface treatment replaces some of the silanol groups with organic groups. The level of hydrophobicity can be altered by varying the type of organic group used to replace the silanol groups as well as by varying the percentage of silanol groups replaced with the organic groups. Both fumed silica and precipitated silica are widely available commercially with different hydrophobic surface treatments.

It is known that aqueous solution droplets can be encapsulated using hydrophobic fumed silica nanoparticles. The encapsulation is done by creating such conditions that small droplets of the aqueous solution form and are sprayed around. These droplets are then contacted with the nanoparticles so that they attach to the droplet surfaces [3]. The resulting stable capsules, which are generally between 0.001-1 mm in diameter, consist of a shell made of fumed silica nanoparticles and the aqueous solution core. The fumed silica shell is porous, i.e., gas, including water vapor, can penetrate the shell, but due to the hydrophobicity of the fumed silica particles, aqueous solution can’t escape from the capsules in liquid form unless the capsules are compressed enough to rupture them. This allows gases to react with the inside of the capsules and for the capsules to be dried without the cores of the capsules contacting each other.

When water is encapsulated with fumed silica in such manner, the resulting composition is usually referred to as dry water. Dry water with different additives added to water is used e.g., in cosmetics, while later developments utilizing concentrated aqueous solutions of, e.g., inorganic salts as the encapsulated liquid are used in applications such as CO2 capture and absorption chemical heat pumps [4, 5].

With the manufacturing method described in the original patent [3], dry water capsules containing up to 90% of the aqueous solution by weight were reported to be achievable. Later, the manufacturing method of dry water was improved by utilizing a hydrophobic hermetically sealed container in which high-shear mixing with for example a blender was used to simultaneously create small droplets of the aqueous solution and pulverize the powder around the container [6]. The nanoparticles would then attach to the surface of the droplets dispersed in air and thus create small capsules. In this method, the turbulence created by the high-shear mixing was responsible for the formation of the tiny droplets while the hydrophobicity of the container walls prevented the aqueous solution from wetting the walls. Similar methods utilizing high-speed blender in a closed container simultaneously creating small liquid droplets and pulverizing the fumed silica powder were later shown in several scientific articles to be capable of encapsulating up to 97- 98.5 wt% of water inside the capsules [7-9], although use of hydrophobic container walls was not mentioned in these later examples.

Dry water-style encapsulation of water has also been achieved with precipitated silica microparticles with a similar method utilizing high-speed blender and hydrophobic silica microparticles [10]. In this case, somewhat higher amount, 5 parts by weight silica, was used for the encapsulation of 95 parts by weight water.

However, encapsulating concentrated aqueous solutions of salts has been found to require significantly higher amounts of silica than in the case of pure water or dilute aqueous solutions. Furthermore, it has only been demonstrated for fumed silica nanoparticles. For example, encapsulation of aqueous 32 wt% Li Br solution with 5 parts by weight fumed silica nanoparticles and 95 parts by weight Li Br solution and encapsulation of 40 wt% aqueous LiCI solution with 7 parts by weight fumed silica nanoparticles and 93 parts by weight solution have been reported [4], Similarly high amounts of silica have been used for encapsulation of concentrated aqueous K2CO3 solutions with fumed silica nanoparticles [5, 11 , 12], The amount of silica required seems to depend on the concentration of the aqueous solution that is encapsulated. A study found that when using a 71 wt% solution of choline dihydrogen phosphate in water, 7.5 parts by weight silica was required for complete encapsulation of the solution with fumed silica while 77 wt% and 83 wt% solutions of choline dihydrogen phosphate in water required 10 and 12.5 parts by weight silica respectively [9]. Our experiments have also shown that if the fumed silica nanoparticles and e.g., concentrated CaCl2 solution are simply mixed in a high-speed blender with no additional steps, much higher amounts of silica are needed for complete encapsulation than what is required when pure water is encapsulated.

The high silica content required for encapsulation of concentrated salt solutions is problematic due to increased cost as well as the space taken by silica instead of the active material. These problems are highlighted in applications where during use, the capsules are dried, i.e., the water from the capsules is evaporated away, resulting in even higher silica content relative to the active material in the composition. If for example 5 parts by weight silica and 95 parts by weight of 32 wt% LiBr solution is used for encapsulation as was done in [4], the resulting dried capsules consisting of fumed silica and anhydrous LiBr have approximately 14 parts by weight fumed silica and 86 parts by weight anhydrous LiBr. Thus, the amount of fumed silica required in the composition is significant and greatly adds to the cost of the material. Due to the extremely low density of fumed silica, which is typically between 50-60 g/l in powder form, the high silica content also significantly decreases the bulk volumetric density of the encapsulated solid in these compositions. Here, bulk volumetric density is defined as the volume taken by the solid divided by the total volume including air gaps/voids.

Applications where aqueous solution capsules are to be dried include the use of hygroscopic salts e.g., for sorption thermochemical energy storage, in absorption chemical heat pumps or for gas dehumidification. Hygroscopic salts can be encapsulated as concentrated aqueous solutions and subsequently dried to create hygroscopic salt particles encapsulated with fumed silica [4], The porous, water vapor permeable silica shells of these capsules prevent agglomeration of the salt particles during drying and subsequent hydration, i.e., when the capsules are contacted with humid gas and absorb water from the surrounding gas. Thus, these silica encapsulated salts are effective materials for example for sorption thermochemical energy storage, where heat is stored by drying (dehydrating) the salts and later released via the reverse hydration reaction, i.e., absorption of water. The dried capsules can also be used for dehumidifying air or other gas phases.

