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Silica encapsulation of liquid PDMS droplets
Colloids and Surfaces A: Physicochemical and Engineering Aspects 142 (1998) 281–285
Silica encapsulation of liquid PDMS droplets M.I. Goller, B. Vincent * School of Chemistry, University of Bristol, Bristol, BS8 1TS, UK Received 28 November 1997; accepted 21 January 1998
Abstract Core-shell particles have been prepared by the precipitation of a silica layer around cross-linked polydimethylsiloxane (PDMS) microgel core particles. The PDMS microgel particles were first encapsulated in a thin layer of silica by precipitation from a supersaturated sodium silicate solution over the pH range 9–10.5. Subsequently, controlled growth of this shell was achieved by the drop-wise addition of tetraethoxysilane, at lower ethanol concentrations (30 vol.% in water) and higher ammonia concentrations (7.6 M ) than are normally used in ‘‘Sto¨ber-type’’ silica particle synthesis. Scanning electron microscopy showed that the PDMS microgel particles were typically about 550 nm in diameter, with silica shells of thickness about 120 nm. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Core/shell; Encapsulation; Microgels; Polydimethylsiloxane Droplets; Silica Particles
1. Introduction The microencapsulation of droplets or particles with a solid shell leads to the formation of coreshell particles. Microencapsulation provides protection and controlled release of core materials. Such particles have, therefore, found a diverse range of applications. Microcapsules are utilised in the pharmaceutical industry for the controlled release and targeting of drugs [1–3]. In the food and detergent industries, microcapsules protect sensitive active agents such as vitamins and enzymes. In controlled release systems, the shell is made selectively and controllably permeable by using a suitable trigger (pH, ionic strength or temperature), which leads to swelling of the shell layer. Alternatively, the capsules can deliver an active component via a pressure release mechanism, * Corresponding author. Tel: +44 117 928 8160; Fax: +44 117 925 0612; e-mail: [email protected]
where the shell is ruptured to release contents such as perfumes and inks. The release kinetics in this case are controlled by the mechanical strength of the shell wall. The formation of a strong shell and subsequent evacuation of the core leads to the production of hollow particles of particular use as fillers in the paint industry [4] and as pigment supports in cosmetics. Core-shell particles have in general been prepared by interfacial polymerisation [5,6 ], but also by emulsion/suspension formation followed by solvent extraction [7,8]. More recently, microcapsules have been prepared by phase separation: multilayered microspheres may form spontaneously when polymer phase separation is induced in the droplets of emulsions containing mixtures of polymers and solvents [9]. Some of the main problems encountered to date in core-shell particle manufacture have been the production of monodisperse, liquid cores, complete and even shell coverage, and the subsequent characterisation of the shell thickness and mor-
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phology. In this work, the first problem was surmounted by the use of polydimethylsiloxane (PDMS) droplets as the liquid polymer cores. Monodisperse PDMS oil-in-water emulsions have been previously produced by this group, utilising a nucleation and growth mechanism, with silane monomers and ammonia as a catalyst in aqueous solution [10,11]. The resulting emulsions are surfactant-free. Goller et al. [12] showed that PDMS ‘‘particles’’ prepared with various amounts of cross-linker (0–90 vol.% methyltriethoxysilane) are also highly monodisperse. As the concentration of cross-linker is increased, the droplets become increasingly microgel-like in character. The same authors [12] showed that solvent uptake by the microgel particles leads to particle swelling, with only a small increase in particle polydispersity. In this paper, we present a route to emulsion/ microgel encapsulation using the concept of sequential polymerisation. PDMS liquid droplets or microgel particles are prepared, and then a cross-linked silica shell is formed, via the interfacial polymerisation of the tetrafunctional silane monomer, tetraethoxysilane.
