Communication
Amphiphilic Patchy Composite Colloidsa Shujiang Ding, Chengliang Zhang, Wei Wei, Xiaozhong Qu, Jiguang Liu,* Zhenzhong Yang*
A new approach to fabricate patchy silica/polymeric gel composite colloids with amphiphilic performance is reported. The amphiphilic performance is rendered by selectively modifying the silica framework with a silane that contains an oleophilic alkyl chain. The patchy composite colloids are dispersible in both water and oil, and may be used as a solid particle surfactant. The modified silica framework can also assist other functional materials to disperse in a desired media. The corresponding silica/carbon composite colloids become amphiphilic after a sequential activation of carbon and modification of silica, and meanwhile possess as good electron conductivity as the as-prepared silica/carbon composite colloids.
Introduction Controlled synthesis of highly dispersible amphiphilic colloidal particles has recently attracted increasing interest because of their special chemical and physical properties. Amphiphilic colloids have many promising applications, such as in the extraction of organic pollutants,[1] controlled release,[2] lipase immobilization,[3] and supporting catalysts.[4] It is important to design the microstructure of a colloid to achieve amphiphilic performance. For example, polymer colloids with a hydrophilic core and a porous oleophilic shell structure are amphiphilic.[5] They are usually Z. Yang, S. Ding, C. Zhang, W. Wei, X. Qu, J. Liu State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China Fax: (þ86) 10-62559373; E-mail:
[email protected];
[email protected] a
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synthesized by a two-step polymerization of a hydrophilic monomer followed by another oleophilic monomer in an inert solvent. Besides a core–shell structure, those colloids with patches on their surface can display amphiphilic performance. Some methods have also been proposed to prepare such amphiphilic patchy colloids. Among them, the co-assembly of functional nanoparticles and amphiphilic polymers to form patchy colloids is of interest since the method works for broad components.[6] In fact, Janus particles with single oleophilic and hydrophilic regions on their two sides are special cases of patchy colloids.[7] Kim et al. have prepared patchy silica colloids with silver nanoparticles on the surface by the in-situ reduction of silver nitrate with butylamine,[8] and the patchy Ag can be changed from individual nanosized particles to a continuous network. Yang et al. have prepared patchy particles by slow evaporation of oil-in-water emulsion droplets.[9] The patchy regions can be further modified to render a different composition thus wettability. It is noticed that the patchy structures are easily disintegrated from the surface especially under strong disturbance such as sonication, thus the amphiphilic performance becomes unstable or even disappears.
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Experimental Part
All reagents were obtained from Sigma–Aldrich and the Beijing chemical works and used as received.
Sample Preparation Sulfonated Polystyrene Hollow Colloids (S0) Commercially available polystyrene hollow colloids (Rohm & Haas, HP-433)[10c,11] were mixed with an emulsion that contained divinylbenzene (DVB), benzoyl peroxide (BPO) initiator, and sodium dodecyl sulfate (SDS) for 12 h. The weight ratio of DVB to the polystyrene hollow colloids was maintained at 2:1. The resultant swollen hollow colloids were heated to 80 8C to allow a polymerization of DVB within the shell. After polymerization, the crosslinked colloids were dried under ambient conditions. The crosslinked polystyrene hollow colloids were sulfonated using concentrated sulfuric acid (98 wt.-%) at 40 8C for 24 h and washed with ethanol and water.
Silica/S0 Composite Colloids A typical recipe and procedure follows. Colloid S0 (0.5 g) and a varied amount of tetraethoxysilane (TEOS) (0.5, 1, and 2 g) were mixed in water (10 g) and ethanol (40 g) under stirring. The pH of the solution was about 5. After the sol–gel process for 12 h, the product was centrifuged.
Amphiphilic Composite Colloids
Dilute dispersions of the spheres in ethanol were dropped onto carbon-coated copper grids for transmission electron microscopy (TEM) characterization (JEOL 100CX operating at 100 KV). Scanning electron microscopy (SEM) measurements were performed with a HITACHI S-4300 apparatus operated at an accelerating voltage of 15 kV. The samples were ambient dried and vacuum sputtered with Pt. FT-IR characterization was conducted on a BRUKER EQUINOX 55 FT-IR spectrophotometer. The samples were prepared by pressing the dried spheres and potassium bromide (KBr) into pellets. Atomic force microscopy (AFM) images were taken by using tapping mode AFM (NanoScope IIIA MultiMode, Digital Instruments) using optical beam deflection to monitor the displacement of a microfabricated silicon cantilever with silicon probes. The scans were 1 mm and 250 nm in size. The emulsion was dropped onto a glass substrate for observation using a microscope (Olympus), and images were obtained by CCD (DH-II).
