WO2016198494A1

WO2016198494A1 - Method for producing inorganic hollow microspheres

Info

Publication number: WO2016198494A1
Application number: PCT/EP2016/063100
Authority: WO
Inventors: Max SIEBERT; Holger Kreusch
Original assignee: Basf Se
Priority date: 2015-06-10
Filing date: 2016-06-09
Publication date: 2016-12-15

Abstract

The invention relates to a method for producing inorganic hollow microspheres, having the following steps: (a) providing a gas flow (G) in a microchannel; (b) providing a flow of an aqueous phase (W) containing water and at least one inorganic precursor compound; providing a flow of an oil phase (O) containing a non-polar solvent and at least one surfactant; (d) combining the gas flow (G) and the flow of the aqueous phase (W) in a first dispersion zone, whereby the gas flow (G) separates and a flow of dispersed gas bubbles is formed in a continuous aqueous phase (G/W emulsion); (e) combining the flow of dispersed gas bubbles in the continuous aqueous phase (G/W emulsion) and the flow of the oil phase (O) in a second dispersion zone, whereby the flow of dispersed gas bubbles in the continuous aqueous phase (G/W emulsion) is constricted such that a flow of dispersed drops, each of which encloses a gas bubble, of the aqueous phase is formed in a continuous oil phase (G/W/O emulsion); (f) activating at least one part of the at least one inorganic precursor compound, whereby a polyaddition and/or polycondensation reaction is triggered in the dispersed drops of the aqueous phase (W), and the inorganic hollow microspheres are formed from the dispersed drops, each of which encloses a gas bubble, of the aqueous phase with the involvement of the at least one inorganic precursor compound; and (g) insulating the inorganic hollow microspheres.

Description

The present invention relates to a process for producing inorganic hollow microspheres, in particular hollow glass microspheres.

Hollow microspheres typically have ball diameters between 1 and 1000 μηη and spherical wall thicknesses ranging from 1 to 10% of the ball diameter. They are characterized in particular by the fact that they have a large surface area and a large volume in relation to their mass. The range of possible spheroidal wall materials ranges from glass, ceramic-forming oxides and mixed oxides, silicates and aluminosilicates, polymers and polycondensates to metals. Micro hollow spheres are suitable for a variety of different applications. Due to their large specific surface they are used for the immobilization of active substances and catalysts. The enclosed interior of the hollow spheres combined with the frequently encountered suitability to store substances allows their use in "drug delivery systems".

Due to their low density compared to solid spheres, hollow microspheres, in particular inorganic hollow microspheres, are widely used as additives or fillers for building and construction materials. The inorganic hollow microspheres are used not only to reduce the density and thus the weight of construction and construction materials, but also to increase the abrasion resistance or to reduce the thermal expansion and to reduce the viscosity of formulations. This often makes it possible to save other - sometimes expensive - additives.

The demand for inorganic hollow microspheres has therefore steadily increased over the past 10 years. A large part of the demand is served by "Cenospheres", which are hollow spheres of aluminosilicate obtained during the washing process of filter dust originating from coal combustion in coal-fired power plants and having grain sizes in the range of 0.01 to 500 μm "Cenospheres" and the limited ability to optimize their properties to meet specific application requirements have led to the development of many alternative manufacturing processes.

Melt-phase processes have long been used to produce inorganic hollow microspheres that simulate the formation process of aluminosilicate hollow spheres in coal combustion in coal-fired power plants. However, here too, the possibilities of influencing the production process and thus deliberately changing certain properties are limited. Modern processes for the production of inorganic hollow microspheres generally use spherical templates for the production of customized hollow microspheres. In this case, the spherical wall of the hollow microspheres is formed regularly at the interface between the liquid or solid spherical template core (disperse phase) and the continuous phase surrounding the template core. The formation of the spherical wall at the interface or surface of the template core may be the result of precipitation reaction, supersaturation or adsorption. Another possibility is the reaction of two components at the phase interface between disperse phase, containing component (1), and continuous phase, containing component (2), to form a spherical wall. In this case remains regularly a liquid or solid template core (" Sacrificial core ") within the formed spherical wall, which must be subsequently removed by dissolution, evaporation or thermolysis to obtain a hollow microsphere (hollow microsphere).

Industrial significance has been achieved by various spraying and dropping methods, fluidized bed processes, emulsion processes and suspension processes. These are the subject of various reviews, see, for example, J. Bertling, J. Blömer, R. Kümmel, Chemie Ingenieur Technik 2003, 75, 669-678; J. Hu, M. Chen, X. Fang, L. Wu, Chem. Soc. Rev. 2011, 40, 5472-54911; and the literature cited therein. However, the use of inorganic hollow microspheres as additives for building and construction materials requires the provision of large quantities at low prices. From this point of view, too, there is great interest in providing new and better methods of producing inorganic hollow microspheres. In the recent past, it has been shown that customized hollow microspheres can also be produced with the aid of gaseous templates in fluidic microsystems. The use of gaseous templates has, inter alia, the advantage that, after the formation of the spherical wall, no liquid or solid template core remains in the spherical interior of the micro (hollow) sphere, which subsequently has to be removed at great expense.

Microfluidic systems (microfluidic systems) contain channels or cavities with cross-sectional dimensions in the submillimeter range, through which not only liquids but also gases can flow.A special feature of the microfluidic systems is that due to the small cross-sectional dimensions Flows are typically laminar, allowing targeted control of flows

From this, the so-called digital microfluidics developed, in which a liquid in the microsystem does not consist of individual phases but of several distinct phases (compartments). The compartmented fluids form a dispersion (emulsion) of two or more fluids that do not mix in the microsystem. The use of the compartmented fluids has the particular advantage that an axial dispersion of the separated phases (almost) completely omitted (see, for example, H. Song et al., Angew Chem 2003, 792, 1145). A microfluidic-based process for producing hollow microspheres with a spherical shell of hydrophobic silica nanoparticles using nitrogen bubbles as gaseous templates is described in MH Lee, V. Prasad, D. Lee, Langmuir Letter 2010, 26, 2227-2230. The preparation of the hollow microspheres in this process comprises the steps of: (a) generating a pressure-regulated continuous nitrogen stream (G), a continuous stream of an oil phase (O) consisting of hydrophobic silica nanoparticles and toluene, and a stream of a aqueous phase (W) consisting of poly (vinyl alcohol) and water, (b) combining the streams in a microfluidic assembly to form a gas-in-oil-in-water emulsion (G / O / W emulsion) from monodisperse micro oil drops (O) each containing a nitrogen bubble (G) enclosed in continuous aqueous phase (W), (c) vaporizing toluene to form a spherical shell of hydrophobic silica nanoparticles, and (d) isolating the hollow microspheres.

