Silicone Nanocapsules Templated Inside the ... - ACS Publications

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Langmuir 2007, 23, 7314-7320

Silicone Nanocapsules Templated Inside the Membranes of Catanionic Vesicles Mariusz Ke¸ pczyn´ski,† Joanna Lewandowska,† Marek Romek,‡ Szczepan Zapotoczny,† Franc¸ ois Ganachaud,§ and Maria Nowakowska*,† Faculty of Chemistry, Jagiellonian UniVersity, Ingardena 3, and Department of Cytology and Histology, Institute of Zoology, Jagiellonian UniVersity, Ingardena 6, 30-060 Krako´ w, Poland, and Laboratoire de Chimie Macromole´ culaire, UMR 5076 CNRS/ENSCM, 8 rue de l’Ecole Normale, 34296 Montpellier cedex 5, France ReceiVed NoVember 27, 2006. In Final Form: March 21, 2007 A simple and effective way to synthesize hollow silicone resin particles of controlled diameter is presented. The synthesis utilizes catanionic vesicles as templates for the polycondensation/polymerization processes of 1,3,5,7tetramethylcyclotetrasiloxane (D4H) within their membranes. Two different surfactant systems were used to form the vesicular templates: mixtures of dodecyltrimethylammonium bromide (DTAB) and sodium dodecylbenzenesulfonate (SDBS) in the cationic (the DTAB/SDBS system) or anionic (the SDBS/DTAB system) rich region of the phase diagram. The templates obtained from these surfactant mixtures form spontaneously unilamellar vesicles in aqueous solution. The vesicular templates swell upon addition of D4H, thus increasing their size. The silicone resin was obtained in acid- or base-catalyzed polycondensation and ring-opening polymerization processes of D4H. In the case of the DTAB/SDBS system the formation of a densely cross-linked silicone material with SiO3/2 units allowed the nanocapsules to retain the vesicular shape after removal of the template, whereas in the SDBS/DTAB system, the polymer produces capsules which are too smooth to support surfactant lysis. The morphology of the silicone nanocapsules was analyzed using transmission electron microscopy (TEM) and, in some cases, atomic force microscopy (AFM). TEM and AFM reveal discrete hollow particles with a small amount of linked or aggregated hollow silica shells.

Introduction Nanostructured materials, especially hollow nanoparticles, are potentially useful for encapsulation of various chemical molecules, living cells, or enzymes.1 They can be applied in drug delivery, controlled release, catalysis, and separation technologies. Polymeric nanocapsules are more attractive for these applications than liposomes or surfactant vesicles due to their considerably higher stability.2 An effective and simple approach for preparation of hollow nanocapsules involves template synthesis. Many templates, such as polymeric microspheres,3 water-in-oil emulsion droplets,4 lyotropic phases exhibiting a multilamellar vesicular structure,5 or vesicular solutions,1,6 were proposed for that purpose. The approach based on application of vesicles as templates seems to be very promising. In templating approaches two main strategies have been explored.7 In the first one, templates are used as directing and structuring agents for the synthesis in solution. Essentially, the interface between the template and the solution represents a specific site * To whom correspondence should be addressed. Phone: +48 12 6632250. Fax: +48 12 6340515. E-mail: [email protected]. † Faculty of Chemistry, Jagiellonian University. ‡ Institute of Zoology, Jagiellonian University. § UMR 5076 CNRS/ENSCM. (1) Hentze, H.-P.; Raghavan, S. R.; McKelvey, C. A.; Kaler, E. W. Langmuir 2003, 19, 1069-1074. (2) Arshady, R. In Microspheres, Microcapsules and Liposomes; Arshady, R., Ed.; Citrus Books: London, 1999; Vol. 1, p 279. (3) Ding, X.; Yu, K.; Jiang, Y.; Hari-Bala, Zhang, H.; Wang, Z. Mater. Lett. 2004, 58, 3618-3621. (4) Park, J.-H.; Oh, C.; Shin, S.-I.; Moon, S.-K.; Oh, S.-G. J. Colloid Interface Sci. 2003, 266, 107-114. (5) Kim, S. S.; Zhang, W.; Pinnavaia, T. J. Science 1998, 282, 1303-1305. (6) Hubert, D. H. W.; Jung, M.; Frederik, P. M.; Bomans, P. H. H.; Meuldijk, J.; German, A. L. AdV. Mater. 2000, 12, 1286-1290. (7) Hubert, D. H. W.; Jung, M.; German, A. L. AdV. Mater. 2000, 12, 12911294.