Results and Discussion
Silica/S0 composite colloid (0.1 g) and a varied amount of octadecyl trichlorosilane (OTS) (0.1, 0.01, 0.001, and 0.0001 g) were mixed in 10 g of hexane at ambient temperature under stirring. After a modification for 6 h, the product was washed with hexane and ethanol, and centrifuged.
Au Nanoparticles Au nanoparticles were prepared by the method reported in the previous literature.[12] Typical preparation procedure: A 30 103 3 M aqueous HAuCl4 solution was added to a 25 10 M solution of tetraoctylammonium bromide in toluene (80 mL). A 0.4 M solution of freshly prepared NaBH4 (25 mL) was added to the stirred mixture. After 30 min the two phases were separated and the toluene phase was subsequently washed with 0.1 M H2SO4, 0.1 M NaOH, and H2O (three times), and then dried over anhydrous NaSO4.[12]
Au Particles Phase Transfer
Characterization
We have previously reported the synthesis of inorganic/ sulfonated polystyrene (SPS) gel interpenetration network composite colloids.[10] They are hydrophilic and dispersible in the aqueous phase. Based on these composite colloids, patchy colloids with amphiphilic performance are expected upon rendering oleophilic performance to the inorganic network. The synthesis is illustrated in Scheme 1. A crosslinked SPS hollow colloid (S0) is derived by sulfonation of the corresponding crosslinked PS hollow colloid. A continuous silica network is formed by a sol–gel process within the SPS gel matrix. Afterwards, an oleophilic moiety is selectively introduced onto the outmost silica network with a silane OTS. Amphiphilic colloids are thus achieved with integrated oleophilic and hydrophilic regions. Silica/S0 composite hollow colloids
An aqueous 0.1 M 4-dimethylaminopyridine (DMAP) solution (1 mL) was added to aliquots (1 mL) of the as-prepared nanoparticle mixtures. Direct phase transfer across the organic/ aqueous boundary was completed within 1 h, with no stirring or agitation required. The pH of the colloidal gold solution was about 10.5.
Silica/Carbon Composite Colloids Silica/S0 composite colloids were first heated to 370 8C in nitrogen at a rate of 2 8C min1 and held for 2 h. The system was further heated to 800 8C and held for 2 h.[11]
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Scheme 1. Illustrative synthesis of a patchy composite colloid. A sulfonated polystyrene (SPS) hollow colloid is achieved by sulfonation of the corresponding PS hollow colloid. After a sol–gel process is undertaken within the SPS matrix, another continuous silica network is formed to derive a SPS/silica composite hollow colloid. By selective modification of the outer surface of the silica network with a silane to render oleophilic performance, the modified composite colloid becomes amphiphilic.
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can be converted into silica/carbon colloids.[11] Using a similar method, the surface wettability, thus the dispersibility, of the silica/carbon colloids can be tuned by further modification. Besides silica, many other functional materials, for example, metals, oxides, and conducting polyaniline can be additionally introduced within the gel matrix.[10,11] How to assist the dispersibility of the functional composite colloids in desired media becomes a main concern. Herein, we demonstrate how to tune the dispersibility of the composite colloids starting from a silica/SPS composite. After a complete sulfonation of the crosslinked PS hollow colloid, the crosslinked SPS gel colloid S0 can well preserve the spherical contour (Figure 1a). The mean shell thickness is 120 nm, and the cavity diameter is 280 nm. Before sulfonation, the shell thickness and cavity diameter is 80 and 320 nm.[10e,11] The colloid S0 will provide a sufficiently robust gel template to guarantee the hollow spherical contour is well retained during further complexation with other materials. In contrast, the SPS gel hollow colloid from the linear PS loses its spherical shape and eventually dissolves. The sulfonic acid group in the gel shell can catalyze the sol–gel process of TEOS to form silica/S0 composite colloids. Excess silicate species can diffuse through the channels within the SPS shell, and create another silica core inside the cavity in addition to within the shell gel matrix (Figure 1b). After silica is incorporated, the average diameter is slightly increased from 520 to 540 nm. The silica content can be controlled by TEOS feeding, whilst the morphology of the composite colloids is affected. When the weight ratio of TEOS/S0 is 1:1, the silicate concentration is at a low level, and silica is dominantly formed within the outer layer of S0. The silica content in the composite colloid is 1.5 wt.-% as determined by TGA in air (Figure S1a). Not all silica is grown on the template colloid S0. In fact, free silica nanoparticles are found in the dispersion medium. The silica hollow colloids become collapsed after the composite colloids are calcined in air (Figure S2a). When the ratio is increased to 2:1, silica is formed within the whole shell. The silica content is about 10 wt.-% (Figure S1b). After the composite hollow colloids are calcined in air, the resultant silica hollow spherical shape is well preserved (Figure S2b). When the ratio is further increased to 4:1, the silica content is 20 wt.-% (Figure S1c), and a core–shell structure is formed (Figure 1b). The surface becomes coarser because of the incorporation of silica. In order to further investigate the shell nature, the composite colloids are calcined in air to remove polymers, which results in silica colloids (Figure 1c). The silica shell thickness is less affected and similar to the composite one. On the other hand, after silica is selectively etched with HF, the resultant hollow colloids are similar to the original S0. It is concluded that Macromol. Rapid Commun. 2009, 30, 475–480 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1. SEM and TEM images of some representative samples. a) SPS hollow colloid S0; b) silica/S0 composite hollow colloid with 20 wt.-% silica; c) silica hollow colloid formed by calcinating the silica/S0 composite hollow colloid in air at 450 8C for 2 h; d) silica/ S0 composite hollow colloid after being modified with OTS.
