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One-step synthesis of hydrophobic multi-compartment organosilica microspheres with highly interconnected macro-mesopores for stabilization of liquid marbles with excellent catalysis Guanqun Du, Junxia Peng, Yuanyuan Zhang, Hongxia Zhang, Jieli Lu, and Yu Fang Langmuir, Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 11, 2017

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One-step synthesis of hydrophobic multi-compartment organosilica microspheres with highly interconnected macro-mesopores for stabilization of liquid marbles with excellent catalysis

Guanqun Du, Junxia Peng*1, Yuanyuan Zhang, Hongxia Zhang, Jieli Lü, and Yu Fang

Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, P. R. China

KEYWORDS: Hydrophobicity, Organosilica microsphere, Multi-compartment, Interconnected macro-mesopores, Multi-emulsion template

∗

Corresponding author. E-mail: [email protected]; Tel: +86-29-81530853; Fax: +86-29-81530727 1

ACS Paragon Plus Environment

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ABSTRACT The combination of emulsion template with polymerization is a very convenient approach for one-step realization of both simple control porous structures via change of emulsion formulation and easy functionalization via concomitant choice of on-demand monomer. A major challenge of this approach is the inherent instability of oil/water interface in emulsions, especially the occurrence of chemical reaction in oil or aqueous phase. This study reports the pioneering preparation of the highly interconnected macro-mesopores and multi-compartment (HIMC) vinyl organosilica microspheres with hydrophobicity by one-step formation of W/O/W emulsions acting as a template. The emulsion system consists of acidified deionized water, stabilizer, and vinyltriethoxysilane (VTEO), in which VTEO can be used to produce organosilica skeleton of the resultant microsphere by sol-gel process. The study demonstrated that the marvelous stability of W/O/W emulsions aid the formation of multi-compartment

organosilica

microspheres

with

highly

interconnected

macro-mesopores by emulsion droplets, rather than single-compartment (SC) microspheres. Meanwhile, the internal porous structure and surface morphology of as-prepared organosilica microspheres could be largely tuned by simple variation of pH value, volume fraction of water phase and stabilizer concentration in the initiating multi-emulsions. Benefiting from such a well-orchestrated structure and existence of numerous vinyl groups on the surface, HIMC organosilica microspheres exhibit highly high hydrophobicity (water contact angle larger than 160°), which make them stabilize liquid marbles with excellent stability and high mechanical robustness. Due to its strong catalyst, Ag nanoparticles within HIMC organosilica microspheres enable Ag/HIMC-vinyl organosilica microspheres-based liquid marble to an efficiently catalytic micro-reactor, realizing the complete degradation of MB to leuco methylene blue by NaBH4 in 10 min. The result of this work could provide some guidance for easy, low-cost, benign preparation of HIMC microspheres, having a potential to be excellent supporter of metal nanoparticles or other functionalized compounds for application in sensing, optoelectronics and catalysis. 2


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1 INTRODUCTION Porous organosilica particulate materials have always attracted much attention due to their usefulness in many applications like adsorption/separation,1,2 catalysis,3,4 sensing and biomedicine.5,6 This is mainly because organosilica materials commonly exhibit excellent thermal and mechanical stability, low toxicity, good biocompatibility and easy surface functionalization.7-9 Generally speaking, properties of porous structure are of pivotal importance, even decided to the performance and usage of materials, wherein a hierarchically porous structure is always highly desirable.10 The hierarchical structures imply that porous materials combine micro- (pore size < 2 nm), meso- (2 nm < pore size < 50 nm), and macro-pores (pore size > 50 nm). Micro- and meso-pores produce high surface areas, and then provide a lot of adsorption and reactive sites, while macro-pores, especially interconnected micron-sized porous structures, can offer faster mass transfer via minimizing channel blocking from tiny cavity.11-13 Therefore, the development of a simple and effective approach for preparation of organosilica particles with the coexistence of micron-sized macro-pores and meso-micropores is very important and an urgent need for high performance of their various applications.14-17 This is because a mixture of micron-size and nano-size pores hold great promise for better coupling of the highly effective mass transfer with large functional surface area. Historically, several interesting methods have been developed for preparing hierarchically porous organosilica microspheres, including templates route,18,19 self-assembly,20-22 aerosol assist,23,24 and so on.25,26 Also, importantly , these methods are also able to further rationally adjust size, geometry, surface functionality of pores in struggling for the higher surface area and faster mass transfer. For example, Zhao's group proposed a strategy that combined the self-assembly of amphiphilic compounds and the templates route, successfully synthesizing a series of the different porous ordered structured silica nanoparticles.26 With easier and exquisite control of the ordered micro-mesopores by the molecular-level self-assembly, however, this strategy is difficult to obtain micron-sized pores that guaranteed more effective accessibility to 3