for growth of material from solution. Growth at the interface leads to coating of the template. Further and complete solidification in the solution will lead to a situation where the templates are essentially used as spatial fillers. This approach was successfully applied to obtain hollow silica particles as a result of controlled growth of silica from the precursor solutions (solgel synthesis) at the outer leaflet of the bilayer of the surfactant vesicles.1,6,8 Silicon alkoxides are hydrolyzed, and the resulting silicic acid polymerizes upon condensation to form silica or hybrid inorganic/organic material.9 Various silica precursors, for instance, tetraethyl orthosilicate (TEOS) or 1,2-bis(trimethoxysilyl)ethane (BTME) have been shown to be useful for successful deposition of silica on the vesicles. The second strategy utilizes the templates as reaction media and focuses on the use of the vesicle bilayer as a compartmentalized reaction space for the polymerization of organic (hydrophobic) monomers solubilized in the vesicle membrane. In this method, polymerization occurs inside the template to produce materials with complex forms: the shape and morphology of the original template are imprinted due to a restrictive growth. This process is referred to as polymerization in vesicles. So far, only radical-type polymerizations have been carried out inside the vesicle bilayer. Although some authors reported that they have synthesized the expected nanocapsules,10,11 more often a “parachute” structure (attributed to polymer chains demixing, thus forming a bead trapped in the bilayer) was observed.12 Controllable nanocapsule synthesis required complex chemistry (8) Lootens, D.; Vautrin, C.; Van Damme, H.; Zemb, T. J. Mater. Chem. 2003, 13, 2072-2074. (9) Djojoputro, H.; Zhou, X. F.; Qiao, S. Z.; Wang, L. Z.; Yu, C. Z.; Lu, G. Q. J. Am. Chem. Soc. 2006, 128, 6320-6321. (10) Kurja, J.; Nolte, R. J. M.; Maxwell, I. A.; German, A. L. Polymer 1993, 34, 2045-2049. (11) Hotz, J.; Meier, W. Langmuir 1998, 14, 1031-1036. (12) Jung, M.; Hubert, D. H. W.; Bomans, P. H. H.; Frederik, P. M.; Meuldijk, J.; van Herk, A. M.; Fischer, H.; German, A. L. Langmuir 1997, 13, 6877-6880.

10.1021/la063442i CCC: $37.00 © 2007 American Chemical Society Published on Web 05/24/2007

Nanocapsules Templated Inside Vesicle Membranes

to prepare original surfactants bearing either a polymerizable group13 or an alkyl/fluorinated tail.14 Recently, Li and co-workers reported a successful method for preparation of hollow polymer spheres of styrene via emulsion polymerization in anionic/ nonionic mixed surfactant templates.15 In our previous paper we have proposed a simple and effective way to obtain the hybrid inorganic/organic nanocapsules by cross-linking/polymerization processes of the functional cyclosiloxane trapped inside the vesicle membranes.16 In the present study, we demonstrate the construction of nanocapsules consisting of densely cross-linked hybrid inorganic/ organic material within the bilayer of surfactant vesicles. The synthesis utilized the catanionic vesicles formed in the dodecyltrimethylammonium bromide (DTAB) and sodium dodecylbenzenesulfonate (SDBS) surfactant systems in the cationic- and anionic-rich regions of the phase diagram. We have characterized the vesicles swollen with 1,3,5,7-tetramethylcyclotetrasiloxane, a monomer which we used in the synthesis, using dynamic light scattering (DLS), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The location of the monomer in the catanionic membrane was discussed on the basis of the interfacial tension of the 1,3,5,7-tetramethylcyclotetrasiloxane (D4H)/water system. Next, we focused on the conditions for the preparations of hollow silicone particles. The structures of the resulting silicone resin were examined with DLS, TEM, and the surfactant lysis technique and in some cases with AFM and size exclusion chromatography (SEC). Experimental Section Materials. SDBS (Aldrich), DTAB (Acros), and Triton-X100 (Aldrich) were used as received. D4H was purchased from ABCR, and its purity was checked by gas chromatography prior to use. Silicon(100) polished wafers (Cemat Silicon S.A., Poland) were cleaned using piranha solution (3:1, v/v, H2SO4/H2O2). Caution: Piranha solution is a Vigorous oxidant and should be used with extreme care! Vesicle Preparation. Vesicular solutions were prepared from surfactant stock solutions in doubly distilled water. Samples were obtained by mixing of the appropriate amounts of the stock solutions followed by sonication with an Ultrasonic Corp. Branson model 450 sonicator for 2 min (power 7) in an ice bath and filtration through a 0.45 µm syringe filter (Millipore). The obtained vesicle dispersion was aged by being stirred for one week at room temperature. For the DTAB/SDBS system the concentrations of DTAB and SDBS were 0.02 and 0.008 mol/L, respectively. For the SDBS/DTAB system the concentrations of SDBS and DTAB were 0.02 and 0.008 mol/L, respectively. Cross-Linking Reaction and Polymerization. D4H was added to the vesicle dispersion. The solution was then stirred for 2 days to solubilize the monomer. Centrifugation at 20 000 rpm for 30 min (J2-MC Centrifuge, Beckman) was applied to remove droplets of the monomer. The polymerization/cross-linking process was started by adjusting the pH of the solution to 3.0 with 0.1 M HCl in the case of SDBS/DTAB system or to 8.2 with 0.1 M NaOH in the case of DTAB/SDBS vesicles. The samples were kept in a water bath thermostated at 25 °C without further mechanical agitation. For characterization of the products, the surfactants were removed by precipitation of the polymeric material in methanol. The precipitant was washed with a mixture of methanol/water (3:1, v/v) followed by centrifugation. The procedure was repeated five times. Finally, (13) Jung, M.; den Ouden, I.; Montoya-Gon˜i, A.; Hubert, D. H. W.; Frederik, P. M.; van Herk, A. M.; German, A. L. Langmuir 2000, 16, 4185-4195. (14) Krafft, M. P.; Schieldknecht, L.; Marie, P.; Giuleri, F.; Schmutz, M.; Poulain, N.; Nakache, E. Langmuir 2001, 17, 2872-2877. (15) Song, L.; Ge, X.; Wang, M.; Zhang, Z.; Li, S. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 2533-2541. (16) Ke¸ pczynski, M.; Ganachaud, F.; Hemery, P. AdV. Mater. 2004, 16, 18611863.