Another application where the encapsulated aqueous solution droplets may wish to be dried is use of organic materials such as erythritol or xylitol as phase change material (PCM), i.e., a thermal energy storage material utilizing the latent heat of crystallization or melting. In such case, a concentrated solution of e.g., erythritol is encapsulated with hydrophobic silica particles and water is subsequently removed from the composition by heating. The resulting erythritol solid encapsulated with silica can then be used as phase change material powder that does not agglomerate even upon melting as the silica shells prevents contact between droplets.

Furthermore, concentrated salt solutions of salts that form salt hydrates can also be encapsulated with silica particles at an elevated temperature and then cooled down to create microencapsulated salt hydrate particles that can be used as PCMs. Alternatively, concentrated salt solutions can be encapsulated with silica and then partly dried to crystallize the salt hydrates inside the capsules.

In general, as low silica content as possible is desired in the above applications, as addition of silica increases cost of the composition and decreases the volumetric density of active material in the material. The latter is especially important in thermal energy storage systems, as the volume available for storing heat may be limited or expensive.

Terms used absorption = physical or chemical phenomenon where atoms, molecules or ions enter some bulk phase, which can be either liquid or solid material adsorption = adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface average particle size = For irregularly shaped objects, defining particle size is somewhat difficult. The term average particle size as used in this patent application is defined as the volume median diameter (VMD) obtained from laser diffraction analysis of the particles. Laser diffraction analysis utilizes diffraction patterns of a laser beam passed through either dry powder blown in air or a liquid suspension of the powder to measure volume-based particle size properties. Laser diffraction analysis results in volume-weighted particle size distribution from which VMD is determined. VMD is such a value that half of the volume of the measured material is in particles with smaller diameter than VMD and the other half of the volume is in particles with larger diameter than VMD. Note that laser diffraction analysis assumes spherical shape, i.e., the results are spherical equivalent diameters. Thus, for powders consisting non-spherical particles, VMD derived from laser diffraction analysis is technically that of an imaginary sample consisting of spheres yielding the same diffraction pattern as the actual sample measured. hygroscopic = ability of a material to attract water molecules from surrounding gas phase either through absorption or adsorption hydrophobic = material that has a tendency to repel and not absorb water hydrophilic = material that is attracted to water molecules microparticles = particles with size ranging from 1 pm to 1000 pm nanoparticles = particles with size ranging from 1 to 1000 nm surfactant = compound that decreases the surface tension between two liquids, between a gas and a liquid, or interfacial tension between a liquid and a solid

Notations wt% = percentage by weight

Summary of the invention

A new method for encapsulation of aqueous solution droplets with hydrophobic fumed silica nanoparticles in which a significantly smaller amount of silica is required than previously possible has been developed and disclosed. Furthermore, the method can be used to encapsulate aqueous solution droplets with low amounts of hydrophobic precipitated silica microparticles in addition to use of fumed silica nanoparticles. Thus, new type of particles comprising an inner part made of an inorganic salt or and organic solid and outer coating made of hydrophobic precipitated silica microparticles can be made with the method.

In the new method, aqueous solution is mixed with hydrophobic silica particles with sufficiently high energy in a high-speed mixer so that a continuous liquid-like mixture forms. Typically, these hydrophobic silica particles are added as solid particles, and can be either precipitated silica or fumed silica particles. For hydrophobic fumed silica nanoparticles, a typical average primary particle size ranges from 1 nm to 50 nm, whereas for hydrophobic precipitated silica particles the average particle size ranges from 1 pm to 40 pm.

Preferably, the mixing takes place at room temperature, or at a slightly elevated temperature between room temperature and 70 °C, such as at a temperature of 65 °C.

This continuous liquid-like mixture behaves similarly to a liquid in that it conforms to the shape of its container and can be poured from the container as a near continuous stream. However, even if the aqueous solution used to make it is completely transparent, the continuous liquid-like mixture is not, instead obtaining the color of the solid silica particles that the aqueous solution is mixed with. Furthermore, it does not stick easily to surfaces and displays an uneven surface. The continuous liquid-like mixture can also be cut into smaller pieces. When this continuous liquid-like mixture is further mixed in a manner that allows the sample to become aerated via expansion to a larger volume, the continuous liquid-like mixture is converted into tiny capsules consisting of a shell made of silica particles and a core made of the aqueous solution. These capsules appear to the naked eye as a fine powder.

The aeration can be done for example by inverting the blender container and shaking it so that the sample expands to the entire container volume and mixes with air. After sufficient time of shaking, all, or at least nearly all, of the continuous liquid-like mixture appears to turn into a fine powder. This material appearing as fine powder to the naked eye consists of aqueous solution droplets encapsulated with silica particles. With pure water as well as concentrated aqueous solutions, the encapsulation with the invented method can be done with as little as 1 part by weight fumed silica nanoparticles and 99 parts by weight aqueous solution, whereas existing methods require about 5 parts silica and 95 parts by weight aqueous salt solution. Furthermore, in addition to fumed silica nanoparticles, the encapsulating shell particles can be hydrophobic precipitated silica, i.e., silica microparticles. If precipitated silica is used, the encapsulation can be done with as little as 2 parts by weight precipitated silica and 98 parts by weight aqueous solution. A slight deviation from these ratios can be allowed, and the new method is typically operated using 0.5-1 .5 parts by weight of fumed silica particles and 98.5-99.5 parts by weight aqueous solution, preferably 0.8-1 .4 parts by weight of fumed silica nanoparticles and 98.6-99.2 parts by weight aqueous solution, or using 1 .5-2.5 parts by weight of precipitated silica and 97.5-98.5 parts by weight of aqueous solution, preferably 1 .8-2.3 parts by weight of precipitated silica and 97.7-98.2 parts by weight of aqueous solution. Higher contents of silica can, however, also be used, despite these lower contents becoming possible using the herein described method.