A disadvantage of this method is that it can in principle only with certain nanoparticles, such as hydrophobic silica nanoparticles, perform.

J.Wan, H.A. Stone, Langmuir 2012, 28, 37-41 describes a process for producing hollow microspheres with a spherical wall made of silicon dioxide, in which individual gas bubbles also serve as templates. The process comprises the steps of: (a) providing a pressure-regulated continuous nitrogen stream (G), (b) introducing the continuous nitrogen stream (G) into a continuous stream of an aqueous phase (W) containing deionized water, glycerol and sodium dodecyl sulfate (SDS (c) introducing said G / W emulsion into a continuous stream of an oil phase (O) containing polydimethylsiloxane (PDMS) and Surfactant 749 producing a stream of monodisperse drops of aqueous phase in continuous oil phase (G / W / O emulsion), each enclosing a nitrogen bubble, (d) introducing a further stream of an oil phase (W / O) laterally containing PDMS and 3,3,3-trifluoropropyltrichlorosilane to the G / W / O emulsion, whereby at the phase boundary between oil and water by a sol-gel reaction between 3,3,3-trifluoropropyltrichlorosilane and un d water a ball wall of silicon dioxide is formed, and (e) isolating the hollow microspheres.

A disadvantage of the last-described method is, on the one hand, that in addition to the hollow microspheres, solid silica spheres (ie spheres consisting solely of silicon dioxide and containing no enclosed nitrogen bubble) are always formed, with the result that the yield of hollow spheres is lower fails and an additional process step is required to separate the solid spheres. On the other hand, even slight changes in the content of 3,3,3-trifluoropropyltrichlorosilane in the oil phase can destroy the G / W / O emulsion. The object of the invention is therefore to provide an improved process for the production of inorganic hollow microspheres. Advantageously, the process should enable the formation of hollow spheres in high selectivity. Preferably, the method should also Allow position of inorganic hollow microspheres with the least possible polydispersity.

The object is achieved by a method for producing inorganic hollow microspheres, comprising the steps:

(a) providing a gas stream (G) in a microchannel;

(b) providing a stream of an aqueous phase (W) containing water and at least one inorganic precursor compound;

(c) providing a stream of an oil phase (O) containing a nonpolar solvent and at least one surfactant; (d) combining the gas stream (G) and aqueous phase stream (W) in a first dispersion zone whereby the gas stream (G) breaks down and a stream of dispersed gas bubbles is formed in a continuous aqueous phase (G / W emulsion);

Merging the stream of dispersed gas bubbles in the continuous aqueous phase (G / W emulsion) and the stream of oil phase (O) in a second dispersion zone, whereby the stream of dispersed gas bubbles in the continuous aqueous phase (G / W emulsion ) is pinched off such that a stream of dispersed droplets of the aqueous phase, each enclosing a gas bubble, is formed in a continuous oil phase (G / W / O emulsion);

Physical or chemical activation of at least part of the at least one inorganic precursor compound, whereby a polyaddition and / or polycondensation reaction is triggered in the dispersed droplets of the aqueous phase (W) and dispersed with the participation of the at least one inorganic precursor compound, respectively forming a bubble enclosing gas bubbles of the aqueous phase the inorganic hollow microspheres; and

(g) Isolation of inorganic hollow microspheres. With the method according to the invention can be produced hollow microspheres whose

Ball walls made of a variety of inorganic solids. In principle, there are no particular restrictions with regard to the selection of inorganic solids. The only requirement is that a corresponding inorganic precursor compound is available, which is sufficiently stable in the aqueous phase, not to completely or partially unreacted before the formation of the G / W / O emulsion, and on any other suitable manner can be selectively activated in order to be converted into an inorganic solid in the aqueous phase can. For the purposes of the present invention, the term "hollow microspheres" is understood as meaning hollow spheres having an outer sphere diameter of between 1 and 1000 μm The spherical walls of the hollow microspheres typically have a wall thickness of between 1 and 40% of the outer sphere diameter "Inorganic hollow microspheres" are understood to mean those hollow microspheres which have a spherical wall of an inorganic material.

In process step (a), the gas stream (G) is preferably passed through a microchannel having an internal cross-sectional diameter of from 1 to 1000 μm, more preferably from 5 to 500 μm and very particularly preferably from 10 to 300 μm.

The term "microchannel" is generally understood to mean a channel with a round or rectangular cross-section, the inner diameter or inner diagonal of which is between 1 and 1000 μm. Preferably, the entire process is carried out in a microfluidic system ("microfluidic system").

In the context of the present invention, the term "microfluidic system" means any microfluidic device having at least three inputs for supplying the fluids and at least one outlet for providing the G / W / O emulsion The "microfluidic system" contains channels or other cavities which have a typical cross-sectional dimension in the micrometer range in at least one direction The "microfluidic system" is preferably designed as a chip The dispersion regions are at a certain distance from the inputs to the fluid The distance is preferably at least 2 mm.

In the gas stream (G), there are basically no particular restrictions on the choice of gases. Typically, the gas stream contains one or more gases selected from the group consisting of air, oxygen, nitrogen, carbon dioxide, hydrogen chloride, hydrogen, helium, neon, and argon. Particularly preferably, the gas stream contains air, oxygen, nitrogen, argon or mixtures thereof, most preferably the gas stream contains carbon dioxide.

The volume flow of the gas stream (G) through the microchannel is preferably from 0.01 to 50 ml-h - particularly preferably 0.1 to 20 ml-h- ¹ and very particularly preferably 0.5 to 10 ml-h- ¹ .

It has been shown that the proportion of hollow spheres can be increased in relation to the regularly produced by-product solid spheres, if the introduction of the gas stream (G) in the microfluidic system is not - as often happens - with a syringe pump, but instead under Use of a flow regulator with integrated thermo or pressure sensor. The most commonly used flow regulators are pressure operated flow regulators (eg, syringe pumps and pressure pumps). Specifically, the pressure-operated flow regulators differ in that some of them directly measure and regulate the pressure within the microfluidic system (eg, pressure pumps) while others (eg, syringe pumps) provide flow rate control. In the syringe pump, which is still popular today, the movable piston of a syringe is slowly pressed in by a stepping motor, thereby injecting the liquid in the syringe into the microfluidic component. However, the syringe pump has the disadvantage of pulsations in the mechanical advancement, each time the stepper motor injects fluid into the microfluidic system in a forward motion. These pulsations are particularly pronounced when large amounts of fluid to be injected with the syringe pump.

While always cause syringe pumps due to their mechanical construction, a more or less pronounced pulsations, the flow controllers offer integrated thermal sensors (commercially available, for example from the company Bronkhorst Mättig GmbH under the name Bronkhorst ^® gas flow measurement controller F211, Bronkhorst ^® digital controller E7100 and the flow regulators with integrated pressure sensors, for example available from Fluigent under the name MFCS ™ Microfluidic Flow Control System, consisting of a precision pressure pump, microfluidic pressure sensor and feedback control loop have the advantage that they have no moving or mechanical parts.