2. Experimental 2.1. Materials Dimethyldiethoxysilane (DMDES ), methyltriethoxysilane (MTES ) and tetraethoxysilane ( TEOS) were acquired from Aldrich Chemical Company Ltd (Gillingham, UK ) and distilled under vacuum prior to use. Ammonia solution (35 wt% NH ), hydrochloric acid (38 wt% HCl ), 3 nitric acid (69 wt% HNO ) and ethanol were sup3 plied by BDH Chemicals Ltd (Poole, UK ). Ethanol was distilled prior to use. Distilled water was deionised (conductivity<5.5×10−6 V−1 m−1), using a Milli-Q filtration system (Millipore Ltd, Watford, UK ). Sodium silicate solution and Dowex 50WX4-400 acid ionexchange resin were purchased from Aldrich Chemical Company Ltd (Gillingham, UK ). The resin was regenerated by consecutive rinsing with hot water, 3 N HCl and distilled water. All the glassware comprising the reaction vessels
(flasks, tubing, stirrers, etc.) was thoroughly cleaned before use, to reduce silica particle formation via secondary nucleation to a minimum. This was achieved by soaking in 4 wt% NaOH solution for 12 h, then 14 wt% HNO solution for 10 min, 3 and finally rinsing with hot tap water and Milli-Q water. 2.2. Emulsion preparation The preparation of monodisperse, surfactantfree PDMS droplets has been discussed in previous papers [10,12]. By varying the monomer concentration ratio MTES:DMDES, the size, viscosity and degree of cross-linking can be tailored to aid the formation of silica shells and modify the end use of the final core-shell particles. In this paper, droplets/microgel particles were prepared using monomer mixtures of MTES:DMDES containing 0–60 vol.% MTES. An aqueous solution containing 1 vol.% NH solution and a total of 1 vol.% 3 monomer were vigorously shaken for 30 s, then left to stand for 18 h. The samples were then dialysed (against water) for 4 days to ensure that excess ammonia and monomer were removed. Average droplet/particle diameters were determined from photon correlation spectroscopy (PCS) using a Brookhaven ZetaPlus (Brookhaven Instruments Corporation, New York) with a helium–neon laser (wavelength=632.8 nm). 2.3. Droplet encapsulation
2.3.1. Silica seeding in ethanol This process is analogous to the growth of large silica particles from seed silica [13]. This method essentially involves the addition of TEOS to dispersions of the PDMS droplets/particles, as seeds. Various concentrations of ethanol, ammonia and dialysed PDMS emulsion/suspension were mixed together (in that order) at 25°C. This was followed by the drop-wise addition of 2–6 vol.% aliquots of TEOS, at a rate of 2 ml h−1. Subsequently, the mixture was stirred for a further 12 h to allow the reaction to proceed to completion, and then the resulting dispersions were dialysed (against distilled water) for 4 days.
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2.3.2. Silica precipitation in water In this case, the PDMS droplets/particles were encapsulated in a thin coating of silica by deposition from a supersaturated sodium silicate solution within a controlled pH range [14]. The experimental procedure used was based on the work of Philipse et al. [15]. An aqueous sodium silicate solution (3 wt%, pH 12) was prepared. The sodium ion concentration was subsequently lowered by the addition of ion exchange resin. At pH 11, the resin was removed by filtration. Care was taken to prevent the pH falling below pH 11, to reduce the possibility of secondary silica particle nucleation occurring. The dialysed PDMS emulsion/suspension (230 ml, 0.1 vol.%—assuming 100% conversion) was vigorously stirred in a three-necked flask. The sodium silicate solution (100 ml ) was then pumped under the meniscus, using a peristaltic pump, at a rate of 1 ml min−1. After 15 min, silicate addition was continued at a rate of 0.5 ml min−1. The pH was controlled within the range 9–10.5 by the addition of ion-exchange resin [15]. The suspension was left for 12 h, decanted (to remove the ion-exchange resin) and dialysed (against distilled water) for 7 days.
2.4. Core-shell particle analysis Core-shell morphology and average particle diameters were assessed using a Hitachi S-2300 scanning electron microscope (Hitachi, Tokyo). In addition, average particle diameters were determined using PCS.
3. Results and discussion It was found that in order to successfully produce PDMS core/silica shell particles, a combination of the methods described in Sections 2.3.1 and 2.3.2 was necessary. In order to understand the reasons for this, we first describe the problems associated with using either method alone.