For the continuous liquid-like mixture that can be converted into capsules to form, the contact angle between the aqueous solution and the solid silica particles has to be tuned to such a value that, during the high speed-mixing, a small decrease in contact angle would result in the formation of a stable air- in-liquid foam while a small increase in contact angle would prevent the formation of a continuous liquid-like mixture. The tuning can be done by either altering the surface tension of the aqueous solution or by altering the hydrophobicity of the fumed silica particles. Surface tension can be altered by addition of a surfactant to the solution. Optionally, the surface tension can be altered by changing the solution temperature. The hydrophobicity of the fumed silica particles can be altered by changing the surface treatment of the silica particles that is used to make them hydrophobic or by changing the pH or salt concentration of the solution.

Thus, the conditions for conversion of the liquid-like mixture into capsules can be achieved by adjusting the contact angle between the silica particles and the aqueous solution by using a surfactant, adjusting the pH of the solution, adjusting the salt concentration of the solution or by adjusting the hydrophobicity of the silica particles or by any combination of these procedures. As mentioned above, it is also possible to achieve the conditions for conversion by adjusting the temperature of the solution.

The method can be used to encapsulate the aqueous solution of nearly any water soluble inorganic salt, such as chlorides, chlorates, perchlorates, bromides, iodides, carbonates or nitrates, preferably the halogenides of alkali metals or earth alkali metals, the suitable salts excluding only particularly alkaline salts, such as hydroxyl salts. Some preferred salts are hygroscopic, such as calcium chloride (CaCl2).

Likewise, the method can be used to encapsulate nearly any organic solids. It is, however, advantageous if the surface tension of the aqueous solution of the solid remains high. Examples of organic solids that can be encapsulated with the method include erythritol and xylitol. A further example is sucrose.

List of figures

Figure 1 . presents an illustration of mean aggregate size and primary particle size in fumed silica aggregates. Note that the size of primary particles vs aggregate size is not in scale as aggregates tend to consist of many more primary particles fused together than depicted here.

Figure 2. presents the contact angle 0 between the liquid and a solid particle of silica (with spherical shape) attached to the liquid-gas interface.

Figure 3. presents the continuous liquid-like mixture formed from 43 wt% CaCl2 solution and Sipernat D10 hydrophobic precipitated silica after highspeed mixing in a blender. The container was slightly tilted to the left so that the translucent parts of the material can be seen on the right-hand side.

Figure 4. presents the continuous liquid-like mixture formed from 43 wt% CaCl2 solution and Aerosil R812S hydrophobic fumed silica nanoparticles after high-speed mixing in a blender. Figure 5. presents the capsules formed from the continuous liquid-like mixture after aeration being poured onto a surface.

Figure 6. presents an optical microscope image of 43 wt% aqueous CaCl2 solution encapsulated with the method described in the patent using 1 part by weight Aerosil R812S fumed silica nanoparticles and 99 parts by weight of the 43 wt% CaCl2 solution.

Figure 7. presents a scanning electron microscope image of erythritol encapsulated with the method described in the patent. Erythritol was encapsulated with 1 .4 parts by weight fumed silica Aerosil R812S and 98.6 parts by weight of 43 wt% aqueous erythritol solution and then dried in an oven at 150 °C.

Figure 8. presents particle size distribution of an aqueous 42.2 wt% CaCl2 solution encapsulated with the method described in the patent at room temperature using 1 part by weight Aerosil R812S fumed silica nanoparticles and 99 parts by weight of solution. The aqueous solution had an ethanol content of 1 .86 wt%. The particle size distribution was measured with laser diffraction device Malvern 2600 Mastersizer. Before measurement, the capsules were dried in an oven at 150 °C. A small sample of the dried capsules was then dispersed in kerosene in a 12.5 ml cuvette with continuous mixing using a magnetic stirrer and measurement was then taken after allowing the capsules to mix with kerosene for a few seconds.

Figure 9. presents particle size distribution of an aqueous 55.1 wt% CaCl2 solution encapsulated with the method described in the patent at approximately 65 °C using 1 part by weight Aerosil R812S fumed silica nanoparticles and 99 parts by weight of solution. The aqueous solution had an ethanol content of 1 .57 wt%. The particle size distribution was measured with laser diffraction device Malvern 2600 Mastersizer. Before measurement, the capsules were dried in an oven at 150 °C. A small sample of the dried capsules was then dispersed in kerosene in a 12.5 ml cuvette with continuous mixing using a magnetic stirrer and measurement was then taken after allowing the capsules to mix with kerosene for a few seconds.

Detailed Description

In an embodiment described above, the new method for producing capsules consisting of aqueous solution droplets coated with hydrophobic silica particles comprises mixing the solid silica particles and a volume of aqueous solution with sufficient energy to create a continuous liquid-like mixture, followed by aeration by expansion of the continuous liquid-like mixture into a larger volume with simultaneous mixing, resulting in conversion from continuous liquid-like mixture into small liquid droplets with average size ranging from 1 pm to 1000 pm encapsulated with silica particles.

When sufficiently hydrophobic fumed silica nanoparticles or hydrophobic precipitated silica microparticles are mixed with an aqueous solution with sufficiently high surface tension in a high-speed mixer, for example a highspeed blender, the aqueous solution can become encapsulated with the silica particles, i.e., liquid-in-air capsules can be formed due to liquid droplets forming due to shear forces and solid silica particles then attaching to the airliquid interface.

Typically, any particles exhibiting a tendency to repel and not absorb water are considered sufficiently hydrophobic.

A sufficiently high surface tension is typically at least 40 mN/m, often more than 50 mN/m depending on the particle hydrophobicity, the used mixing method and the particle weight percentage.