Langmuir, Vol. 23, No. 13, 2007 7315 the product was dried for 24 h in a vacuum. A white powder was finally obtained. In the case of the DTAB/SDBS system the nanocapsules were separated from the template and salt by dialysis (Spectra/Por 6 membrane, 1000 molecular weight cutoff, Spectrum Laboratories, Inc.) against a 0.01 mol/L solution of DTAB or a 1.5% solution of Triton X-100. Light Scattering Measurements. A Malvern ZetaSize model 4 light scattering apparatus with a Malvern correlator was used for dynamic light scattering measurements at a scattering angle of 90° and a temperature of 25 °C. The z-averaged hydrodynamic mean diameters (dz), polydispersity (PD), and distribution profiles were calculated using the software provided by Malvern. Transmission Electron Microscopy. TEM was carried out on a Jeol 100 SX transmission electron microscope (Japan) at an accelerating voltage of 80 kV. Micrographs were taken at 35000× or 56000× magnification, and then they were scanned and processed with CorelDraw 9.0 software (Corel Corp., Ottawa, Canada). A negative staining technique was used to visualize the vesicles. A 200 mesh copper grid coated with Formvar/carbon film (Pacific Grid-Tech) was dipped in the sample dispersion and left for 20 min. The excess of the sample was blotted with filter paper. The samples were stained with a 1% solution of uranyl acetate in water and then allowed to dry. Atomic Force Microscopy. Tapping mode AFM images were acquired with a NanoScope IV multimode atomic force microscope (Veeco, Santa Barbara, CA) by using a 10 µm (E) scanner equipped with an open liquid cell, if necessary. Microfabricated silicon cantilevers (model TESP, force constant around 40 N/m, Nanosensors, Wetzlar, Germany) were used for measurements in air. For the measurements in water silicon nitride cantilevers (NP series, Veeco) of nominal spring constant 0.12 N/m were used. The rms amplitude of the cantilever (1.0 V in air and 0.4 V in water) and the amplitude damping (5%) were minimized to reduce the peak normal forces applied to the surface. The samples for the measurements in air were prepared by depositing a drop of an aqueous dispersion of the vesicles after polymerization on a cleaned silicon surface and subsequent evaporation of water under a gentle stream of nitrogen. The measurements in liquid were performed in a drop of the respective vesicle solution deposited on a cleaned silicon surface. UV-Vis Spectroscopy. UV measurements were carried out with a Hewlett-Packard 8452A diode-array spectrophotometer equipped with an HP 89090A Peltier temperature control accessory using a quartz cuvette of 1 cm optical path length. FTIR Spectroscopy. FTIR spectra were obtained on a Nicolet Avatar 320 FTIR spectrometer. For each sample 128 scans were recorded with a resolution of 4 cm-1. The KBr pressed disk technique (0.5 mg of sample and 150 mg of KBr) was used for solid substances. Liquid substances were deposited between two KBr plates. Size Exclusion Chromatography. SEC measurements were performed on polymers precipitated from the dispersion by addition of methanol and next dissolved in a THF/toluene mixture (7:3, v/v). Samples were analyzed using Waters Associates equipment (Waters 515 HPLC pump and Waters 2420 refractive index detector). The measurements were performed in a THF/toluene mixture (7:3, v/v) at a 1 mL min-1 flow with three µ-Styragel columns (mixed, 500 and 100 Å) calibrated with polystyrene standards.