3.1. Silica seeding method Bogush et al. [13] have described a method for producing larger silica particles from silica seeds. Essentially, the reaction medium for both the seed and the subsequent growth stages is ethanol, to which water and ammonia are added as catalysts. However, the PDMS droplets, which are formed in water, are soluble in ethanol. In fact, the maximum concentration of ethanol in water for which the droplets retain their identity is around 40%, for the uncross-linked PDMS droplets (0% MTES ); this increases to around 60% for PDMS microgel particles containing 50% MTES. Therefore, the question then becomes: what is the maximum concentration of ethanol in water that the silica growth (i.e. the shell formation) stage can in fact be carried out at? Bridger et al. [16 ] have prepared small (~60 nm diameter) silica particles at (maximum) water concentrations in ethanol in the range of 25–30 M; this translates to minimum ethanol concentrations in water of ~45%. We have shown, in these studies, that if the ammonia concentration is increased, then stable silica particles may be produced at ethanol concentrations in water as low as 25%. Table 1 shows some typical particle sizes for silica particles produced at different ethanol and ammonia concentrations. In principle, therefore, an ethanol in water concentration window does exist (i.e. from ~25% to ~40–60% ethanol, depending on the amount of cross-linking MTES present) for carrying out the growth of silica shells around PDMS droplets and microgel particles. However, a second problem Table 1 Hydrodynamic diameter (D) of dialysed silica particles (0.015 M TEOS ) prepared in simultaneously decreasing ethanol concentrations [EtOH ] and increasing ammonia concentrations [EtOH ] vol.%
[NH ] M 3
D nm−1
70 40 35 30 25 20
3 6 6.5 7 7.5 8
294 270 237 245 132 —
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then arises: solubilisation of the TEOS monomer, used in the shell-formation stage, within the core droplets/microgel particles! This severely reduces the concentration of TEOS in the aqueous phase, available for silica formation and subsequent precipitation on to the PDMS cores, and, of course, one has the problem of TEOS as an impurity within the cores. This partitioning of the TEOS into the droplets from the water phase is reduced, the higher the degree of cross-linking of the PDMS droplets (i.e. the higher the ratio of MTES to DMDES used in the core stage polymerisation). Nevertheless, it was this solubilsation of the TEOS problem that led to the concept of laying down an impermeable, ‘‘protective skin’’ around the droplets/microgel particle cores, prior to growth of the silica shells, in order to minimise this solubilisation problem. We shall return to this point in the next section. 3.2. Silica precipitation method The precipitation of silica on to the droplets/ particles in an aqueous solution of sodium silicate led to the formation of thin layers, typically a few nanometres in thickness, but the dispersions of these core/shell particles in water were colloidally stable, suggesting that, as with the original core particles themselves, the core/shell particles are electrostatically stabilised, through the surface charge groups (i.e. ionised silanol groups). SEM was used in an attempt to study the structure of these core-shell particles, but the shells proved to be too thin to withstand the high vacuum in the SEM sample chamber. In order to grow thicker shells in a controlled way it was found that a combination of the formation of a protective silica ‘‘skin’’ around the droplets, followed by further silica growth using the modified Bogush procedure, as described in the previous section, was the most effective route. This led to the production of core-shell particles with shells of sufficient thickness to be viewed successfully in the SEM. Fig. 1 is a typical photomicrograph of one of these core-shell systems. In this case, the cores are PDMS microgel particles, with an average diameter of 550 nm and a polydispersity of 7%, prepared using 60 vol.% MTES.
Fig. 1. Scanning electron micrograph of core-shell particles (some deliberately broken by applied pressure). PDMS core (1 vol.% monomer, 60:40 methyltriethoxysilane: dimethyldiethoxysiloxane, 1 vol.% NH solution), and silica shell. 3
After the initial ‘‘skin’’ step, described above, further development of the shell was achieved in a 30 vol.% ethanol solution (27 M H O) containing 2 the particles at a concentration of c. 0.02 vol.%, and the following additives: 0.13 M TEOS and 7.6 M NH .. 3 Fig. 1 shows a mixture of broken core-shell particles, unbroken core-shell particles, and also some very much smaller silica particles. The broken particles appear hollow inside due to the evaporation of the PDMS under the vacuum in the SEM. The silica particles are by-products formed by homogeneous nucleation. The size of the core-shell particles is c. 900 nm. The shell appears to have a thickness of ~200 nm, which is consistent with the original cores having a diameter of ~550 nm (see above). This value for the shell thickness is lower than would be expected if a 100% conversion of all the TEOS monomer present had ended up in the silica shells. Some, of course, goes to forming the ‘‘free’’ silica particles (880 nm). The production of the core-shell particles shown in Fig. 1 was achieved by the use of cross-linked (60 vol.% MTES ) microgel particles as the core. Attempts to formulate core-shell particles with uncross-linked PDMS liquid droplet cores (i.e. containing no MTES ) were, unfortunately, not successful. It appears that the PDMS droplets cannot survive the shell formation process. Even though they are not totally dissolved in 30 vol.%
M.I. Goller, B. Vincent / Colloids Surfaces A: Physicochem. Eng. Aspects 142 (1998) 281–285
ethanol, we have shown (using PCS) that about a 10% decrease in PDMS droplet diameter occurs on increasing the ethanol concentration from 0 to 30%; this is probably sufficient to disrupt the shellformation process.