For this encapsulation to occur, the contact angle between the silica particles and the liquid (with air or other gas as the surrounding phase) must be high enough and the weight ratio of silica to liquid needs to be high enough [14, 15]. Here, the contact angle 0 is defined as seen in Fig. 2. The surrounding gas in Fig. 2 is normally air mixed with vapor of the liquid, i.e., air that is in equilibrium with the liquid phase, but it can be virtually any other gas or gas mixture. The gas phase usually has very little effect on the contact angle. In this patent application air can always be replaced with any other gas or gas mixture.

Typically, the contact angle is above 90°.

If the contact angle is above 90°, it means that the solid prefers to be “wet” by air rather than the liquid, i.e., the solid prefers contact with air over contact with the liquid. The formation of liquid-in-air capsules is known to require contact angles significantly above 90° [15]. If the contact angle is progressively decreased from values that are sufficiently high for liquid-in-air capsule formation, the same mixing conditions (mixing speed, blender type, air-to-liquid volume ratio, silica-to-liquid weight ratio etc.) that previously led to liquid-in-air capsules at least partially forming will eventually result in an air-in-liquid foam once the contact angle is sufficiently low. The required decrease in contact angle can be achieved either by altering the aqueous solution properties by e.g., adding surfactant to the solution or by decreasing the hydrophobicity of the silica particles.

Examples of suitable surfactants include organic molecules with an anionic functional group, such as a hydroxyl group, e.g. methanol or ethanol; a sulfate group, e.g. ammonium lauryl sulfate, sodium dodecyl sulfate (SDS), sodium lauryl ether sulfate (SLES) or sodium myreth sulfate; a sulfonate group, e.g. an alkylbenzene sulfonate, perfluorooctanesulfonate or perfluorobutanesulfonate; a phosphate group, e.g. alkyl ether phosphates or alkyl-aryl ether phosphates; or a carboxylate group, e.g. sodium stearate; fatty acid derivatives, such as their ethoxylates or esters; and amino acid derivatives.

The needed amount of surfactant depends on the selected surfactant and its characteristics. For example ethanol can be added in concentrations of up to 10 % by weight, preferably 0.25-10 % by weight. The sulfate surfactants, such as SDS, in turn, are stronger, and require smaller amounts. The amount of SDS is preferably only up to 0.15 % by weight, more preferably 0.015-0.15 % by weight.

The altering of silica particle hydrophobicity can be achieved either by altering the surface treatment of the silica particles or by changing either the solution pH or salt concentration in the solution.

Possible surface treatments are mentioned above, and examples of changes to the surface treatment include varying the type of organic group used to replace the silanol groups as well as by varying the percentage of silanol groups replaced with the organic groups. The organic group can be varied by varying the treating agent with which the silanol groups react. One example of a treating agent is hexamethyldisilazane which converts the surface silanol groups into trimethylsilyl groups. Other examples of treating agents include polydimethylsiloxane, alkylsilanes such as octylsilane, octamethylcyclotetrasiloxane, trimethylchlorosilane, dimethyldichlorosilane and vinylsilanes such as methacrylsilane. Other similar compounds can also be used.

An advantageous pH for altering the silica particle hydrophobicity is a pH raised to a level of 7-10. The precise pH level is typically obtained by testing.

An advantageous salt concentration for altering the silica particle hydrophobicity is a salt concentration increased to a level of up to 65 wt%, preferably 30-58 wt%, most suitably 43-56 wt%.

The term foam as used here is defined as substance with air bubbles inside a continuous liquid phase. More precisely, this air is typically trapped in numerous small pockets inside the continuous liquid phase, which pockets can be irregularly shaped. In this case the air bubbles are stabilized with solid silica particles at the air-liquid interface. Such foams stabilized by solid particles are called Pickering foams. Transition from liquid-in-air capsules to air-in-liquid Pickering foam with decreasing contact angle has been shown in several scientific articles [14-16]. The Pickering foam is easy to visually differentiate from the liquid-in-air capsules as the foam forms a continuous mixture while the capsules appear to the naked eye as a fine powder.

It has been found that when contact angle is adjusted to the range where transition from liquid-in-air capsules to air-in-liquid foam occurs, i.e., to the transient contact angle range, high-speed mixing can result in the formation of a continuous liquid-like mixture that can subsequently be converted into liquid-in-air capsules by expansion into a larger air volume and simultaneous mixing, i.e., aeration of the continuous liquid-like mixture.

The exact structure of this continuous liquid-like mixture that is convertible into capsules is yet unknown. The term “liquid-like” is used here as the continuous liquid-like mixture behaves similarly to a liquid in that it conforms well to the shape of its container and can be poured from the container as a near continuous stream. An exception to this is small containers or tubes of less than 1 cm in diameter, in which the continuous liquid-like mixture can behave more like a gel in that it can get stuck and not flow to the bottom of the container. This effect becomes more pronounced and applies to larger containers as silica content is increased.

However, the continuous liquid-like mixture is not transparent even if made from a transparent aqueous solution, although parts of it are translucent as can be seen on the right-hand side of Fig. 3. As mentioned previously, upon further reduction in contact angle from the transient range, a Pickering foam that also appears to the naked eye as a continuous mixture is known to form. However, the continuous liquid-like mixture does not stick as easily to surfaces as a Pickering foam would. In addition, unlike a liquid or for example an emulsion, the continuous liquid-like mixture does not display an even interface with air as can be seen in Fig. 3 and Fig. 4.