As a result, extremely stable volume flows in the channels of a microfluidic system are achieved, which are essentially free of pulsation. According to the manufacturer, such stable volume flows can be achieved that their standard deviation is less than 0.1%.

Preferably, in the method according to the invention, a flow regulator with integrated thermo or pressure sensor is used. This makes it possible to keep the volume flow of the gas stream (G) so stable that the weight ratio of hollow spheres to solid spheres is regularly greater than 80:20. Preferred embodiments of the method according to the invention achieve values of greater than 95: 5. The volume flow of the gas stream (G) accordingly varies during the execution of the method preferably by less than 0.1 ml-h - more preferably less than 0.05 ml-h - ¹ and most preferably less than 0.01 ml-h -

In process step (b), the stream of an aqueous phase (W) is preferably conducted through one or two microchannels with cross-sectional diameters of 1 to 1000 μm, particularly preferably 10 to 1000 μm and very particularly preferably 20 to 600 μm. The ratio of the volume flows between gas stream and stream of an aqueous phase is preferably from 10: 1 to 1: 2, particularly preferably from 5: 1 to 1: 1, very particularly preferably from 2: 1 to 1: 1. The volume flow of the flow of an aqueous phase (W) through the microchannel is preferably from 0.01 to 50 ml-h - particularly preferably 0.1 to 20 ml-h- ¹ and very particularly preferably 0.5 to 10 ml-h - The aqueous phase (W) generally contains at least 1.0% by weight of water, preferably at least 10% by weight of water, more preferably at least 15% by weight of water, based on the total mass of the aqueous phase.

The aqueous phase (W) generally contains from 1, 0 to 99 wt .-% of the at least one inorganic precursor compound, based on the total mass of the aqueous phase. The aqueous phase (W) preferably contains from 10 to 90% by weight and more preferably from 20 to 85% by weight of the at least one inorganic precursor compound.

In general, the viscosity of the aqueous phase (W) of 0.1 to 200 mPa ^«s. The viscosity by Vis is preferably from 0.5 to 50 mPa ^'s, more preferably from 0.8 to 25 and most preferably from 1, 0 to 10 mPa ^«s.

The temperature of the aqueous phase is 2 to 90 ° C, preferably 10 to 80 ° C, particularly preferably 25 to 75 ° C.

The aqueous phase (W) may contain other components in addition to water and at least one inorganic precursor compound. Basically, these are not required. Depending on which inorganic precursor compound is added to the aqueous phase (W), however, it may be advantageous or even necessary to increase the viscosity and / or targeted change of other physical quantities of the aqueous phase (W) a cosolvent and / or a To add anionic surfactant. This is the case in particular if otherwise no stable stream of dispersed droplets of aqueous phase enclosing a gas bubble (G / W / O emulsion) can be formed. In one embodiment of the invention, the aqueous phase (W) contains an anionic surfactant. Suitable anionic surfactants are, for example, alkylcarboxylates, alkylbenzenesulfonates, alkanesulfonates and fatty alcohol sulfates. Furthermore, alkyl phosphonates and fatty alcohol phosphates and zwitterionically charged surfactants may be included. If anionic surfactants are added, these are preferably selected from the group of fatty alcohol sulfates and alkyl sulfonates. The anionic surfactants can be used in an amount of 0 to 10 wt .-%, based on the total weight of the aqueous phase.

Suitable co-solvents are, for example, monohydric, dihydric and polyhydric alcohols and mixtures thereof. Suitable monohydric alcohols are those having 1 to 6 carbon atoms, such as, for example, methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, isobutanol and tert-butanol. Suitable dihydric alcohols are, in particular, those having 2 to 6 carbon atoms, in particular ethylene glycol, diethylene glycol, triethylene glycol and propylene glycol. Suitable polyhydric alcohols are those having 3 to 6 carbon atoms, such as For example, glycerol and sorbitol, of which glycerol is particularly preferred. The cosolvent can be used in an amount of up to 80% by weight, preferably from 0 to 40% by weight, based on the total weight of the aqueous phase. In one embodiment, the aqueous phase (W) contains, in addition to water and at least one inorganic precursor compound as a further component, a nonionic surfactant. The aqueous phase particularly preferably contains a nonionic surfactant selected from the group consisting of polyalkylene glycol ethers, alkylglucosides, alkylpolyglucosides and alkylphenol ethoxylates. Further nonionic surfactants are, for example, surfactants based on silicones and (per) fluorinated surfactants. Most preferably, the aqueous phase (W) alkyl polyglucosides, such as Triton ^® CG-110 (Dow Chemicals), or Glucopon ^® DK225 (BASF) contains. The nonionic surfactant may be used in an amount of 0 to 10% by weight based on the total weight of the aqueous phase. The aqueous phase (W) preferably contains from 0.001 to 2.0 wt .-% and particularly preferably from 0.01 to 0.5 wt .-% of a non-ionic surfactant, based on the total weight of the aqueous phase.

In process step (c), the stream of an oil phase (O) is preferably conducted through one or two microchannels with cross-sectional diameters of 1 to 1000 μm, particularly preferably 10 to 1000 μm and very particularly preferably 20 to 600 μm.

The ratio of the volume flows of oil phase (O) to aqueous phase (W) per channel is preferably from 200: 1 to 2: 1, particularly preferably from 100: 1 to 3: 1, very particularly preferably from 50: 1 to 4: 1.

The volume flow of the flow of an oil phase (O) through the microchannel (s) is preferably from 0.05 to 200 ml- ^h_1 , more preferably from 0.1 to 100 ml-h- ¹ and most preferably from 0.5 to 20 ml-h - ¹ per channel.

The oil phase (O) usually contains as nonpolar solvent mineral oils, vegetable oils, animal oils, silicone oils, perfluorinated oils, for example perfluoroether or perfluoroalkanes or partially fluorinated alkanes or mixtures thereof. Suitable mineral oils are for example Cobersol ^® B25, B40 and H120. Suitable vegetable oils are, for example, jojoba oil, olive oil, sunflower oil, soybean oil, peanut oil, rapeseed oil, almond oil, palm oil, coconut oil, castor oil, wheat germ oil, grapeseed oil and thistle oil. Suitable animal oils are especially trans and cod liver oil. Suitable silicone oils are, for example, polydimethylsiloxane (PDMS), suitable oils are for example perfluoriere Fluoronox ^® L20 / 1, Fluoroinert ^® FC-40 (3M) or Krytox ^® GPL-oils (DuPont). Preferably, the oil phase contains one or more non-polar solvents selected from the group consisting of linear and branched Cs-C2o alkanes.