Results and Discussion DTAB/SDBS Vesicles. Vesicle Formation and Monomer Swelling. It was shown earlier that thermodynamically stable vesicles can be formed spontaneously in an aqueous solution of the mixture of DTAB and SDBS in the cationic-rich region of the phase diagram.17 We have noticed that short sonication of the surfactant mixture aided the vesicular dispersion in reaching the equilibrium state.16 After sonication and aging of the dispersion for one week, the measured hydrodynamic diameters (dz) of the (17) So¨derman, O.; Herrington, K. L.; Kaler, E. W.; Miller, D. D. Langmuir 1997, 13, 5531-5538.

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Figure 1. Distribution profiles of the hydrodynamic diameters obtained from the light scattering measurements for the DTAB/ SDBS vesicles with different ratios of D4H to surfactant.

vesicles were found to be in the range of 46-49 nm, and the variances were less than 0.2. Our goal was to utilize the DTAB/SDBS vesicle bilayer as the reaction medium for the polymerization of D4H solubilized in the membrane to produce materials with the shape and morphology of the original template. Therefore, it is necessary to demonstrate that the monomer really resides in the bilayer shell, rather than in other micellar structures that might be formed due to the surfactant rearrangement, and that the polymer is formed within the bilayer with preservation of the equilibrium structure. The penetration of hydrophobic molecules into the vesicle bilayer affects the vesicle stability and its morphology.18 Thus, the solubilization capacity of bilayer membranes is of great importance for preparation of the hollow nanocapsules. There are several effects which can influence the vesicle morphology during the process of solubilization: (i) increase of the vesicle size, (ii) reorganization into a lamellar structure, or (iii) disintegration of the bilayer after addition of a too high amount of solutes. These effects can be most directly monitored by means of the microscopy technique or, more indirectly, with the aid of scattering techniques. The scattering measurements were carried out to determine the amount of monomer which can be introduced to the bilayer without inducing changes in the vesicle morphology. A series of samples with the monomer:surfactant molar ratio varying from 0 to 1 and a constant total concentration of surfactant were prepared. The hydrodynamic diameters of the vesicles were measured using the DLS technique. Figure 1 shows the distribution profiles of the hydrodynamic diameters of the DTAB/SDBS vesicles with different amounts of monomer determined by dynamic light scattering measurements. DLS analysis of the mixed DTAB and SDBS solution carried out at room temperature reveals the presence of dispersed vesicles. After addition of a small amount of the monomer (the monomer:surfactant molar ratio was equal to 0.25), the vesicles still have unilamellar structures, but their size increases. Solubilization of more monomer (ratio 0.5) results in further growth of the vesicles. However, the addition of even more monomer leads to the drastic increase of the vesicle size and to the appearance of the second population of particles with a larger diameter. We attributed the appearance of the second peak to the formation of monomer droplets (o/w emulsion) in the system. The size of the monomer droplets is strongly dependent on the method used for preparation of the dispersion. We have noticed that a short vortexing of the dispersion after addition of the (18) Mao, M.; Huang, J.; Zhu, B.; Yin, H.; Fu, H. Langmuir 2002, 18, 33803382.

Figure 2. TEM micrograph of DTAB/SDBS vesicles before (A, bar 100 nm) and after (B, bar 100 nm) addition of 50 mol % D4H with respect to surfactant and (C, bar 200 nm) silicone nanocapsules isolated from surfactants by dialysis.