As the structure is not transparent when thicker samples are taken, and because the structure seems to break when pressed to form a thinner, more transparent layer, optical microscopy does not yield conclusive results on the inner structure of the continuous liquid-like mixture. The continuous liquid-like mixture is known to consist of the aqueous solution, solid silica particles and air mixed with vapor of the aqueous solution as they are the only components inside the mixing container. The continuous liquid-like mixture is believed to contain air bubbles encapsulated with the silica particles and possibly also some aqueous solution droplets encapsulated with silica that are located inside the bubbles. Typically, the air bubbles are in the form of small irregularly shaped pockets, trapping the air. In addition, some capsules covered with silica have been observed to be present outside the continuous liquid-like mixture after the high-speed mixing. Due to the likely presence of air bubbles inside this continuous liquid-like mixture, it could be classified as an air-in-liquid foam, or simply foam. However, as previously mentioned, its properties are atypical for a foam in that it is not sticky, suggesting that the surface is also covered with fumed silica particles. Furthermore, it is often easy to cut small pieces from this continuous liquid-like mixture and the cut pieces are again not as sticky as one would expect for a pure foam. The latter phenomenon suggests that it is possible that the continuous liquid-like mixture only appears continuous and may consist of several regions of continuous liquid phase separated by thin layers of silica particles.

The formed continuous liquid-like mixture is clearly distinguishable from dry water-style powder since it does not consist solely or even mostly of clearly distinguishable capsules separated from one another. It behaves similarly to a liquid in that it conforms to the shape of a container and can be poured from one container to another as a continuous flow, i.e. with no air visible air gaps between capsules that would be expected if encapsulation had already occurred in significant amounts.

The conversion from this continuous liquid-like mixture to liquid-in-air capsules is possible due to the increase in the air-to-liquid volume ratio that occurs during the aeration. It has also been found that with this method of producing dry water-type capsules, significantly lower weight ratio of silica particles to aqueous solution is required for complete encapsulation of the aqueous solution than with other methods. Generally, only 1 part by weight fumed silica nanoparticles is required for encapsulation of 99 parts by weight aqueous solution, or 0.5-1 .5 parts by weight of fumed silica particles and 98.5-99.5 parts by weight aqueous solution, preferably 0.8-1 .4 parts by weight of fumed silica nanoparticles and 98.6-99.2 parts by weight aqueous solution. The aqueous solution can include any inorganic or organic solutes even in very high concentrations as long as the contact angle between the solid silica particles and the aqueous solution is appropriate.

The capsules formed in this manner in some cases have a narrower particle size distribution than with traditional method of simply mixing the fumed silica particles and liquid in a high-speed mixer. Typically, the method of this invention results in particles having a size distribution of 1 pm to 1000 pm. Furthermore, the capsules are often more spherical than those made with the traditional method. Images of capsules created with the described method can be seen in Fig. 5-7. Particle size distributions of capsules created with the described method can be seen in Fig. 8-9.

The advantage of the new method is clear when low weight ratios of silica to aqueous solution wish to be used for encapsulation. With the traditional method described for example in [6], if insufficient amount of fumed silica particles is used, for example 1 part by weight fumed silica per 99 parts per weight of the aqueous solution, only part of the aqueous solution becomes encapsulated with the rest of the liquid sitting underneath the capsules formed. Thus, even the part that does become encapsulated has a significantly higher weight ratio of fumed silica to aqueous solution than desired. As discovered, if the contact angle is progressively decreased from these conditions where incomplete encapsulation is achieved, once the transient contact angle range is reached, a majority of the liquid and silica particles form what appears to be one continuous liquid-like mixture. This continuous liquid-like mixture mostly mixes at the bottom of the mixing container during the high-speed mixing and if mixing is stopped, the continuous liquid-like mixture remains at the bottom. The container can then be shaken so that the continuous liquid-like mixture expands into the entire container and after sufficient time of mixing, complete or near complete conversion into capsules occurs.

However, sufficient air-to-liquid volume ratio has to be present in the container for the conversion to occur. In other words, the container needs have sufficient amount of extra space above the continuous liquid-like mixture so that it can be properly aerated by shaking. Note that mixing can also be provided by other means than shaking if the air-to-liquid volume can be increased sufficiently. However, using simple mixing with a blade, the continuous liquid-like mixture often stays at the bottom and thus, sufficient aeration does not occur.

A preferred air-to-liquid volume ratio is 5:1 or more, preferably 8:1 or more, more preferably 10:1 or more, and most suitably 15:1 or more.

Note also that the solutes in the aqueous solution may themselves act as surfactants or they might increase the surface tension of the aqueous solution or they might affect the hydrophobicity of the silica particles. Thus, concentration of the aqueous solution also affects the amount of surfactant and hydrophobicity of fumed silica particles required for successful conversion. In addition, for example increasing pH decreases the hydrophobicity of the silica particles, Thus, controlling pH can be used to create appropriate conditions for the continuous liquid-like mixture to form.

Some examples of the encapsulation process are listed below to illustrate the method further.

Example 1

50.0 g of 43 wt% CaCl2 solution was first mixed with 1 .2 ml ethanol acting as the surfactant. The resulting solution was poured onto a -400 ml container (height -8 cm) of blender model Bosch SilentMixx MMB66G7M and 0.55 g of Aerosil R812S fumed silica nanoparticles was then added. This corresponds to approximately 1 part by weight nanoparticles and 99 parts by weight of the aqueous solution. Air-to-liquid volume ratio was approximately

^ml ~

10. The mixture was then mixed at full power for 30 seconds at room temperature. During the mixing, a white continuous liquid-like mixture was formed. After the blender was stopped, the container was turned upside down and shaken vertically by hand to allow the continuous liquid-like mixture to expand into the entire container volume while under mixing. After approximately 15 seconds of shaking by hand, all of the continuous liquid-like mixture had transformed into what appeared to be a fine powder, although the time required for complete transition varied between different batches. After drying at 150 °C, the powder was confirmed via optical microscopy and laser diffraction measurements to consist of capsules with an average particle size of -210 pm. Particle size distribution of the dried powder was also measured and the results are shown in Fig. 8.

Example 2

CaCl2 was dissolved in water to create a 56 wt% CaCl2 solution at 65 °C.