The oil phase (O) generally contains from 60 to 99.99% by weight of one or more nonpolar solvents, based on the total mass of the oil phase. The oil phase (O) contains preferably 80 to 99.99% by weight, particularly preferably 90 to 99.99% by weight and very particularly preferably 95 to 99.99% by weight of one or more nonpolar solvents.

The oil phase (O) preferably contains one or more nonionic surfactants having an HLB value between 1, 5 and 8. Preferred nonionic surfactants are selected from the group consisting of fatty alcohols, sterols, sorbitan fatty acid esters and polyalkylene ethers. Most preferably, the oil phase (O) contains one or more nonionic surfactants selected from the group consisting of sorbitan isostearate (SPAN ™ 120), sorbitan trioleate (SPAN ™ 85), sorbitan monooleate (SPAN ™ 80), sorbitan tristearate (SPAN ™ 65), sorbitan stearate (SPAN ™ 60), sorbitan sesquioleate (SPAN ™ 83), glyceryl monostearate, sorbitan monopalmitate (SPAN ™ 40) sorbitan laurate (SPAN ™ 20), cetyl alcohol, stearyl alcohol and polyglycerol polyricinoleate ("PGPR") or mixtures thereof.

The oil phase (O) usually contains 0.0001 to 2.0% by weight of these nonionic surfactants, based on the total weight of the oil phase. The amount of nonionic surfactant added is preferably 0.001 to 1.0% by weight, more preferably 0.005 to 0.5% by weight.

The interfacial tension between the aqueous phase and the oil phase is generally from 20 to 80 mN-m. The interfacial tension refers to forces that occur between two different phases that are in contact with each other. They form a common interface that is under interfacial tension. The interfacial tension between the aqueous phase and the oil phase is preferably from 25 to 60 mN-m, more preferably from 30 to 60 mN-m. It has been found that the proportion of hollow spheres in relation to the by-produced solid spheres can also be increased if the nonionic surfactant is used in amounts which are preferably not above its critical micelle concentration (CMC). Preferably, the nonionic surfactant is used in an amount which is below its critical micelle concentration in the corresponding nonpolar solvent. Accordingly, the oil phase (O) most preferably contains a surfactant amount comprised between 0.001% by weight and the critical micelle formation concentration.

The critical micelle formation concentration is the concentration of a surfactant in a certain continuous phase, above which aggregates of surfactant molecules - so-called micelles - begin to form. The critical micelle concentration is not only dependent on the surfactant itself, but also in particular on the phase surrounding the surfactant. The critical micelle formation concentration of many common nonionic surfactants has been determined in recent decades for various non-polar solvents. The critical micelle formation concentration can therefore usually be found in the specialist literature (see, for example, L. Peltonen, "The Behavior of Sorbitan Surfactants at the Water-Oil Interface: Straight-Chained Hydrocarbons from Pentanes to Dodecanes as on Oil Phase", J. Colloid 2001, 240, 272-276; H. Zhou et al., "A facile microfluidic strategy for measuring inter- Physal Lett., Appl. Phys. Lett., 2013, 103, 234192.) or from the Internet pages of surfactant suppliers (see, for example, http://www.sigmaaldrich.com/life-science / biochemicals / biochemical-products If critical micelle formation concentrations are not known for specific nonionic surfactants and / or for particular nonpolar solvents, the corresponding concentration can be determined simply by standard methods experimentally using the Noüy ring method (EN 14370) or the spinning drop method (HH Hu, DD Joseph, "Evolution of a Liquid Drop in a Spinning Drop Tensiometer", J. Colloid Interface Sci. 1994, 162, 331 -339) the interfacial tension is measured and then the critical micelle concentration is calculated (see also other methods: CE Lin, "Determination of critical micelle concentration of surfactants by capillary electrophore - sis ", J. Chromatogr. A. 2004, 1037, 467-478; H. Zhou et al., "A facile microfluidic strategy for measuring interfacial tension", Appl. Phys. Lett. 2013, 103, 234192; A. Dominguez et al., "Determination of Critical Micelle Concentration of Some Surfactants by Three Techniques", J. Chem. 1997, 74, 1227.)

The viscosity of the oil phase (O) is generally from 0.01 to 50 mPa ^«s. The viscosity is preferably from 0.1 to 10 mPa ^'s, more preferably from 0.2 to 5 ^mPa' s, and most preferably from 0.5 to 4 mPa ^«s. The vapor pressure is generally less than 1 mbar (20 ° C). The boiling point is generally 80 to 250 ° C, more preferably 100 to 250 ° C.

Details regarding the operation of such "microfluidic systems", e.g. their coupling to liquid reservoirs, the generation and control of liquid flows by pumping devices, the fluidic influencing of the fluids and the performance of measurements are known per se and will therefore not be described in detail here. Detailed accounts of these and other fundamental aspects can be found, for example, in the following books and reviews: K. E. Herold, A. Rasooly (editor). Lab-on-a-Chip Technology: Fabrication and Microfluidics, Caister Academic Press 2009; P. Tabeling (editor), Introduction to Microfluidics, Oxford University Press 2006; P. Day ef al. (editor), Microdroplet Technology: Principles and Emerging Applications in Biology and Chemistry, Springer-Verlag Berlin Heidelberg 2012; B. Lin (editor), Microfluidics: Technologies and Applications, Springer-Verlag Berlin Heidelberg 2011; G.M. Whitesides, "The Origins and the Future of Microfluidics", Nature 2006, 442, 368-373; T. M. Squires, S.R. Quake, "Microfluidics: Fluid Physics at the Nanoliter Scale", Reviews of Modern Physics 2005, 77, 977-1026; R. Seemann et al., "Droplet based microfluidics", Rep. Prog. Phys. 2012, 75, 16601; J. B. Baret, "Surfactants in droplet-based microfluidics", Lab Chip. 2012, 12, 422-433; Y. Zhu, Q. Fang, "Analytical detection techniques for droplet microfluidics - A review", Analytica Chimica Acta 2013, 787, 24-35; S.-Y. Teh et al., "Droplet microfluidics", Lab Chip. 2008, 8, 198-220; C.N. Baroud et al., "Dynamics of Microfluidic Droplets", Lab Chip. 2010, 10, 2032-2045.