monomer reduces the size of the droplets. On the contrary, the method of preparation has no influence on the size of the monomer-loaded vesicles. The results presented in Figure 1 were obtained for the system in which only stirring was applied. The monomer droplets can be easily removed from the system by centrifugation. Parts A and B of Figure 2 present the TEM micrographs of the DTAB/SDBS vesicles, as formed, and the same vesicles swollen with D4H. The vesicles obtained in the preparation process are not ideally spherical (Figure 2A) as those observed for phospholipids.19 Instead, structures with angular geometries are present. The sizes of the vesicles determined by TEM are roughly in the range of 30-75 nm, which seems to be consistent with the results obtained from dynamic light scattering. Figure 2B illustrates the effect of D4H (molar ratio of 0.5) on the vesicle morphology at room temperature. Some tendency to form a smoother structure is noticed. Also their size increases; their diameters are in the range of 40-90 nm. That observation is again in agreement with the light scattering measurements (see Figure 1). The results of our studies correspond quite well with those of German et al.,20 who demonstrated using cryo-TEM analysis that solubilization of styrene in dioctadecyldimethylammonium bromide (DODAB) bilayers smoothes the curvature and bilayer undulations are facilitated. In the next step attempts to image the unloaded vesicles and monomer-loaded ones in water using AFM were undertaken (see Figure 3). Such objects are very fragile, and they may be easily damaged or moved by the scanning AFM tip. The scanning conditions were optimized to be very mild, and even so, consecutive scans usually showed damage of the soft objects. The imaged vesicles are significantly flattened due to both adsorption on the surface and pressure applied by the tip. The average height of the unloaded vesicles was 4 nm, while for monomer-loaded ones we measured 3.5 nm. The lateral size of those features varies between 30 and 50 nm, which is consistent with the DLS data. Interestingly, the monomer-loaded vesicles seemed to be less stable mechanically, since they were (19) Ke¸ pczyn´ski, M.; Nawalany, K.; Jachimska, B.; Romek, M.; Nowakowska, M. Colloids Surf., B 2006, 49, 22-30. (20) Jung, M.; Hubert, D. H. W.; van Veldhoven, E.; Frederik, P. M.; Blandamer, M. J.; Briggs, B.; Visser, A. J. W. G.; van Herk, A. M.; German, A. L. Langmuir 2000, 16, 968-979.

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Figure 3. Height images from tapping mode AFM measurements in water of DTAB/SDBS vesicles before (A) and after (B) addition of 50 mol % D4H with respect to surfactant immobilized on the silicon surface. The image size is 0.502 µm × 0.502 µm.

disintegrated much faster under the same scanning conditions than the unloaded vesicles. This conclusion is also in agreement with the result of surfactant lysis (see below) and can be explained considering that the presence of monomer molecules weakens the mutual interaction between the surfactant molecules forming vesicles and destabilizes the self-assembled structures. The literature on solubilization of hydrophobic solutes in vesicle membranes is very scarce. Bru¨ckner and Rehage21 investigated the solubilization of toluene in dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) giant vesicles. On the basis of morphological studies, they could distinguish three domains of solubilization for toluene in DPPC vesicles. At lower toluene concentrations, i.e., [toluene]:[DPPC] < 4.5, the vesicles kept their spherical shape. In an intermediate range, 4.5 < [toluene]:[DPPC] < 18, fluctuations of shape were observed, leading to nonspherical structures with low symmetry. Addition of more toluene to the suspension induced the formation of an o/w emulsion which coexists with vesicles. It has also been noticed that solubilization of styrene in DODAB vesicles modulates the bilayer properties and hence their morphology.20 However, at a molar ratio of [styrene] to [DODAB] equal to 2 there was no considerable change in the vesicle size distribution, as observed by DLS. Fu et al.18 have studied the effect of toluene and octane on the vesicular system identical to that used in our work. They observed quite different effects of each of these solutes on the catanionic vesicles, which were explained considering that the solutes are solubilized in different sites in the bilayer. The solubilization of toluene into the catanionic vesicle bilayer leads to the enlargement and deformation of the vesicles (for [toluene]:[surfactant] < 1.4) and the formation of a lamellar structure (for [toluene]:[surfactant] > 3.5). However, after solubilization of octane at that concentration the catanionic vesicles retain their unilamellar structure. The results of octane and toluene solubilization clearly indicate that the effects of solutes on the vesicle are strongly dependent on the solute nature and the position which they occupy in the bilayer, which was (21) Bruckner, E.; Rehage, H. Prog. Colloid Polym. Sci. 1998, 109, 21-28.