54.3 g of this solution was mixed with 1.1 ml of ethanol in the blender container (same blender and container as in Example 1 ) which had been preheated to 65 °C. 0.62 g of Aerosil R812S that had also been pre-heated to 65 °C was quickly added and the mixture was mixed at full power in the blender for 30 seconds. The weight ratio was approximately 1 part by weight nanoparticles and 99 parts by weight of the aqueous solution. Air-to-liquid volume ratio was 10 as in Example 1. During the high-speed mixing, a white continuous liquid-like mixture formed and the container was then turned upside down and shaken vertically by hand for -15 seconds until all of the continuous liquid-like mixture had transformed into what appeared to be a fine powder, although the time required for complete transition varied between different batches. Based on laser diffraction measurements, the resulting capsules in the powder had an average particle size of -120 pm after drying at 150 °C. Particle size distribution of the dried powder was also measured and the results are shown in Fig. 9. Example 3

49.5 g of 43 wt% CaCl2 solution was first mixed with 0.6 ml ethanol acting as the surfactant. The resulting solution was poured onto a -400 ml container (height -8 cm) of blender model Bosch SilentMixx MMB66G7M and 1 .165 g of Sipernat D10 precipitated silica microparticles was added. This corresponds with approximately 2.3 parts by weight nanoparticles and 97.7 parts by weight of the solution. Air-to-liquid volume ratio was approximately blender at full power for

10 seconds at room temperature. During the mixing, a white continuous liquid-like mixture was formed. After the blender was stopped, the container was turned upside down and lightly shaken vertically by hand to allow the continuous liquid-like mixture to expand into the entire container volume while under mixing. After approximately 15 seconds of shaking by hand, most of the continuous liquid-like mixture had transformed into what appeared to be a fine powder, although some macroscopic encapsulated liquid droplets still remained. The time required for the transition to occur varied between different batches. The formed powder was mixed again for 10 seconds with the blender at full power and then again handshaken until conversion into powder occurred. Almost all of the solution had now been microencapsulated with only a very low number of macroscopic droplets remaining.

Based on laser diffraction measurements, the resulting capsules in the powder had an average particle size of 250 pm after drying at 150 °C.

Example 4

Such hygroscopic salts as CaCl2 encapsulated with fumed silica using the method described in this patent can be used as sorption thermochemical energy storage material. This is described next.

Capsules from Examples 1 , 2 and 3 were put under repeated water desorption-absorption cycles in a simultaneous thermal analysis (STA) apparatus where 10 mg samples were cycled from a ~43 wt% solution to anhydrous state 10 times with dehydration performed at 120 °C and hydration at 30 °C with water vapor pressure in the STA chamber set to -1 .4 kPa. No difference was observed in reaction kinetics between the cycles, indicating that agglomeration did not occur in any significant amount. In addition, the heats of hydration remained constant, i.e., the amount of stored heat was not reduced.

Notably, during the absorption steps, the sample from Example 2 expanded by over 40 % over the volume it originally had after its preparation. The results show that the formed capsules can withstand expansion beyond their original volume without breaking and thus, they can keep their high surface area in contact with surrounding gas phase.

Example 5

38.86 g of 43 wt% aqueous erythritol solution was poured onto a -400 ml container (height -8 cm) of blender model Bosch SilentMixx MMB66G7M and 0.53 g of Aerosil R812S fumed silica nanoparticles was added. This corresponds with approximately 1 .4 part by weight nanoparticles and 98.6 parts by weight of the solution. Air-to-liquid volume ratio was approximately (400-35) mi ₁₀ .p mixture was then mixed at full power for 30 seconds at room temperature. During the mixing, a white continuous liquid-like mixture was formed. After the blender was stopped, the container was turned upside down and shaken vertically by hand to allow the continuous liquid-like mixture to expand into the entire container volume while under mixing. After approximately 30 seconds of shaking by hand, all of the continuous liquid-like mixture had transformed into what appeared to be a fine powder. The capsules were dried and melted at 150 °C, and subsequently cooled to room temperature. The composition remained in fine powder form, proving that the silica particles prevented agglomeration of the molten erythritol. Example 6

45 g of water was mixed with 0.6 ml ethanol and poured onto a -400 ml container (height -8 cm) of blender model Bosch SilentMixx MMB66G7M and 0.38 g of Aerosil R812S fumed silica nanoparticles was added. This corresponds with approximately 0.8 part by weight nanoparticles and 99.2 parts by weight of water. Air-to-liquid volume ratio was approximately seconds at

room temperature. During the mixing, a white continuous liquid-like mixture was formed. After the blender was stopped, the container was turned upside down and shaken vertically by hand to allow the continuous liquid-like mixture to expand into the entire container volume while under mixing. After approximately 30 seconds of shaking by hand, all of the continuous liquid-like mixture had transformed into what appeared to be a fine powder.

Example 7

35 g of 43 wt% CaCl2 solution poured onto a -400 ml container (height -8 cm) of blender model Bosch SilentMixx MMB66G7M and 0.32 g of Aerosil R812S fumed silica nanoparticles was then added. This corresponds to 0.9 parts by weight of fumed silica to 99.1 parts by weight of the solution. Air-to- liquid volume ratio was approximately

The mixture was then mixed at full power for 30 seconds at room temperature. Large portion of the solution was left as free liquid sitting at the bottom of the container with white powder sitting on top of the liquid. Mixing at full power for 5 more minutes did not change the result. More fumed silica was added in small increments of 0.15-0.20 g with mixing always performed in the blender for 30 seconds at full power after each addition. Some free liquid remained at the bottom of the container until the composition was approximately 9.4 parts by weight of fumed silica and 90.6 parts by weight of aqueous solution.