In process step (d), in the first dispersion zone, by merging gas stream (G) and stream of an aqueous phase (W), a stream of dispersed gas bubbles in the continuous aqueous phase (G / W emulsion) is formed. The first dispersion zone can have different concrete configurations. FIG. 1 shows by way of example three widespread embodiments of the first dispersion zone. The first dispersion zone can be designed as a T-shaped intersection (see FIG. 1 a). The first dispersion zone here consists of two microchannels which are T-shaped interconnected. The horizontal microchannel (Mw.ein) carries the flow of an aqueous phase (W) in the arrow direction to the T-shaped intersection, while the microchannel (Mcein) the gas flow (G) perpendicular to the flow direction of the aqueous phase (W) in the T-shaped Introduces a crossing. The dimensions of the microchannels are chosen so that gas bubbles can form. This depends in particular on the ratio of the cross-sectional diameter between the feed opening to the T-shaped intersection (dcein) and outlet opening of the T-intersection (dc / w.off) to the microchannel (Mc / w.off). For only if the quotient dc.a dG w.a. <1, preferably <0.7, more preferably <0.5, can the gas flow (G) demolish structurally such that isolated gas bubbles are formed and a stream from dispersed gas bubbles in a continuous aqueous phase (G / W emulsion) is formed. As can be seen from FIG. 1 a), this G / W emulsion is directed in the direction of the arrow out of the first dispersion zone. Which physical forces to take into account when pinching off the individual gas bubbles and how these interact are known in principle. More details can be found, for example, in: T. Thorsen et al., Phys. Review Letters 2001, 86, 4163-4166; P. Garstecki et al., "Formation of droplets and bubbles in a microfluidic T-junction", Lab Chip. 2006, 8, 437-446; CN Baroud et al., "Dynamics of Microfluidic Droplets", Lab Chip. 2010, 10, 2032-2045; and J. Wan, HA Stone, "Coated Gas Bubbles for the Continuous Synthesis of Hollow Inorganic Particles", Langmuir 2012, 28, 37-41.

Another possible embodiment of the first dispersion zone is shown schematically in FIG. 1 b). In this case, the first dispersion zone consists of a planar channel system, which is characterized in that a horizontal microchannel crosses a microchannel perpendicular to it, so that the channel system has three supply openings. In this embodiment, therefore, three streams are merged into one, which is why this is often referred to by a so-called flow-focusing device (English: "flow-focusing device") or simply by flow-focusing. The horizontal microchannel (Mcein) introduces the gas flow (G) into the intersection in the direction of the arrow. In the microchannels (Mw.ein) arranged perpendicular to the flow direction of the gas stream (G), streams of an aqueous phase (W) are guided in the direction of the arrow to the center of the intersection, the gas stream (G) being subjected to shearing forces and at the entry opening of the intersection (diameter: dc w.aus, length: /w.aus) breaks off and in the horizontal microchannel (MG w, off) a stream of dispersed gas bubbles in continuous aqueous phase (G / W emulsion) is formed, which in the arrow direction from the current-focusing arrangement exit. In the current-focusing arrangement (Figure 1 b)) not only two, but four sizes are taken into account, so that gas bubbles are formed. In addition to the quantities dcein and dG w, from in this design of the first dispersion zone, the diameter dcw.sub.out and the length b / w, from the outlet opening and the flow rates must also be taken into account. What influence these variables have on the formation of gas bubbles and how these concretely too To this extent, it is known, for example, from the following publications: P. Garstecki et al., "Formation of droplets and bubbles in microfluidic systems", Bull. Pol. Ac: Tech. 2005, 53, 361-372; CN Baroud et al., "Dynamics of Microfluidic Droplets", Lab Chip. 2010, 10, 2032-2045; and J. Wan, HA Stone, "Coated Gas Bubbles for the Continuous Synthesis of Hollow Inorganic Particles", Langmuir 2012, 28, 37-41.

A further embodiment of the first dispersion zone is shown in FIG. 1 c). The first dispersion zone here consists of an inner cylindrical microchannel and a coaxially arranged outer microchannel. In the outer microchannel, the flow of an aqueous phase (W) is guided in the direction of the arrow, while the gas stream (G) is introduced into the aqueous phase (W) via the inner microchannel (Mcein) in such a way that it flows into the inlet opening (Dcein ) leave and form isolated gas bubbles. In contrast to the other abovementioned embodiments of the first dispersion zone, the diameters of the inlet opening (dc.ein) and outlet opening (cross-sectional diameter dc / w.sub.out) are largely without influence on the gas bubble formation. Rather, in addition to the material properties of the gas phase and the aqueous phase, in particular the flow rate of the flow of an aqueous phase is crucial here. Only below a certain limit speed do isolated gas bubbles form. Further details on the influence of other physical quantities can be found in the articles cited above and S. Takeuchi et al., "An Axisymmetric Flow-Focusing Microfluidic Device", Advanced Materials 2005, 17, 1067-1072.

The generation of a stream of dispersed gas bubbles in continuous aqueous phase (G / W emulsion) is not only possible with the three described embodiments of the first dispersion phase, but can of course also with other - mostly differing only in some details of these embodiments - microfluidic Applications are achieved. For example, the microfluidic arrangements described in the following literature are also suitable: DE 10 2006 036 815 (A1), DE 10 2005 037 401 (B4).

In general, the cross section of the microchannels, the viscosity of the aqueous phase and the flow rates of the fluids conducted to the first dispersion zone are selected such that the gas bubbles produced have a diameter of 1 to 1000 μm. Preferably, the respective diameters of 5 to 500 μηη, more preferably from 10 to 300 μηη, most preferably from 30 to 200 μηη. Depending on the cross-sectional diameters of the microchannels, between 1 and 20 gas bubbles per microchannel may form per second.

The metastable state of the dispersed gas phase / continuous aqueous phase composition achieved with the formation of the G / W emulsion has a sufficiently long lifetime. The dispersed gas bubbles remain until the formation of the spherical wall as delimited compartments. The gas bubbles are almost exactly the same size and almost equidistant from each other. As a result, they reach the second dispersion zone according to process step (e) at always the same time intervals. In process step (e), the stream of dispersed gas bubbles in a continuous aqueous phase (G / W emulsion) in a second dispersion zone is combined with a stream of an oil phase (O). The second dispersion zone, like the first dispersion zone in process step (d), may have various concrete configurations. FIG. 2 shows, by way of example, a concrete embodiment of the second dispersion zone. The embodiment shown in FIG. 2 is essentially identical to the possible embodiment of the first dispersion zone already described above and shown in FIG. 1 b). In the horizontal microchannel (MGw, a), the stream of dispersed gas bubbles generated in process step (d) in a continuous aqueous phase (G / W emulsion) is introduced in the direction of the arrow into the second dispersion zone. In the microchannels (Mo.ein) arranged perpendicular to the flow direction of the G / W emulsion, streams of an oil phase (O) are introduced in the direction of the arrow into the second dispersion zone. In this case, the stream of dispersed gas bubbles in the continuous aqueous phase (G / W emulsion) is pinched off in such a way that individual drops of the aqueous phase are formed, which in turn each contain a gas bubble. The stream of emulsified, in each case a bubble enclosing drops of aqueous phase in the continuous oil phase (G / W / O emulsion) is formed, which emerges in the direction of arrow from the second dispersion zone. In principle, the same applies to the dimensions of the second dispersion phase, as has already been explained above in connection with the first dispersion zone (FIG. 1 b)).