connected with the interfacial tension of the substance/water. Because the interfacial tension of the toluene/water interface (35 mN/m22) is lower than that of the octane/water interface (50.8 mN/m22), toluene is surface active at the water/hydrocarbon interface and therefore solubilized in the palisade layer between the surfactant headgroups.18 We have measured the interfacial tension in the D4H/water system and found it to be 27.3 mN/m (see the Supporting Information). Thus, D4H is more surface active than toluene and is mainly solubilized at the interface of the surfactant aggregate. The solubilized D4H molecules move the surfactant headgroups further apart. Because the vesicles are positively charged (due to the excess of cationic surfactant), this results in a decrease of the repulsions between the surfactant headgroups as well as in a decrease of the charge density. The lower surface charge density favors the formation of an aggregate with a small curvature (large radius).23 Therefore, the catanionic vesicles would enlarge after solubilization of D4H and finally lose their integrity after addition of a certain amount of the monomer. Polycondensation/Polymerization Processes. Cross-linking/ polymerization processes were performed using samples obtained by centrifugation of the 0.5:1 system. Hydrolysis of Si-H bonds followed by condensation processes, both catalyzed by hydroxyl anions (the pH of the solution was raised to about 8), was used to synthesize silicone resin inside the vesicle bilayer. The formation of nanocapsules can be confirmed by surfactant lysis, e.g., with Triton X-100 (Figure 4).24 Addition of a 15% solution of Triton X-100 to the DTAB/SDBS vesicle leads primarily to an incorporation of the surfactant in the vesicle assembly. After a certain critical concentration is exceeded, the surfactant provokes a breakup of the bilayers to form mixed micelles, as revealed by the decreased turbidity of the dispersion. The transition to mixed micelles of both the unloaded and monomer-loaded vesicles occurs when the molar ratio of Triton (22) Handbook of Chemistry and Physics, 55th ed.; CRC Press Inc.: Cleveland, OH, 1974. (23) Bergstrom, M. Langmuir 1996, 28, 2454-2463. (24) Bucak, S.; Robinson, B. H.; Fontana, A. Langmuir 2002, 18, 8288-8294.

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X-100 to the total concentration of vesicle surfactant exceeds 0.9. However, in the case of the monomer-laden vesicles the turbidity of the dispersion decreases gradually with increasing concentration of the surfactant. This can be explained assuming that the incorporation of the monomer into the membrane, especially when the monomer is solubilized at the interface, reduced the stability of the vesicular assembly. On the other hand, the cross-linked vesicles are characterized by a similar turbidity in the presence of Triton X-100 at various concentrations (a decrease in absorbance is only due to dilution). The particle size distributions of cross-linked material before and after lysis display similar profiles (see ref 16), indicating that the nanocapsules are not affected by surfactant addition. The hydrodynamic diameter of the nanocapsules was determined by DLS to be 96 nm and was higher than that for the monomer-loaded vesicles. The nanocapsules obtained can be easily isolated from the template by dialysis against a diluted solution of DTAB or Triton X-100. Dialysis against pure water leads to coagulation and precipitation of the silicon material. Figure 2C shows typical TEM micrographs of the isolated nanocapsules. The size of the nanocapsules is in the range of 50-150 nm. TEM reveals either discrete hollow particles or networks of linked or aggregated hollow silica shells. These nanocapsules could be directly imaged by AFM in the dry state (Figure 5). The images show hemispherical structures with a polydisperse distribution of lateral sizes (30-90 nm). The horizontal sizes of the features are 3-10 nm. The imaged structures are much more robust compared to pure vesicles and monomer-loaded ones. The polymerized vesicles do not disintegrate after drying and survive multiple scanning (Figure 5A). During drying, they only collapse, forming biconcave disks (red blood cell shape) characteristic of dried capsules (see Figure 5B). The same structure was observed using scanning electron microscopy for polymer-containing vesicles.10 It has been established that D4H can participate in several possible reactions which are catalyzed by hydroxyl anions. First, the Si-H bonds hydrolyze readily under basic conditions. That process occurs rapidly and is associated with the release of gas product. Then the cross-linking reaction takes place by the basecatalyzed polycondensation of siloxane, as shown in Scheme 1. Anionic ring-opening polymerizations of D4H and of the hydrolyzed monomer certainly can take place under these conditions,25 but this process is slow compared to polycondensation.