silica and the SPS gel form a bi-continuous interpenetration network. In order to render oleophilic performance to the silica/S0 composite colloids, the colloids are modified with OTS in hexane. The oleophilic solvent hexane can ensure that the modification only occurs on the silica surface rather than
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on the SPS surface. At the beginning of the modification, the silica/S0 composite colloids aggregate heavily. With prolonged modification, the aggregation becomes partially dispersible. Eventually, the silica/S0 composite colloids become completely dispersible in hexane. It can be seen from Figure 1d that the colloids are individuals without aggregation and the surface becomes much coarser. The introduced OTS moiety is confirmed by FT-IR spectra (Figure S3). The characteristic bands at 670 and 1 173 cm1 are assigned to the sulfonic acid group ( –SO3H) (trace S3a).[10d,e,11] Some new peaks at 463, 796, and 1 089 cm1 are assigned to the Si –O –Si bond for the silica/S0 composite colloids (trace S3b).[13] The broad absorption band at 3 424 cm1 is a result of the overlapping of the O –H stretching vibration of Si –OH and adsorbed water.[14] After being modified with the silane, the intensity of the characteristic bands at 2 852 and 2 925 cm1 (assigned to the stretch vibration of C –H ( –CH2 –)) is increased, which indicates the presence of OTS. The oleophilicity of the composite colloids can be tuned by adjusting the weight ratio of OTS to the silica/S0 composite colloid. When the weight ratio is within 1:100–1 000, the modified colloids are well dispersible in both water and hexane. In comparison, the composite colloid before modification is only dispersible in water not in oil. When the amount of OTS is increased to a weight ratio of 1:10, the colloids are only dispersible in hexane not in water. When the weight ratio is increased from 1:100 to 1:10, the area ratio of the peak at 2 925 cm1 ( –CH2 –) to that at 3 026 cm1 ( –CH – – CH –) is greatly increased (traces S4a, S4b). When the weight ratio is further increased to 1:1, the area ratio reaches a plateau (trace S4c), which indicates that all the Si –OH has been modified with OTS. In comparison, no OTS characteristic bands are observed when S0 colloids are treated with the silane. In order to determine locations of the SPS and OTS moiety, hydrophilic Au nanoparticles with a 4-dimethylaminopyridine corona are used to selectively label the SPS region. In fact, the Au nanoparticles can be absorbed onto both the unmodified silica and SPS regions.[12] For example, the nanoparticles are easily absorbed onto the silica surface and thus homogeneously distributed (Figure S5). The case is similar for the silica/SPS composite colloids (Figure 2a, 2b). However, it is essentially different for the composite colloids after being modified. Au nanoparticles form some patches on the surface rather than being homogeneously distributed (Figure 2c, d), which implies that the silica region has been covered with an OTS alkyl moiety. Even at a very high OTS/colloid weight ratio of 1:1, Au nanoparticles can also form patches. This indicates that only the silica region is covered with OTS, and the SPS region is less influenced. We also confirm the location of the SPS and OTS moiety by AFM. In Figure S6, the pale patches are assigned to the OTS-modified silica
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Figure 2. a,b) SEM and TEM images of the silica/S0 composite colloid with Au nanoparticles adsorbed on the surface. c,d) The modified silica/S0 composite colloid with OTS, followed by an absorption of Au nanoparticles.