The gas bubble trapped in the droplet of the aqueous phase subsequently remains unchanged. The drops of the aqueous phase are emulsified by the continuous oil phase.

In process step (f) at least a portion of the at least one inorganic precursor compound is activated in the aqueous phase so that a polyaddition and / or polycondensation reaction is triggered therein and a solid in the form of a spherical shell forms in the aqueous phase ,

In the context of the present invention, the expression "in (the) aqueous phase" includes not only the aqueous phase itself, but also the phase interface to the oil phase and the phase boundary to the gas phase The activation of at least a portion of the inorganic precursor compounds can be accomplished in a very different manner and is primarily tuned to the chosen inorganic precursor compound For example, an option may be to increase the pH or decrease the pH of the chosen inorganic precursor compound, and this possibility may be considered in particular when some or all of the inorganic precursor compounds have a pH-dependent reactivity having. In a preferred embodiment, the activation of at least part of the at least one inorganic precursor compound takes place in that the pH values in the oil phase are lowered by at least 1.5 pH units and at least part of the at least one inorganic precursor compound is Compound is protonated.

The present invention thus also provides a process for producing inorganic hollow microspheres, comprising the steps:

(a) providing a gas stream (G) in a microchannel;

(c) providing a stream of an oil phase (O) containing a nonpolar solvent and at least one surfactant;

(d) combining gas stream (G) and aqueous phase stream (W) in a first dispersion zone whereby the gas stream (G) is drained off and a stream of dispersed gas bubbles is formed in a continuous aqueous phase (G / W emulsion);

(e) combining the stream of dispersed gas bubbles in the continuous aqueous phase (G / W emulsion) and the stream of an oil phase (O) in a second dispersion zone, whereby the stream of dispersed gas bubbles in the continuous aqueous phase (G / W emulsion) is pinched off such that a stream of dispersed droplets of the aqueous phase, each enclosing a gas bubble, is formed in a continuous oil phase (G / W / O emulsion);

(f1) activation by lowering the pH in the oil phase (O) by at least 1.5 pH units, whereby an acid-mediated polyaddition and / or polycondensation reaction is triggered in the dispersed droplets of aqueous phase (W) is formed and the at least one inorganic precursor compound from the dispersed, in each case a bubble enclosing droplets of aqueous phase, the inorganic hollow microspheres; and (g) isolating the inorganic hollow microspheres.

In this preferred embodiment, the inorganic precursor compound is preferably selected from the group consisting of siloxanes (Si (OR) ₄ ), alkyl siloxanes

(R ¹ xSi (OR ² ) _4-x or metal alkoxides (eg Ti (OR) ₄ ) and mixtures thereof The inorganic precursor compound is particularly preferably selected from the group consisting of soda waterglass, potassium silicate, lithium waterglass, ammonium waterglass and potassium methyl waterglass and mixtures from that. In a variant of this embodiment of the invention, the lowering of the pH in the oil phase (O) by at least 1.5 pH units is particularly preferably carried out by adding to the stream of dispersed droplets of the aqueous Phase in the continuous oil phase (G / W / O emulsion), a stream of another oil phase (W / O) containing a non-polar solvent and a protonic acid is supplied.

Suitable protic acids are selected from the group consisting of hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, acetic acid, methanesulfonic acid, phosphorous acid, sulfurous acid and p-toluenesulfonic acid.

In another variant of this embodiment of the invention, the lowering of the pH in the oil phase (O) by at least 1.5 pH units is particularly preferably characterized in that the stream of dispersed, each enclosing a gas bubble drops of the aqueous Phase in the continuous oil phase (G / W / O emulsion) in a further oil phase (W / O) containing a non-polar solvent and a protonic acid is passed.

In this embodiment of the invention, the lowering of the pH in the oil phase by at least 1.5 pH units is particularly preferably effected by adding to the stream of dispersed droplets of the aqueous phase in the continuous oil which in each case include a gas bubble Phase (G / W / O emulsion), a stream of another oil phase (W / O) containing a non-polar solvent and a protonic acid is supplied.

In a further preferred embodiment, the activation of at least part of the at least one inorganic precursor compound takes place in that the pH in the oil phase is increased by at least 1.5 pH units and at least part of the at least one inorganic precursor compound is deprotonated.

(a) providing a gas stream (G) in a microchannel;

(d) combining gas stream (G) and aqueous phase stream (W) in a first dispersion zone whereby the gas stream (G) is drained off and a stream of dispersed gas bubbles is formed in the continuous aqueous phase (G / W emulsion); Merging the stream of dispersed gas bubbles in the continuous aqueous phase (G / W emulsion) and the stream of oil phase (O) in a second dispersion zone, whereby the stream of dispersed gas bubbles in the continuous aqueous phase (G / W emulsion ) is pinched off such that a stream of dispersed droplets of the aqueous phase, each enclosing a bubble of gas, is formed in the continuous oil phase (G / W / O emulsion);

Activation by increasing the pH in the oil phase by at least 1.5 pH units, whereby a base-mediated polyaddition and / or polycondensation reaction is triggered in the dispersed droplets of the aqueous phase (W) and with the participation of at least one inorganic precursor compound is formed from the dispersed droplets of the aqueous phase, each enclosing a gas bubble, the hollow inorganic microspheres; and

Isolation of the inorganic hollow microspheres

In this preferred embodiment, the inorganic precursor compound is preferably selected from the group consisting of silicic acid and its low molecular weight condensates (such as di- or tri-silicic acid, meta-silicic acid), metal salts, e.g. Cersalts and mixtures thereof. The inorganic precursor compound is particularly preferably selected from the group consisting of silicic acids, ammonium cerium nitrate, cerium chloride, cerium carbonate, cerium oxalate, cerium hydroxide and mixtures thereof. Activation of at least a portion of the inorganic precursor compound can also be accomplished by increasing the temperature or by irradiating UV light.

In another preferred embodiment, the activation of at least a portion of the at least one inorganic precursor compound is effected by at least the temperature of the stream of dispersed droplets of aqueous phase in continuous oil phase (G / W / O emulsion), each including a gas bubble 15 ° C is increased and at least a portion of the at least one inorganic precursor compound forms radicals.