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the surface through the porous matrix. Gi-Ra Yi et al. used the large polystyrene (PS) beads as hard templates to synthesize an ordered macro-porous silica particles after removal of templates by burning out PS beads.27 But the removal of the sacrificial hard templates usually need etch (for inorganic beads) or calcination (for organic beads). These fierce manners for removing template not only yield increasing costs, but also readily cause collapse and/or deformation of the porous structure. Among these strategies, multi-emulsion droplets can serve as the ideal template for porous microspheres because of their intrinsic multi-compartment structure and easiness in elimination of templates.28-31 As for inorganic oxide porous particles prepared by the multi-emulsion template approach, researchers usually encounter three fundamental problems. Multi-emulsions are firstly thermodynamic instability system, which spontaneously tend to phase separation through different destabilization mechanisms, such as coalescence and Ostwald ripening. Particularly, the coalescence between the inner droplets very often produces single compartment (that is hollow) structure, and further cause the collapse of porous structural microspheres. Secondly, most of the precursors of oxide are reactive with water, which result in difficulty in preparing aqueous multi-emulsions. Thirdly, large amounts of alcohols produced in hydrolysis of metal alkoxides usually used precursors, and will destroy the multi-emulsions because of its tendency to mix both oil and water phase.32 Nevertheless, few effort has been made to acquire hierarchical silica microspheres by multi-emulsion droplets templating.33-35 Vilanova and co-workers prepared silicone multi-compartment micro-size particles by using polydimethylsiloxane as silica precursors and a mixture of two types of surfactant in a W1/O/W2 emulsion as template.36 Wu et al. reported multi-compartment silica microspheres of sub-100 nm size via sol-gel reaction of TEOS in W/O/W nano-emulsion created by ultrasonication using a rich variety of surfactants, oils, and solution conditions.37 However, the multi-emulsion template-based organosilica microspheres involved in previous reports exhibit the independent multi-compartment or

single

compartment

(hollow)

structures.

Actually,

the

interconnected

multi-compartment structure in microspheres originated from multi-emulsion 4


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templating is not reported till date. But the highly interconnected multi-compartment structure has superior performances for practical applications in sensing, catalysis, and adsorption. The fundamental cause is the uncontrolled stability of the O/W interface, especially in the continuously evolving system due to chemical reaction in water/oil phase. Therefore, it is still a great challenge to obtain the highly interconnected and multi-compartment porous organosilica microspheres by using a multi-emulsion template. The key point is how to accomplish the controllable and stable O/W interface. Liquid marbles, as a pioneer work proposed by Aussillous and Quéré in 2001, are aqueous liquid droplets encapsulated with some hydrophobic or super-hydrophobic solid particles.38,39 In view of their perfect non-wetting performance, they can behave like a soft solid moving quickly on solid substance and even water substance without any leakage.40,41 Therefore the liquid marbles have attracted increasing attention due to their potential applications in cosmetics, transport and microfluidics, miniature reactors, personal and health care products, sensors, accelerometers, and gas storage.42 Various super-hydrophobic particles, such as silica, graphene, Fe3O4, and polystyrene, were reported to be used for stabilization of the liquid marbles. Up to date, the silica-based stabilizer for formation of liquid marbles in previous reports, mainly include silica particles that must be post-synthesis modified by the fluorinated compounds. In this study, for the first time, the hydrophobic macro-mesoporous organosilica microsphere was successfully prepared with highly interconnected macro-mesopores by one-step W/O/W emulsion template. Fortunately, the marvelous stability of the interfacial films formed by the special structured stabilizer enables multi-chambers of emulsion to successfully transfer into the corresponding porous microspheres. Due to their high hydrophobicity, the as-prepared organosilica microspheres with and without Ag nanoparticles can also stabilize liquid marbles, wherein Ag composite organosilica microsphere-based liquid marbles show an excellent degradable property for methylene blue. This paper reports the details. 5