All above-mentioned processes occur simultaneously and lead to the formation of a densely cross-linked silicone material with SiO3/2 units as was confirmed by Fourier-transform infrared (FTIR) spectra.16 Using polycondensation instead of common radical polymerization allowed us to avoid a polymer demixing problem, which led to formation of a bead trapped in the bilayer (so-called parachute structure).12 Since the polycondensation reaction proceeds uniformly in the whole volume of the template, there is no preferential formation of latex particles inside the bilayer. SDBS/DTAB Vesicles. Vesicle Formation. There are no literature data on the phase diagram for the SDBS and DTAB surfactant system in the anionic-rich region. By analogy to a similar system containing cetyltrimethylammonium tosylate (CTAT) and SDBS,26 we have assumed the formation of vesicles also in that region. Simple intermixture of the surfactant stock solutions followed by two weeks of stirring produced a characteristic turbid sample. The light scattering measurements confirmed the presence of vesicles of diameter equal to 123.3 ( 0.6 nm and polydispersity equal to 0.312 ( 0.007. Like in the case of DTAB/SDBS vesicles, the dispersion was subjected to short sonication followed by aging for one week. The blue transparent solution was obtained by that procedure. The diameter of the vesicles varied in the range 52-60 nm in different preparations, and the PDs were always lower than 0.2. The formation of the vesicles in this system was also confirmed with TEM visualization. Figure 6A depicts the typical morphology of the SDBS/DTAB vesicles obtained from the negative staining TEM. As can be seen they are spherical to a higher extent than the previous system. Monomer Swelling. As in the case of the DTAB/SDBS system, these vesicles also swell upon addition of monomer and are capable of solubilizing a certain amount of D4H before phase separation occurs. To study the effect of the monomer on the vesicle size, five vesicular solutions were prepared by adding D4H to the vesicle dispersion at molar ratios of 0:1, 0.24:1, 0.42: 1, 0.60:1, and 0.85:1 (with respect to the total surfactant concentration). After 2 days of stirring, light scattering measurements were performed for these solutions (Figure 7). DLS results indicate that in the range of D4H concentration from 0 to 0.011 M (0.42:1 system) there is one peak in the size distribution profile, which shifts only slightly to the higher diameter range with an increase in the monomer concentration. After addition of a larger amount of monomer more significant changes in the size distributions are observed. A second peak appears in a larger diameter region, and shifts of the first peak are more pronounced. Thus, the excess of monomer forms a second population of larger sized droplets, which become more visible at higher monomer: surfactant ratios. Figure 6B presents the TEM micrograph of the SDBS/DTAB vesicles swollen with D4H (molar ratio of 0.6:1). The sizes of the vesicles determined by TEM are roughly in the range of 40-90 nm, which is consistent with the results of dynamic light scattering. Polymerization/Cross-Linking Processes. Polymerization/ cross-linking processes were performed using samples obtained by centrifugation of the 0.6:1 system. The processes were initiated by the addition of a 0.1 M solution of HCl followed by SEC analysis, IR spectroscopy, and surfactant lysis. The SEC analysis of the polymeric material extracted after different times from SDBS/DTAB vesicles (the SEC traces are shown in the Supporting Information, Figure s1) was carried out. It was found that the molar masses of polymer increase,

(25) Barre`re, M.; Ganachaud, F.; Bendejacq, D.; Dourges, M.-A.; Maitre, C.; He´mery, P. Polymer 2001, 43, 7239-7246.

(26) Kaler, E. W.; Herrington, K. L.; Murthy, A. K.; Zasadzinski, J. A. N. J. Phys. Chem. 1992, 96, 6698-6707.

Figure 4. Optical density traces for the surfactant titration of the DTAB/SDBS vesicles (O), the monomer-loaded vesicles (∆), and the cross-linked vesicles (0).

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Figure 5. Height images from tapping mode AFM measurements in air of silicone nanocapsules isolated from the DTAB/SDBS templates, which were deposited on the silicon surface: (A) immediately after depositing, image size 0.506 µm × 0.506 µm, (B) one week after depositing, image size 0.45 µm × 0.45 µm. Scheme 1. Main Reactions in the Hydrolysis Condensation of D4H Catalyzed by Hydroxide Anion in the DTAB/SDBS Vesicles

jumping, and the kinetics is slow in comparison to that observed for polymerization of D4H in an emulsion.27 After two weeks the polymerization seems to be completed, and the resulting polymer is characterized by Mw ) 169 000 g/mol, Mn ) 94 000 g/mol, and DP ) 1.8. Further duration of the polymerization process results in insoluble polymeric material. Cationic emulsion polymerization of D4H in aqueous solution is well-known.27,28 No cross-linking reaction was observed, confirming good stability of the Si-H bond under these experimental conditions. The stability of the Si-H bond under our experimental conditions was checked by IR spectroscopy. The IR spectra of the polymeric material isolated from vesicles after various periods of time from the beginning of the polymerization show several changes compared to the spectrum of the monomer (see the Supporting Information, Figure s2). The absorption at 2170 cm-1, which is highly distinctive for the (27) Maisonnier, S.; Favier, J.-C.; Masure, M.; He´mery, P. Polym. Int. 1999, 48, 159-164. (28) Gupta, S. P.; Moreau, M.; Masure, M.; Sigwalt, P. Eur. Polym. J 1993, 29, 15-22.

Figure 6. TEM micrographs of SDBS/DTAB vesicles before (A, bar 100 nm) and (B, bar 100 nm) after addition of 60 mol % D4H with respect to surfactant and (C, bar 100 nm) the sample after four weeks of polymerization.