The present invention thus also relates in particular to a process for the production of inorganic hollow microspheres, comprising the steps:

(a) providing a gas stream (G) in a microchannel;

(b) providing a stream of the aqueous phase (W) containing water and at least one inorganic precursor compound;

Providing a stream of an oil phase (O) containing a non-polar solvent and at least one surfactant; (d) combining gas stream (G) and aqueous phase stream (W) in a first dispersion zone whereby the gas stream (G) is drained off and a stream of dispersed gas bubbles is formed in the continuous aqueous phase (G / W emulsion);

(e) combining the stream of dispersed gas bubbles in the continuous aqueous phase (G / W emulsion) and the oil phase (O) stream in a second dispersion zone, whereby the stream of dispersed gas bubbles in the continuous aqueous phase (G / W emulsion) is pinched off such that a stream of dispersed droplets of the aqueous phase enclosing a gas bubble in continuous oil phase (G / W / O emulsion) is formed;

(f3) activation by raising the temperature by at least 15 ° C and free radical formation from at least a portion of the at least one inorganic precursor compound, whereby in the dispersed droplets of the aqueous phase (W) a radical-mediated polyaddition and / or polycondensation reaction is triggered and the inorganic hollow microspheres are formed with the participation of the at least one inorganic precursor compound from the dispersed droplets of the aqueous phase enclosing a respective gas bubble; and

(g) Isolation of inorganic hollow microspheres.

The present invention also relates in particular to a process for producing inorganic hollow microspheres, comprising the steps:

(a) providing a gas stream (G) in a microchannel;

(c) providing a stream of an oil phase (O) containing a non-polar solvent and at least one surfactant;

(d) combining the gas stream (G) and the aqueous phase stream (W) in a first dispersion zone whereby the gas stream (G) is drained off and a stream of dispersed gas bubbles is formed in the continuous aqueous phase (G / W emulsion) ;

(e) combining the stream of dispersed gas bubbles in the continuous aqueous phase (G / W emulsion) and an oil phase (O) stream in a second dispersion zone, whereby the stream of dispersed gas bubbles in the continuous aqueous phase (w / w) Emulsion) is pinched off such that a stream of disperse gated, in each case a gas bubble enclosing drops of the aqueous phase in continuous oil phase (G / W / O emulsion) is formed;

(f4) activation by irradiation of UV light and radical formation from at least a part of the at least one inorganic precursor compound, whereby a radical-mediated polyaddition and / or polycondensation reaction is triggered in the dispersed droplets of the aqueous phase (W) and with the participation of at least one inorganic precursor compound from the dispersed droplets of the aqueous phase, each enclosing a gas bubble, the inorganic hollow microspheres are formed; and

(g) isolating the inorganic hollow microspheres.

After the hollow spheres have been formed and the microemulsion system has optionally been cleaned or thermally post-treated, the hollow spheres are separated from the microemulsion system in a further after-treatment step.

Hereinafter, FIGS. 1 to 9 will be briefly described. FIG. 1 shows various specific embodiments of the first dispersion zone of the microfluidic structure in which streams of dispersed gas bubbles in a continuous aqueous phase (G / W emulsion) can be formed; Figure 1 a) shows schematically the dispersion formation at a T-shaped crossing; FIG. 1 b) shows schematically the formation of monodisperse gas bubbles on a flow-focusing arrangement; and FIG. 1 c) shows schematically the dispersion formation in an arrangement.

FIG. 2 shows a concrete embodiment of the second dispersion zone of the microfluidic structure, in which streams of emulsified droplets of aqueous phase enclosing a gas bubble in continuous oil phase (G / W / O emulsion) can be formed.

Figure 3 shows schematically a way of activating the inorganic precursor compound in which activation is triggered by introducing into the G / W / O emulsion a stream of an oil phase 2 containing a proton acid downstream of the second dispersion zone and proton acid protonates at least a portion of the at least one inorganic precursor compound.

FIG. 4 shows a microfluidic structure for producing hollow microspheres. The first flux-focusing arrangement (first dispersion zone) shown on the left serves to generate a stream of dispersed gas bubbles in continuous aqueous phase (G / W emulsion) and the second flow-focusing arrangement (second dispersion zone) to produce a stream of emulsified, in each case a gas bubble enclosing drops of aqueous phase in a continuous oil phase (G / W / O emulsion). FIG. 5 shows a scanning electron micrograph of a mixture of silicon dioxide-based hollow microspheres (partially crushed by exertion of pressure) and silicon dioxide-based micro-solid particles or microparticles. FIG. 6 shows the microchannel (cross-sectional diameter: 200 μm) of a microfluidic structure with a flow of a continuous oil phase flowing in it, containing emulsified droplets of aqueous phase, each including a gas bubble.

FIG. 7 shows a microfluidic application for producing hollow microspheres directly in the microchannel. The left and middle flux-focusing arrangements serve to generate a stream of emulsified droplets of aqueous phase in continuous oil phase (G / W / O emulsion) which in each case surround a gas bubble; the third current-combining arrangement, shown on the right, serves to initiate a solution which activates the inorganic precursor compound.

FIG. 8 shows a scanning electron micrograph of silicon dioxide-based hollow microspheres.

FIG. 9 shows a scanning electron micrograph of a sliced silicon dioxide-based hollow microspheres.

The present invention is further illustrated by the following examples.

example 1

Composition of the streams used: gas phase: air Aqueous phase: 80.0% by weight of an aqueous sodium metasilicate solution (27% by weight)

Sodium metasilicate (soda waterglass) in water)

-19.9% by weight of water

- 0.1 wt .-% Triton ^® CG110 oil phase: - 99.0 wt .-% Cobersol ^® H 120

- 1, 0 wt .-% Span 80

In a microfluidic application, as shown in Figure 4, has a laminar air stream (GAS; 2.5 ml / h; cross-sectional diameter of the microchannel 25 μηη), which (by using a thermal mass flow controller Bronkhorst ^® Digital controller E7100 and Bronkhorst ^® gas flow measurement controller F211) was adjusted (with a stream of an aqueous phase W; 2.5 ml / h; cross-sectional diameter of the microchannels 50 μηη) together in a first dispersion zone. This formed a stream of dispersed gas bubbles in continuous aqueous phase (G / W emulsion, cross-sectional diameter of the microchannel: 100 μηη). The stream of dispersed gas bubbles in continuous aqueous phase was introduced directly into a stream of an oil phase (O, 7.5 ml / h, cross-sectional diameter of the microchannels: 100 μηη), resulting in a stream of individual, one gas bubble included emulsified droplets of aqueous phase in continuous oil phase (G / W / O emulsion, cross-sectional diameter of the microchannel: 100 μηη), which was then passed into HCI-acidic isopropanol (6 M HCl in isopropanol). In the process, HCI-acidic isoporopanol dissolves in the oil phase. At the phase boundary between oil phase and aqueous phase, this triggers a polycondensation reaction with the participation of sodium metasilicate, which leads to the formation of a spherical wall. The resulting silica-based hollow microspheres were washed several times with isopropanol for analysis to remove the oil derived from the oil phase. Subsequently, the silica-based hollow microspheres were dried. FIG. 5 shows a scanning electron micrograph (Phenom Pro X, standard sample holder with aluminum carrier and carbon contact pad, resolution about 17 nm, acceleration voltage 5, 10 or 15 kV, CeB6 electron source (thermionic), coupling with energy-dispersive spectrometer (EDS)) of the hollow spheres , In addition to smaller amounts of silica-based microparticles, the resulting silicon dioxide-based hollow microspheres can be seen. To prove that the balls are actually hollow, parts of the balls were destroyed by applying pressure to the slides.