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2 EXPERIMENTAL SECTION 2.1. Materials 1, 8-Dibromooctane (98%) and sodium hydride (60% suspension in oil) were purchased from Alfa Aesar. Glycidol (96%) was purchased from Sigma-Aldrich. 15-Crown-5 ether (97%) was purchased from Xiya Reagent Co., Ltd. Dimethylamine (40% in water) was purchased from Aladdin Reagent Inc. Cholesterol (95%), potassium hydroxide (≥85.0%), tetrahydrofuran (≥99.0%), methanol (≥99.5%), hexane (≥97.0%), ethyl acetate (≥99.5%), dichloromethane (≥99.5%), petroleum ether (60-90

), acetonitrile(≥99.5%), hydrochloric acid (HCl, 36-38%), ammonia (NH4OH,

25-28%), ethanol (≥99.7%), sodium borohydride (NaBH4, 96%) and silver nitrate (AgNO3, ≥99.8%) were obtained from Sinopharm Chemical Reagent Co., Ltd. Vinyltriethoxysilane (VTEO, ≥98.0%), methylene blue (MB, ≥70.0%), and polyvinylpyrrolidone (PVP) were purchased from Tokyo Chemical Industry Co., Ltd. Water was used throughout secondary distillation and then adjusted to different pH by hydrochloric acid according to the experimental need. All chemicals, except those specially indicated, were used without further purification. 2.2 Preparation of W/O/W emulsions and HIMC-vinyl organosilica microspheres Preparation of the starting multi-emulsions. The starting W/O/W emulsion for this experiment was prepared by one-step direct emulsifying of the solution of stabilizer (Chol-OH) in VTEO with water, using a homogenizer (T10, IKA, Germany) set at 11400 rpm for 2 min. The stabilizer (Chol-OH), a low mass molecular amphiphilic compound, was synthesized according to our previous work.43 In a typical experiment, a given amount of Chol-OH was first dissolved in VTEO as oil phase, and then the resulting solution was homogenized with deionized water with different pH value. Moreover, effects of stabilizer concentration (varying from 0% to 25%), water content (varying from 80% to 95%), and pH value of the aqueous phase (varying from 1 to 3) on the stability and structure of multi-emulsion were systematically investigated, the selected dosages in all tried tests are summarized in Table 1S. The pH value of aqueous phase was controlled by adding different amount of 1 mol/L of HCl stocking 6


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solution. Unless stated otherwise, the concentration of Chol-OH was based on the oil phase (w/v), and the volume fraction of water was based on total volume (v/v). Preparation of HIMC-vinyl organosilica microspheres. The HIMC-vinyl organosilica microspheres were obtained after the hydrolysis and condensation (sol-gel) process of VTEO (oil phase), and followed by the washing and drying process. In a typical experiment, the initiating W/O/W emulsions were putted into an ammonia bath produced by a certain amount of concentrated ammonia water in a closed container at room temperature for 48 h. The ammonia gas volatilized from the ammonia bath would diffuse into the multi-emulsions and catalyse the sol-gel reaction of the oil phase (VTEO). And then, the completion of hydrolysis and condensation of VTEO yielded the HIMC-vinyl organosilica microspheres, which were then collected by centrifuged. After successive washes with ethanol and dichloromethane, these microspheres were dried at 35

overnight under vacuum. These as-prepared

organosilica microspheres were washed to make sure they are free of stabilizer and the regeneration of the stabilizer was realized by drying its ethanol and dichloromethane solution in a rotary evaporator. 2.3 Formation of HIMC-vinyl organosilica microspheres-based liquid marbles The liquid marbles were formed by rolling sessile water droplets onto the powder bed of as-prepared HIMC-vinyl organosilica microspheres with hydrophobicity. In all tried tests, the volume of the dispensed water droplets was kept at 10 µL using a micropipette. In addition, the water encapsulated in liquid marble was dyed by methylene blue (MB) for clearer observation than that of pure water. By gently rolling the solution of methylene blue on these organosilica microspheres bed, the liquid was then entirely encapsulated by some hydrophobic HIMC-vinyl organosilica microspheres, and finally the resultant light blue liquid marble was obtained. 2.4 Formation of Ag/HIMC-vinyl organosilica microspheres-based catalytic liquid marbles and their catalytic degradation of MB Formation of Ag/HIMC-vinyl organosilica microspheres-based catalytic liquid marbles was in the same way as the above-mentioned HIMC-vinyl organosilica microspheres-based liquid marbles, and the degradation of MB was carried out in 7