4. Conclusions Core-shell particles have been produced with a cross-linked PDMS core and a solid silica shell. The synthesis of these particles has been aided by the production of silica particles at low ethanol concentrations (30 vol.%).
Acknowledgment This work was supported by the award of an EPSRC grant.
References [1] J.P. McGee, S.S. Davis, D.T. O’Hagan, J. Control. Release 34 (1995) 77–86. [2] D.D. Lewis, in: M. Chasin, R. Langer ( Eds.),
285
Biodegradable Polymers and Drug Delivery Systems, Marcel Decker, New York, 1990, pp. 1–4. [3] J. Filipovic-Grcic, D. Maysinger, I. Jalsenjak, J. Microencapsulation 12 (1995) 343–362. [4] Y. Nakahara, Y. Tanaka, Y. Ehara, F. Nakahara, Shikizai 61 (1988) 488. [5] A.R. Bachtsi, C.J. Boutris, C. Kiparissides, J. Appl. Polym. Sci. 60 (1996) 9–20. [6 ] B. Miksa, S. Slomkowski, Colloid Polym. Sci. 273 (1995) 47. [7] M. Okubo, T. Nakagawa, Colloid Polym. Sci. 272 (1994) 530. [8] G. Crotts, T.G. Park, J. Control. Release 35 (1995) 91–105. [9] E. Mathiowitz, J.S. Jacob, Y.S. Jong, G.P. Carino, D.E. Chickering, P. Chaturvedi, C.A. Santos, K. Vijayaraghavan, S. Montgomery, M. Bassett, C. Morrell, Nature 386 (1997) 410. [10] T.M. Obey, B. Vincent, J. Colloid Interface Sci. 163 (1994) 454–463. [11] K.R. Anderson, T.M. Obey, B. Vincent, Langmuir 10 (1994) 2493–2494. [12] M.I. Goller, T.M. Obey, D.O.H. Teare, B. Vincent, M.R. Wegener, Colloids Surf. A 123–124 (1997) 183–193. [13] G.H. Bogush, M.A. Tracy, C.F. Zukoski IV, J. Non-Cryst. Solids 104 (1988) 95–106. [14] R.K. Iller, The Chemistry of Silica, Wiley, New York, 1979. [15] A.P. Philipse, A. Nechifor, C. Patmamanoharan, Langmuir 10 (1994) 4451–4458. [16 ] K. Bridger, D. Fairhurst, B. Vincent, J. Colloid Interface Sci. 68 (1979) 190.