Si-H bond,29 decreased slightly after two weeks of polymerization. Concomitantly, the band at 1086 cm-1, characteristic of the asymmetric Si-O-Si stretching vibration in cyclic tetramers,30 becomes broader and splits into two absorption maxima, the first at 1103 cm-1 and the second at 1045 cm-1. They are characteristic of linear polysiloxanes. After four weeks of polymerization, the band at 2170 cm-1 decreased by half and further broadening of the band for Si-O-Si vibration with maxima at 1120 and 1041 cm-1 can be noticed. The pattern of (29) Lipp, E. D.; Smith, A. L. In The analytical chemistry of silicones; Smith, A. L., Ed.; Wiley-Interscience Publications: New York, 1991; Vol. 112, Chapter 11. (30) Anderson, D. R. In Analysis of silicones; Smith, A. L., Ed.; WileyInterscience Publications: New York, 1974; Chapter 10.

7320 Langmuir, Vol. 23, No. 13, 2007

Figure 7. Distribution profiles of the hydrodynamic diameters received by light scattering measurements for the SDBS/DTAB vesicle with different amounts of D4H added. Scheme 2. Reactions of D4H in the SDBS/DTAB Bilayer Catalyzed by Acid

Ke˛pczyn´ ski et al.

Figure 8. Optical density traces for the surfactant titration of the SDBS/DTAB vesicles (]) and the monomer-loaded vesicles after different times of cross-linking/polymerization processes: one week (0), two weeks (O), four weeks (∆).

resistant against dialysis, and we were not able to remove the salt from the system. Thus, the precipitated salt is present in the picture.

Conclusions

these bands is characteristic of monoalkylsiloxane polymers of indefinite structure (silsesquioxanes), RSiO3/2.30 All these observations imply that during the polymerization of D4H slow hydrolysis of Si-H bonds followed by the condensation process occurs. The condensation process between two propagating polymer chains is responsible for the jumps of the molar masses observed by SEC. Schematically it is depicted in Scheme 2. As in the case of the DTAB/SDBS template the durability of the created silicone structure can be easily tested using the surfactant lysis procedure. The pathway of vesicle disintegration was followed by measurements of the turbidity of the dispersion with the aid of a UV spectrometer. The optical density of the samples at λ ) 300 nm was measured and is plotted versus the ratio of concentration of Triton to the total concentration of the surfactants in Figure 8. As can be expected the titration with a 15% Triton-X100 solution of the SDBS/DTAB vesicles alone and swollen with D4H results in reversion of the vesicle dispersions to a transparent micellar solution. In the case of the monomerladen vesicles subjected to polymerization/cross-linking processes the addition of detergent also reduces the turbidity of the dispersion. However, the turbidity of the samples obtained after titration increases with increasing time of the polymerization/ cross-linking processes. This suggests that the synthesis of polymeric material, cross-linked enough to retain its integrity during the detergent lysis, is possible in these experimental conditions. However, this is quite a slow process. The morphology of the monomer-laden vesicles after four weeks of polymerization is presented in Figure 6C. The image shows a polydisperse distribution of hemispherical structures with sizes in the range of 60-100 nm. The structures were not

Silicone hollow particles were synthesized inside the membrane of an equilibrated surfactant vesicle using cross-linking/polymerization processes of 1,3,5,7-tetramethylcyclotetrasiloxane. Two different surfactant systems were used to form the vesicular templates: mixtures of DTAB and SDBS in the cationic- or anionic-rich region of the phase diagram. The monomer was solubilized readily in both types of vesicles. The solubilization of D4H in the catanionic vesicle bilayer led to enlargement of the vesicle at low concentration of the monomer and the formation of an o/w emulsion at higher concentrations. The DTAB/SDBS and SDBS/DTAB vesicles were capable of solubilizing, respectively, about 50 and 60 mol % monomer with respect to the surfactant contents, without significant changes of the vesicular morphology. The solubilization capacity of the cationic vesicle for D4H was lower compared to that reported for organic compounds, such as toluene or styrene, but it was sufficient for the preparation of stable nanocapsules, and as a result we were able to get much thinner capsule walls. AFM imaging and surfactant lysis experiments have shown that the monomer-loaded vesicles are less stable than the genuine ones. By using polycondensation instead of common radical polymerization, the problem of a so-called parachute structure was avoided. The silicone hollow spheres formed are typically stable for weeks. The sizes of the hollow silicone particles obtained were in the range of 50-150 nm. Acknowledgment. We thank the Polish Ministry of Science and Higher Education for financial support in the form of Grant N204 131 32/3320 and the French Ministry of Research and Education for financing the one-year postdoctoral work of M.K. Supporting Information Available: Surface and interfacial tension measurements, SEC traces for the polymeric material extracted from SDBS/DTAB vesicles after different times of polymerization, and IR spectra of the polymeric material isolated from SDBS/DTAB vesicles after different times of polymerization. This material is available free of charge via the Internet at http://pubs.acs.org. LA063442I