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as-prepared catalytic liquid marbles. The mixed solution of MB aqueous solution (4 mmol/L) and NaBH4 aqueous solution (0.4 mol/L) at volume ratio of 1:1 was first obtained. Then, four 10 µL droplets of the as-obtained mixed solution was successively dropped on these composited organosilica microspheres bed, subsequently forming the Ag/HIMC-vinyl organosilica microspheres-based catalytic liquid marbles by simple rolling. 5 µL internal solutions were, respectively extracted out from the four catalytic liquid marbles at the selected time intervals, and these 5 µL extracted internal solutions were diluted 110 times in water to measure the optical absorbance at wavelength of 664 nm (the characteristic absorption wavelength of MB solution) by a UV-vis spectrophotometer (Hitachi U-3900/3900H). In order to evaluate the catalytic efficiency of the catalytic liquid marbles, the degradation of MB by NaBH4 in the liquid marbles without Ag nanoparticles and in bare solution droplets without Ag/HIMC-vinyl organosilica microspheres were carried out as references, and the experimental conditions of all references are the same as the Ag/HIMC-vinyl organosilica microspheres-based catalytic liquid marbles. All catalytic experiments were repeated at least three times. 2.5 Characterization Confocal Laser Scanning Microscopy: The confocal microscopy images were taken on an Olympus Confocal Laser Scanning Microscopy microscope (FV1200). Before examination, Nile Red dye was first dissolved at a concentration of 1h10-3 M in VTEO which acted as oil phase prior to emulsification. Laser with wavelength of 488 nm was used to excite the Nile Red molecules dissolved in the oil phase. Optical micrographs: The measurements were taken on a Metallurgical Microscope (CEWEI, LW300LJT). Before examination, the W/O/W emulsions, without any further dilution, were placed on a slide glass holder, and then covered with a thin glass slide. SEM: The surface morphology structure of HIMC-vinyl organosilica microspheres were observed using a TM3030 scanning electron microscopy spectrometer at an accelerating voltage of 15 kV, specimens of these organosilica microspheres were not coated with Au before observation. 8


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FE-SEM: The internal pore structure of the sample was imaged to obtain the fracture surfaces of selected organosilica microspheres using a Hitachi, Ltd. SU8020 ﬁeld emission gun scanning electron microscope (FE-SEM, Zeiss UltraPlus Analytical). The measurement was performed under high vacuum. Prior to measurement, the sample was coated with a thin layer of platinum. TEM: The TEM images were obtained with a JEM-2100 at 100 kV after dispersing the sample in ethanol and deposition of the carbon grid. XRD: X-ray powder diffraction (D8 Advance, Bruker, Germany) was performed using Cu Kα radiation (30 kV, 15 mA). 7KH ORZ DQJOH ;5' SDWWHUQV ZHUH FROOHFWHG LQ WKH ORZ DQJOH UDQJH IURP

° WR ° of 2θ.

Contact-angle tests: The contact angles of as-prepared organosilica microspheres were measured using a Data physics OCA20 contact-angle system at ambient temperature. Prior to measurement, the organosilica microspheres were deposited into a thin sheet. BET: The surface area and pore volume analyses were performed on Micrometerics ASAP 2020 apparatus. The specific surface area of the sample was calculated based on the Brunauer-Emmett-Teller (BET) equation at P/P0 between 0.05 and 0.3. The total pore volume was the N2 adsorbed amount at a P/P0 of 0.99. The pore diameter was

calculated

from

the

branch

of

the

desorption

isotherm

using

a

Barrett-Joyner-Halenda (BJH) method. Before analysis, the samples were degassed at 200

.