Silica encapsulation of liquid PDMS droplets M.I. Goller, B. Vincent * School of Chemistry, University of Bristol, Bristol, BS8 1TS, UK Received 28 November 1997; accepted 21 January 1998
Abstract Core-shell particles have been prepared by the precipitation of a silica layer around cross-linked polydimethylsiloxane (PDMS) microgel core particles. The PDMS microgel particles were first encapsulated in a thin layer of silica by precipitation from a supersaturated sodium silicate solution over the pH range 9–10.5. Subsequently, controlled growth of this shell was achieved by the drop-wise addition of tetraethoxysilane, at lower ethanol concentrations (30 vol.% in water) and higher ammonia concentrations (7.6 M ) than are normally used in ‘‘Sto¨ber-type’’ silica particle synthesis. Scanning electron microscopy showed that the PDMS microgel particles were typically about 550 nm in diameter, with silica shells of thickness about 120 nm. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Core/shell; Encapsulation; Microgels; Polydimethylsiloxane Droplets; Silica Particles
1. Introduction The microencapsulation of droplets or particles with a solid shell leads to the formation of coreshell particles. Microencapsulation provides protection and controlled release of core materials. Such particles have, therefore, found a diverse range of applications. Microcapsules are utilised in the pharmaceutical industry for the controlled release and targeting of drugs [1–3]. In the food and detergent industries, microcapsules protect sensitive active agents such as vitamins and enzymes. In controlled release systems, the shell is made selectively and controllably permeable by using a suitable trigger (pH, ionic strength or temperature), which leads to swelling of the shell layer. Alternatively, the capsules can deliver an active component via a pressure release mechanism, * Corresponding author. Tel: +44 117 928 8160; Fax: +44 117 925 0612; e-mail: [email protected]
where the shell is ruptured to release contents such as perfumes and inks. The release kinetics in this case are controlled by the mechanical strength of the shell wall. The formation of a strong shell and subsequent evacuation of the core leads to the production of hollow particles of particular use as fillers in the paint industry [4] and as pigment supports in cosmetics. Core-shell particles have in general been prepared by interfacial polymerisation [5,6 ], but also by emulsion/suspension formation followed by solvent extraction [7,8]. More recently, microcapsules have been prepared by phase separation: multilayered microspheres may form spontaneously when polymer phase separation is induced in the droplets of emulsions containing mixtures of polymers and solvents [9]. Some of the main problems encountered to date in core-shell particle manufacture have been the production of monodisperse, liquid cores, complete and even shell coverage, and the subsequent characterisation of the shell thickness and mor-
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phology. In this work, the first problem was surmounted by the use of polydimethylsiloxane (PDMS) droplets as the liquid polymer cores. Monodisperse PDMS oil-in-water emulsions have been previously produced by this group, utilising a nucleation and growth mechanism, with silane monomers and ammonia as a catalyst in aqueous solution [10,11]. The resulting emulsions are surfactant-free. Goller et al. [12] showed that PDMS ‘‘particles’’ prepared with various amounts of cross-linker (0–90 vol.% methyltriethoxysilane) are also highly monodisperse. As the concentration of cross-linker is increased, the droplets become increasingly microgel-like in character. The same authors [12] showed that solvent uptake by the microgel particles leads to particle swelling, with only a small increase in particle polydispersity. In this paper, we present a route to emulsion/ microgel encapsulation using the concept of sequential polymerisation. PDMS liquid droplets or microgel particles are prepared, and then a cross-linked silica shell is formed, via the interfacial polymerisation of the tetrafunctional silane monomer, tetraethoxysilane.
2. Experimental 2.1. Materials Dimethyldiethoxysilane (DMDES ), methyltriethoxysilane (MTES ) and tetraethoxysilane ( TEOS) were acquired from Aldrich Chemical Company Ltd (Gillingham, UK ) and distilled under vacuum prior to use. Ammonia solution (35 wt% NH ), hydrochloric acid (38 wt% HCl ), 3 nitric acid (69 wt% HNO ) and ethanol were sup3 plied by BDH Chemicals Ltd (Poole, UK ). Ethanol was distilled prior to use. Distilled water was deionised (conductivity<5.5×10−6 V−1 m−1), using a Milli-Q filtration system (Millipore Ltd, Watford, UK ). Sodium silicate solution and Dowex 50WX4-400 acid ionexchange resin were purchased from Aldrich Chemical Company Ltd (Gillingham, UK ). The resin was regenerated by consecutive rinsing with hot water, 3 N HCl and distilled water. All the glassware comprising the reaction vessels
(flasks, tubing, stirrers, etc.) was thoroughly cleaned before use, to reduce silica particle formation via secondary nucleation to a minimum. This was achieved by soaking in 4 wt% NaOH solution for 12 h, then 14 wt% HNO solution for 10 min, 3 and finally rinsing with hot tap water and Milli-Q water. 2.2. Emulsion preparation The preparation of monodisperse, surfactantfree PDMS droplets has been discussed in previous papers [10,12]. By varying the monomer concentration ratio MTES:DMDES, the size, viscosity and degree of cross-linking can be tailored to aid the formation of silica shells and modify the end use of the final core-shell particles. In this paper, droplets/microgel particles were prepared using monomer mixtures of MTES:DMDES containing 0–60 vol.% MTES. An aqueous solution containing 1 vol.% NH solution and a total of 1 vol.% 3 monomer were vigorously shaken for 30 s, then left to stand for 18 h. The samples were then dialysed (against water) for 4 days to ensure that excess ammonia and monomer were removed. Average droplet/particle diameters were determined from photon correlation spectroscopy (PCS) using a Brookhaven ZetaPlus (Brookhaven Instruments Corporation, New York) with a helium–neon laser (wavelength=632.8 nm). 2.3. Droplet encapsulation
2.3.1. Silica seeding in ethanol This process is analogous to the growth of large silica particles from seed silica [13]. This method essentially involves the addition of TEOS to dispersions of the PDMS droplets/particles, as seeds. Various concentrations of ethanol, ammonia and dialysed PDMS emulsion/suspension were mixed together (in that order) at 25°C. This was followed by the drop-wise addition of 2–6 vol.% aliquots of TEOS, at a rate of 2 ml h−1. Subsequently, the mixture was stirred for a further 12 h to allow the reaction to proceed to completion, and then the resulting dispersions were dialysed (against distilled water) for 4 days.