Example 2 Composition of the streams used: gas phase: air

Aqueous phase: 80.0% by weight of an aqueous sodium metasilicate solution (27% by weight)

Sodium metasilicate (soda waterglass) in water)

-19.9% by weight of water

- 0.1 wt .-% Triton ^® CG110

Oil phase: - 99.0 wt .-% Cobersol ^® H120

0.1% by weight of Span 80

In a microfluidic application, as shown in Figure 6, has a laminar air stream (GAS; 2.5 ml / h; cross-sectional diameter of the microchannel 25 μηη) having a (by using a thermal mass flow controller Bronkhorst ^® Digital Controller E7100 and Bronkhorst ^® gas flow measurement controller F211) guaranteed constant mass flow showed, (with a stream of an aqueous phase W; 2.5 ml / h; cross-sectional diameter of the microchannels 50 μηη) in a first dispersion zone merged, resulting in a stream of dispersed Gas bubbles in aqueous continuous phase (cross-sectional diameter of the microchannel: 100 μm). The flow of dispersed gas bubbles in continuous aqueous phase was introduced in a second dispersion zone into a stream of an oil phase (O, 7.5 ml / h, cross-sectional diameter of the microchannels: 100 μηη), whereby a nearly perfect stream of emulsified, respectively a droplet of aqueous phase enclosing a gas bubble in continuous oil phase (cross-sectional diameter of the microchannel: 100 μm) was formed (see FIG. 7). The thus generated stream was then introduced into a continuous stream of HCI-acidic isopropanol (W / O; 6 M HCl in isopropanol, 20 ml / hr, cross-sectional diameter of the microchannel: 200 μηη). In this case, HCl acid isoporopanol dissolves in the oil phase and at the O / W phase boundary a polycondensation reaction involving sodium metasilicate was induced, which leads to the formation of a spherical wall. The resulting silica-based hollow microspheres were washed several times for analysis to remove the oil with isopropanol. Subsequently, the silica-based hollow microspheres were dried. FIG. 8 shows a scanning electron micrograph of the silicon dioxide-based hollow microspheres obtained thereby. It can also be seen that virtually no silicon dioxide-based microparticles are formed as by-products. FIG. 9 shows that the microspheres actually obtained are hollow spheres.

Claims

claims

1. A process for producing inorganic hollow microspheres, comprising the steps of:

(a) providing a gas stream (G) in a microchannel;

(d) combining gas stream (G) and aqueous phase stream (W) in a first dispersion zone whereby the gas stream (G) breaks down and a stream of dispersed gas bubbles is formed in a continuous aqueous phase (G / W emulsion) becomes;

(e) combining the stream of dispersed gas bubbles in the continuous aqueous phase (G / W emulsion) and the oil phase (O) stream in a second dispersion zone, whereby the stream of dispersed gas bubbles in the continuous aqueous phase (G / W emulsion) is pinched off such that a stream of dispersed droplets of the aqueous phase, each enclosing a gas bubble, is formed in a continuous oil phase (G / W / O emulsion);

(F) activation of at least part of the at least one inorganic precursor compound, whereby in the dispersed droplets of the aqueous phase (W) a polyaddition and / or polycondensation reaction is triggered and with participation of the at least one inorganic precursor compound from the dispersed, in each case a gas bubble enclosing aqueous phase droplets are formed the inorganic hollow microspheres; and

(g) Isolation of inorganic hollow microspheres.

2. The method according to claim 1, characterized in that the activation of at least part of the at least one inorganic precursor compound by lowering the pH of the oil phase (O) by at least 1.5 pH units is carried out.

3. The method according to any one of claims 1 or 2, characterized in that the lowering of the pH by introducing the stream of emulsified, each enclosing a gas bubble droplets of the aqueous phase in the continuous oil phase (G / W / O- Emulsion) into another oil phase (W / O) containing a non-polar solvent and a protic acid.

4. The method according to any one of claims 1 or 2, characterized in that the lowering of the pH by adding a further oil phase (W / O), containing a non-polar solvent and a protic acid, dispersed to the stream, in each case a gas bubble enclosing drops of aqueous phase in continuous oil phase (G / W / O emulsion) takes place.

5. The method according to any one of claims 1 to 4, characterized in that the inorganic precursor compound is selected from the group consisting of sodium silicate, potassium silicate, lithium water glass, ammonium water glass, potassium methyl waterglass and mixtures thereof.

6. The method according to claim 1, characterized in that the activation of at least a portion of the at least one inorganic precursor compound by increasing the pH of the oil phase (O) by at least 1.5 pH units is carried out.

7. The method according to claim 1, characterized in that the activation of at least a portion of the at least one inorganic precursor compound by irradiation of UV light and radical formation of at least a portion of the at least one inorganic precursor compound takes place.

8. The method according to any one of claims 1 to 7, characterized in that the gas Ström (G) one or more gases selected from the group consisting of air, oxygen, nitrogen, carbon dioxide, hydrogen chloride, hydrogen, helium, neon, and Contains argon.

9. The method according to any one of claims 1 to 8, characterized in that the gas Ström (G) has a volume flow, the fluctuations of less than 0.01 ml-h - ¹ has.

10. The method according to any one of claims 1 to 8, characterized in that the stream of an aqueous phase (W) additionally contains one or more nonionic surfactants.

11. The method according to any one of claims 1 to 10, characterized in that the stream of an aqueous phase (W) having a viscosity of 0.8 to 25 mPa ^«s.

12. The method according to any one of claims 1 to 11, characterized in that the ratio of the volume flows between the gas stream (G) and stream of an aqueous phase (W) of 10: 1 to 1: 2.

13. The method according to any one of claims 1 to 12, characterized in that the at least one nonionic surfactant in the oil phase (O) has an HLB value between 1, 5 and 8.0.

14. The method according to any one of claims 1 to 13, characterized in that the at least one nonionic surfactant in the oil phase (O) is selected from the group consisting of fatty alcohols, sterols, sorbitan fatty acid esters and polyalkylene ethers.

15. The method according to any one of claims 1 to 14, characterized in that the amount of non-ionic surfactant in the oil phase (O) is between 0.02 wt .-% and the critical micelle concentration.