35 g of 43 wt% CaCl2 solution was poured onto the same -400 ml blender container and 0.32 g of Aerosil R812S fumed silica nanoparticles was then added. This corresponds to approximately 0.9 parts by weight nanoparticles and 99.1 parts by weight of the aqueous solution. Air-to-liquid volume ratio was again ^f4°°~^ ^ml ~ 16. The mixture was then mixed at full power for 30 seconds at room temperature. Large portion of the solution was left as free liquid sitting at the bottom of the container with white powder sitting on top of the liquid. Ethanol was then added in 0.2 ml increments. After each addition the mixture was mixed in the blender at full power for 30 seconds. After a total of 0.4 ml of ethanol had been added, notable amount of free liquid solution still existed at the bottom after mixing and the mixture could not be converted into powdery form.

Once the total ethanol content was increased to 0.6 ml, a white continuous liquid-like mixture was formed upon mixing in the blender. After the blender was stopped, the container was turned upside down and shaken by hand to allow the continuous liquid-like mixture to expand into the entire container volume while under mixing. After a few seconds of shaking by hand, all of the continuous liquid-like mixture had transformed into what appeared to be a fine powder.

Thus, complete encapsulation of 43 wt% CaCl2 solution was achieved with 0.9 parts by weight fumed silica with our method whereas 9.4 parts by weight fumed silica was needed for full encapsulation of 43 wt% CaCl2 solution with previously known method.

Example 8

35 g of 43 wt% CaCl2 solution was poured onto a -400 ml container (height -8 cm) of blender model Bosch SilentMixx MMB66G7M and 0.96 g of hydrophobic Sipernat D10 precipitated silica microparticles was then added. This corresponds to approximately 2.7 parts by weight nanoparticles and 97.3 parts by weight of the aqueous solution. Air-to-liquid volume ratio was approximately

~ 16. The mixture was then mixed at full power for 2 minutes at room temperature. After mixing, notable amount of free liquid solution existed at the bottom of the container. Turning the container upside down and shaking vertically by hand did not lead to full encapsulation of the liquid solution either.

When otherwise identical experiment was performed with 0.96 g of the slightly less hydrophobic Sipernat D17 precipitated silica microparticles instead of the more hydrophobic Sipernat D10 particles, a continuous liquidlike mixture was formed. Inverting the container and shaking now led to formation of a fine powder, i.e., full encapsulation of the solution.

References

[1] Evonik, Aerosil - Fumed Silica: Technical Overview, 2015. https ://prod ucts .evon i k.com/assets/45/92/244592. pdf, alternatively available at https://products.evonik.com/assets/45/92/Technical_Overview_AEROSIL_Fu med_Silica_EN_EN_244592.pdf.

[2] Evonik Industries, SIPERNAT® Specialty Silica - Product Overview 101 , (2019), Available at: https://products.evonik.com/assets/46/41/PO_101_SIPERNAT_Specialty_Sili ca_EN_EN_244641 .pdf.

[3] D. Schutte, F.-T. Schmitz, H. Brunner, Predominantly aqueous compositions in a fluffy powdery form approximating powdered solids behavior and process for forming same, 1968. US 3393155 A https://lens.0rg/121 -577-977-987-691 .

[4] G. Bolin, D. Glebov, Salt coated with nanoparticles, United States Patent US 9,459,026 B2, 2016.

[5] L. Sun, S. Wang, Z. Liu, A.T. Smith, Y. Zeng, L. Sun, W. Wang, Dry hydrated potassium carbonate for effective CO 2 capture, Dalt. Trans. 49 (2020) 3965-3969. https://doi.org/10.1039/C9DT01909J. [6] O. Takashi, K. Nobuyoshi, T. Emiko, Y. Toshio, Process for producing dry water, (2004). US 20040028710 A1 https://lens.org/121-182-065-578-725.

[7] L. Forny, I. Pezron, K. Saleh, P. Guigon, L. Komunjer, Storing water in powder form by self-assembling hydrophobic silica nanoparticles, Powder Technol. 171 (2007) 15-24. https://doi.Org/10.1016/J.POWTEC.2006.09.006.

[8] A. Boonyasittikul, D. Charnvanich, W. Chongcharoen, Effect of the ratio between hydrophobic mesoporous silica (Aerosil®R812S) and water on the formation and physical stability of water-entrapped self-assembly particle, Part. Sci. Technol. 39 (2021) 781-789. https://doi.Org/10.1080/02726351 .2020.1821141 .

[9] K. Shirato, M. Satoh, “Dry ionic liquid” as a newcomer to “dry matter,” Soft Matter. 7 (2011 ) 7191-7193. https://doi.org/10.1039/C1SM05999H.

[10] Y. Li, D. Zhang, D. Bai, S. Li, X. Wang, W. Zhou, Size Effect of Silica Shell on Gas Uptake Kinetics in Dry Water, Langmuir. 32 (2016) 7365-7371 . https://d0i.0rg/l 0.1021/acs.langmuir.6b01918.

[11] M. Al-Wabel, J. Elfaki, A. Usman, Q. Hussain, Y.S. Ok, Performance of dry water- and porous carbon-based sorbents for carbon dioxide capture, Environ. Res. 174 (2019) 69-79. https://doi.Org/10.1016/J.ENVRES.2019.04.020.

[12] X. Rong, H. Yang, N. Zhao, Rationally Turning the Interface Activity of Mesoporous Silicas for Preparing Pickering Foam and “Dry Water,” Langmuir. 33 (2017) 9025-9033. https://d0i.0rg/l 0.1021/acs.langmuir.7b01702.

[13] B.P. Binks, B. Duncumb, R. Murakami, Effect of pH and Salt Concentration on the Phase Inversion of Particle-Stabilized Foams, Langmuir. 23 (2007) 9143-9146. https://doi.org/10.1021/LA701393W. [14] B.P. Binks, R. Murakami, Phase inversion of particle-stabilized materials from foams to dry water, Nat. Mater. 2006 511. 5 (2006) 865-869. https://doi.Org/10.1038/nmat1757.