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M.I. Goller, B. Vincent / Colloids Surfaces A: Physicochem. Eng. Aspects 142 (1998) 281–285
2.3.2. Silica precipitation in water In this case, the PDMS droplets/particles were encapsulated in a thin coating of silica by deposition from a supersaturated sodium silicate solution within a controlled pH range [14]. The experimental procedure used was based on the work of Philipse et al. [15]. An aqueous sodium silicate solution (3 wt%, pH 12) was prepared. The sodium ion concentration was subsequently lowered by the addition of ion exchange resin. At pH 11, the resin was removed by filtration. Care was taken to prevent the pH falling below pH 11, to reduce the possibility of secondary silica particle nucleation occurring. The dialysed PDMS emulsion/suspension (230 ml, 0.1 vol.%—assuming 100% conversion) was vigorously stirred in a three-necked flask. The sodium silicate solution (100 ml ) was then pumped under the meniscus, using a peristaltic pump, at a rate of 1 ml min−1. After 15 min, silicate addition was continued at a rate of 0.5 ml min−1. The pH was controlled within the range 9–10.5 by the addition of ion-exchange resin [15]. The suspension was left for 12 h, decanted (to remove the ion-exchange resin) and dialysed (against distilled water) for 7 days.
2.4. Core-shell particle analysis Core-shell morphology and average particle diameters were assessed using a Hitachi S-2300 scanning electron microscope (Hitachi, Tokyo). In addition, average particle diameters were determined using PCS.
3. Results and discussion It was found that in order to successfully produce PDMS core/silica shell particles, a combination of the methods described in Sections 2.3.1 and 2.3.2 was necessary. In order to understand the reasons for this, we first describe the problems associated with using either method alone.
3.1. Silica seeding method Bogush et al. [13] have described a method for producing larger silica particles from silica seeds. Essentially, the reaction medium for both the seed and the subsequent growth stages is ethanol, to which water and ammonia are added as catalysts. However, the PDMS droplets, which are formed in water, are soluble in ethanol. In fact, the maximum concentration of ethanol in water for which the droplets retain their identity is around 40%, for the uncross-linked PDMS droplets (0% MTES ); this increases to around 60% for PDMS microgel particles containing 50% MTES. Therefore, the question then becomes: what is the maximum concentration of ethanol in water that the silica growth (i.e. the shell formation) stage can in fact be carried out at? Bridger et al. [16 ] have prepared small (~60 nm diameter) silica particles at (maximum) water concentrations in ethanol in the range of 25–30 M; this translates to minimum ethanol concentrations in water of ~45%. We have shown, in these studies, that if the ammonia concentration is increased, then stable silica particles may be produced at ethanol concentrations in water as low as 25%. Table 1 shows some typical particle sizes for silica particles produced at different ethanol and ammonia concentrations. In principle, therefore, an ethanol in water concentration window does exist (i.e. from ~25% to ~40–60% ethanol, depending on the amount of cross-linking MTES present) for carrying out the growth of silica shells around PDMS droplets and microgel particles. However, a second problem Table 1 Hydrodynamic diameter (D) of dialysed silica particles (0.015 M TEOS ) prepared in simultaneously decreasing ethanol concentrations [EtOH ] and increasing ammonia concentrations [EtOH ] vol.%
[NH ] M 3
D nm−1
70 40 35 30 25 20
3 6 6.5 7 7.5 8
294 270 237 245 132 —