region, and the dark curved areas are assigned to the SPS region. To demonstrate the amphiphilic performance of the modified silica/S0 composite colloids (OTS/colloid weight ratio 1:100), their dispersibility in water and hexane is further investigated. The colloids can be dispersed both in water and hexane, respectively (Figure 3aI, II), which
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Figure 3. Optical pictures of some samples: aI, II) The modified colloid dispersed in water and hexane, respectively. aIII, IV) Hexane-in-water (hexane/water ¼ 1/3 w/w) and water-inhexane (hexane/water ¼ 3/1 w/w) emulsions in the presence of the modified composite colloid. bI) The as-prepared silica/carbon composite colloid dispersed in a water/hexane mixture, the colloid is favorably dispersed in the bottom water phase. bII) The silica/carbon colloid after being modified with OTS, favorably dispersed in the top hexane phase. bIII) The OTSmodified silica/carbon colloid after being activated can be well dispersed both in water and hexane, and a stable water-inhexane emulsion is formed. (The OTS-modified silica/carbon colloid, water, and hexane weight ratio is 0.1:1:3.)
indicates that the modified silica/S0 composite colloids are amphiphilic. The dispersibility remains unchanged under sonication, which indicates the amphiphilic performance is permanent. In the presence of colloids, for a mixture of hexane/water ¼ 1/1 v/v, water is the continuous phase, which indicates the colloids are more hydrophilic (Figure S7). In the presence of 2.5 wt.-% of composite colloids, an oil-in-water emulsion (hexane/water ¼ 1/3 w/w) is formed (Figure 3aIII). A trace amount of methylorange was added into the water phase only for guidance of the eyes. When the oil content is increased to hexane/ water ¼ 3/1 w/w, the continuous phase is reversed to form a water-in-oil emulsion (Figure 3aIV). The emulsion type is confirmed by measuring the conductivity of the emulsions. Microscope images of the two emulsions show the droplets (Figure S8a, S8b) are 5–20 and 10–40 mm in diameter, respectively. After the emulsions are dried, the Macromol. Rapid Commun. 2009, 30, 475–480 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
corresponding spherical shapes are preserved (Figure S9a, b), which indicates the modified composite colloids predominantly exist at the interface of the water and oil. For the silica/S0 composite colloids that contained 1.5 wt.-% of silica, the amphiphilic performance is similar to those that contained 20 wt.-% of silica. This is consistent with the continuous silica network at the outer surface even at a low silica content. The patchy silica can be used to assist other materials to disperse in desired media. In order to demonstrate the concept, we show the synthesis of amphiphilic carbon colloids. Silica/S0 composite hollow colloids can be transformed into silica/carbon composite hollow colloids at high temperature.[11] They are dispersible in water in an aggregated form (Figure 3bI), which indicates their hydrophilicity becomes weaker. When carbon is activated with H2O2 and concentrated sulfuric acid, some polar groups are derived (Figure S10a). Some new peaks at 1 760, 1 631, 1 593, and 1 385 cm1 are assigned to the polar groups of the carboxy, carbonyl, and sulfate groups.[15] The activated silica/carbon composite hollow colloids become highly dispersible in water again. The three characteristic peaks at 463, 802, and 1 109 cm1 are assigned to silica.[12] The broad absorption band at 3 424 cm1 is a result of the overlap of the OH stretching vibration of silanols and adsorbed water.[13] When the as-prepared carbon/silica colloids are modified with OTS, two new strong peaks appear at 2 922 and 2 853 cm1 (Figure S10b) assigned to C –H ( –CH2 –). The colloids become easily dispersible in hexane (Figure 3bII). After the OTS-modified silica/carbon composite colloids are activated, some new peaks appear at 1 734, 1 614, and 1 385 cm1, which are assigned to the polar groups (Figure S10c, S10d).[15] They become dispersible both in water and oil. In the presence of such colloids, a stable water-in-oil emulsion is formed (Figure 3bIII). Both the activated and the OTS-modified carbon/silica composite hollow colloids possess conductivity of the same level (0.01 S cm1) as the as-prepared silica/carbon composite colloids.
Conclusion We have demonstrated a new approach to fabricate an amphiphilic silica/polymeric gel and the corresponding carbon composite colloids with patches on the surface. By selectively modifying the silica patches, another oleophilic moiety is introduced onto the surface that is responsible for the amphiphilic performance. Such composite colloids can be dispersed in both water and oil, and stable emulsions are formed using them as solid emulsifiers. Using the patchy structure can assist functional materials such as carbon to be dispersible in desired media.
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Acknowledgements: This work was supported by the NSF of China (50573083, 50733004, and 20720102041), foundations from Chinese Academy of Sciences, and China Ministry of Science and Technology (2003CB615600, 2006CB605300). Received: October 19, 2008; Revised: December 8, 2008; Accepted: December 8, 2008; DOI: 10.1002/marc.200800657 Keywords: amphiphilic; colloids; composites; dispersible; patchy
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