[15] L. Forny, K. Saleh, R. Denoyel, I. Pezron, Contact Angle Assessment of Hydrophobic Silica Nanoparticles Related to the Mechanisms of Dry Water Formation, Langmuir. 26 (2009) 2333-2338. https://doi.org/10.1021/LA902759S.

[16] B.P. Binks, A.J. Johnson, J.A. Rodrigues, Inversion of ‘dry water’ to aqueous foam on addition of surfactant, Soft Matter. 6 (2009) 126-135. https://d0i.0rg/l 0.1039/B914706C.

Claims

Claims

1 . A method for producing capsules consisting of aqueous solution droplets coated with hydrophobic silica particles, comprising mixing the solid silica particles and a volume of aqueous solution with sufficient energy to create a continuous liquid-like mixture, followed by aeration by expansion of the continuous liquid-like mixture into a larger volume with simultaneous mixing, resulting in conversion from continuous liquid-like mixture into small liquid droplets with average size ranging from 1 pm to 1000 pm encapsulated with silica particles.

2. A production method as claimed in claim 1 where the hydrophobic silica particles are hydrophobic fumed silica nanoparticles with average primary particle size ranging from 1 nm to 50 nm.

3. A production method as claimed in claim 2 where the amount of fumed silica is 0.5-1 .5 parts by weight of fumed silica particles and 98.5-99.5 parts by weight aqueous solution, preferably 0.8-1 .4 parts by weight in 98.6-99.2 parts by weight aqueous solution, more preferably 1 part by weight fumed silica in 99 parts by weight of aqueous solution.

4. A production method as claimed in claim 1 where the hydrophobic silica particles are hydrophobic precipitated silica microparticles with an average particle size ranging from 1 pm to 40 pm.

5. A production method as claimed in claim 4 where the amount of precipitated silica is 1 .5-2.5 parts by weight in 97.5-98.5 parts by weight of aqueous solution, preferably 1 .8-2.3 parts by weight precipitated silica in 97.7-98.2 parts by weight of aqueous solution, more preferably 2 parts by weight precipitated silica in 98 parts by weight aqueous solution.

6. A production method as claimed in claim 1 where the aqueous solution contains a salt, preferably being a hygroscopic salt, such as calcium chloride (CaCl2), or a water soluble organic solids such as erythritol or xylitol, most suitably at a concentration of up to 60wt%.

7. A production method as claimed in claim 1 where the expansion of the continuous liquid-like mixture into a larger volume with simultaneous mixing is achieved by shaking a container with sufficiently high air-to- liquid volume ratio, preferably an air-to-liquid volume ratio of 5:1 or more, more preferably 8:1 or more, even more preferably 10:1 or more, and most suitably 15:1 or more.

8. A production method as claimed in claim 1 , wherein required conditions for formation of a continuous liquid-like mixture that is convertible to liquid-in-air capsules are achieved by adjusting the contact angle between the silica particles and the aqueous solution by using a surfactant, adjusting the pH of the solution, adjusting the salt concentration of the solution or by adjusting the hydrophobicity of the silica particles or by any combination of these procedures, or, alternatively, by adjusting the temperature of the solution.

9. A production method as claimed in claim 1 , wherein a surfactant is added to the aqueous solution before forming the continuous liquidlike mixture, selected from organic molecules with an anionic functional group, such as a hydroxyl group, e.g. methanol or ethanol; a sulfate group, e.g. ammonium lauryl sulfate, sodium dodecyl sulfate (SDS), sodium lauryl ether sulfate (SLES) or sodium myreth sulfate; a sulfonate group, e.g. an alkylbenzene sulfonate, perfluorooctanesulfonate or perfluorobutanesulfonate; a phosphate group, e.g. alkyl ether phosphates or alkyl-aryl ether phosphates; or a carboxylate group, e.g. sodium stearate; fatty acid derivatives, such as their ethoxylates or esters; and amino acid derivatives.

10. A production method as claimed in claim 1 , wherein the pH of the aqueous solution is raised to a level of 7-10 before forming the continuous liquid-like mixture.

11 . A production method as claimed in claim 1 , wherein an inorganic salt is added to the aqueous solution before forming the continuous liquidlike mixture, and the salt concentration is adjusted to up to 65 wt%, preferably 30-58 wt%, most suitably 43-56 wt%.

12. A production method as claimed in claim 1 , wherein the hydrophobicity of the silica particles is altered before mixing them with the aqueous solution by changing the surface treatment of the silica particles, preferably by varying the type of organic group used to replace the silanol groups or by varying the percentage of silanol groups replaced with the organic groups, or, alternatively, by varying the treating agent with which the silanol groups react, particularly preferred treating agents reacted with the silanol group being hexamethyldisilazane, polydimethylsiloxane, alkylsilanes such as octylsilane, octamethylcyclotetrasiloxane, trimethylchlorosilane, dimethyldichlorosilane and vinylsilanes such as methacrylsilane.

13. Capsules consisting of aqueous solution droplets coated with hydrophobic silica particles, produced using the method of any of claims 1 to 12, with an average size ranging from 1 pm to 1000 pm.

14. Capsules as claimed in claim 13, wherein the droplets are coated with hydrophobic fumed silica nanoparticles having an average particle size ranging from 1 nm to 50 nm.

15. Capsules as claimed in claim 13, each capsule being a particle comprising an inner part and an outer coating, wherein said inner part comprises at least one salt or organic solid and said outer coating comprises hydrophobic precipitated silica microparticles having an average particle size from 1 pm to 40 pm, wherein the particle has an average particle size from 5 pm to 1000 urn.