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then arises: solubilisation of the TEOS monomer, used in the shell-formation stage, within the core droplets/microgel particles! This severely reduces the concentration of TEOS in the aqueous phase, available for silica formation and subsequent precipitation on to the PDMS cores, and, of course, one has the problem of TEOS as an impurity within the cores. This partitioning of the TEOS into the droplets from the water phase is reduced, the higher the degree of cross-linking of the PDMS droplets (i.e. the higher the ratio of MTES to DMDES used in the core stage polymerisation). Nevertheless, it was this solubilsation of the TEOS problem that led to the concept of laying down an impermeable, ‘‘protective skin’’ around the droplets/microgel particle cores, prior to growth of the silica shells, in order to minimise this solubilisation problem. We shall return to this point in the next section. 3.2. Silica precipitation method The precipitation of silica on to the droplets/ particles in an aqueous solution of sodium silicate led to the formation of thin layers, typically a few nanometres in thickness, but the dispersions of these core/shell particles in water were colloidally stable, suggesting that, as with the original core particles themselves, the core/shell particles are electrostatically stabilised, through the surface charge groups (i.e. ionised silanol groups). SEM was used in an attempt to study the structure of these core-shell particles, but the shells proved to be too thin to withstand the high vacuum in the SEM sample chamber. In order to grow thicker shells in a controlled way it was found that a combination of the formation of a protective silica ‘‘skin’’ around the droplets, followed by further silica growth using the modified Bogush procedure, as described in the previous section, was the most effective route. This led to the production of core-shell particles with shells of sufficient thickness to be viewed successfully in the SEM. Fig. 1 is a typical photomicrograph of one of these core-shell systems. In this case, the cores are PDMS microgel particles, with an average diameter of 550 nm and a polydispersity of 7%, prepared using 60 vol.% MTES.
Fig. 1. Scanning electron micrograph of core-shell particles (some deliberately broken by applied pressure). PDMS core (1 vol.% monomer, 60:40 methyltriethoxysilane: dimethyldiethoxysiloxane, 1 vol.% NH solution), and silica shell. 3
After the initial ‘‘skin’’ step, described above, further development of the shell was achieved in a 30 vol.% ethanol solution (27 M H O) containing 2 the particles at a concentration of c. 0.02 vol.%, and the following additives: 0.13 M TEOS and 7.6 M NH .. 3 Fig. 1 shows a mixture of broken core-shell particles, unbroken core-shell particles, and also some very much smaller silica particles. The broken particles appear hollow inside due to the evaporation of the PDMS under the vacuum in the SEM. The silica particles are by-products formed by homogeneous nucleation. The size of the core-shell particles is c. 900 nm. The shell appears to have a thickness of ~200 nm, which is consistent with the original cores having a diameter of ~550 nm (see above). This value for the shell thickness is lower than would be expected if a 100% conversion of all the TEOS monomer present had ended up in the silica shells. Some, of course, goes to forming the ‘‘free’’ silica particles (880 nm). The production of the core-shell particles shown in Fig. 1 was achieved by the use of cross-linked (60 vol.% MTES ) microgel particles as the core. Attempts to formulate core-shell particles with uncross-linked PDMS liquid droplet cores (i.e. containing no MTES ) were, unfortunately, not successful. It appears that the PDMS droplets cannot survive the shell formation process. Even though they are not totally dissolved in 30 vol.%
M.I. Goller, B. Vincent / Colloids Surfaces A: Physicochem. Eng. Aspects 142 (1998) 281–285
ethanol, we have shown (using PCS) that about a 10% decrease in PDMS droplet diameter occurs on increasing the ethanol concentration from 0 to 30%; this is probably sufficient to disrupt the shellformation process.
4. Conclusions Core-shell particles have been produced with a cross-linked PDMS core and a solid silica shell. The synthesis of these particles has been aided by the production of silica particles at low ethanol concentrations (30 vol.%).
Acknowledgment This work was supported by the award of an EPSRC grant.
References [1] J.P. McGee, S.S. Davis, D.T. O’Hagan, J. Control. Release 34 (1995) 77–86. [2] D.D. Lewis, in: M. Chasin, R. Langer ( Eds.),
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