3 RESULTS AND DISCUSSION The HIMC-vinyl organosilica microspheres were prepared in two continuous steps that include the formulation of a W/O/W emulsion and the sol-gel process in the oil phase of the multi-emulsions, which is schematically illustrated in Figure 1. After direct and one-step homogenization of the acidified deionized water (pH=1-3) and VTEO solution containing a stabilizer of Chol-OH, the stable W/O/W emulsions can 9


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Figure 1. An illustration of the one-step emulsification to prepare W/O/W emulsions and the fabrication procedures of highly interconnected macro-mesoporous and multi-compartment (HIMC) vinyl organosilica microspheres by using the W/O/W emulsion template.

be formed. When the hydrolysis and condensation of VTEO in W/O/W emulsions occurred until conclusion by introduction of NH3 for 48 h at room temperature, a HIMC-vinyl organosilica microspheres can be obtained after the simple washing and drying process. The achievement of HIMC structures can be made by a W/O/W emulsions template coupled with the classical sol-gel process, which is reasonably deduced by the utilization of a specially designed amphiphilic cholesterol derivative, Chol-OH. The interfacial film formed by self-assembly of Chol-OH at oil/water interface shows marvelous stability against occurrence of coalescence among droplets with the change of time and microenvironments during reaction. This is the reason for attainment of HIMC-vinyl organosilica microspheres, rather than single-compartment (hollow) microspheres. To further confirm the decisive role of the special interfacial film formed by Chol-OH, the similar W/O/W emulsion containing the totally similar formulation, except for complex surfactants of Tween 80 and Span 80, can be chosen as a control example. Although it is demonstrated that the system emulsified by combination of Tween 80 and Span 80 shows good stability, W/O/W emulsion 10


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droplets were destroyed with the sol-gel processing of VTEO, there was no achievement of the associated HIMC-vinyl organosilica microspheres (Figure S1, see the ESI†). 3.1 Formation and formulation of the W/O/W emulsions Unlike the traditional two-step homogenizing method inevitably accompanied by two different types of surfactants,35,36 the multi-emulsions in this study can be generated through a one-step homogenization of the suitable water and oil phase and by using one single stabilizer, Chol-OH. The simplicity of the prepared procedures is favorable to adjust the internal structures of the resulting porous materials. Optimization of the concentration of stabilizer, the volume fraction of water, and the pH value of the aqueous phase were systematically investigated in the system (Table 1S). It was demonstrated that systems with less than 3 of pH value in aqueous phase and 80% or

Figure 2. An example of W/O/W emulsion by one-step emulsification with Chol-OH as the stabilizer (25% of Chol-OH, 90% of water, pH=2). (a) A digital picture; (b) Optical microscopic image; (c) Confocal fluorescence image. For confocal fluorescence image, the oil phase, vinyltriethoxysilane (VTEO), was labeled by addition of Nile Red (1×10-3 M) acted as fluorescent probe.

more volume fraction of water exist as the stable multi-emulsions. Taking multi-emulsions with 90% of aqueous phase and 25% of stabilizer as an example, Figure 2a clearly shows that there was no obvious phase separation observed, indicating the formation of stable emulsion. Furthermore, the microscopic image shows a clear multi-compartment structure of emulsion droplets (Figure 2b), signifying that the multi-emulsions were formed. The type of W/O/W emulsions was 11


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then investigated by a fluorescent microscopy measurement, as shown in Figure 2c. From this figure, it can clearly be observed that unlabeled lots of close-packed water droplets (black parts) dispersed in the red labeled oil droplets, indicating that a water-in-oil-in-water type of multi-emulsion was formed. 3.2 Preparation and structure of HIMC-vinyl organosilica microspheres Generally, the sol-gel reaction of the VTEO as oil phase was initiated by putting the system under an ammonia atmosphere. The W/O/W emulsion was kept in this atmosphere for 48 h to ensure complete reaction, then resulting in the generation of the porous organosilica skeleton, which lays the foundation for further obtaining the expected porous vinly organosilica microspheres. The parameters including pH value of aqueous phase, the concentration of stabilizer and the water volume fraction of W/O/W emulsion will affect their formation, structure and stability. Correspondingly, these are also critical factors of surface morphology and internal porous properties of the resulting HIMC-vinyl organosilica porous microspheres. Thus, effects of the pH value, the volume fraction of water phase, and the concentration of stabilizer on the preparation of the HIMC-vinyl organosilica microspheres were discussed. All attempts were to determine the correlations between behaviors of W/O/W emulsion and structural performances of HIMC-vinyl organosilica microspheres.

Figure 3. SEM images of HIMC-vinyl organosilica microspheres with different water contents under the same pH value of aqueous phase (pH=1) and 15 % stabilizer: (a) 80%, (b) 90%, (c) 95%.

As is well known, W/O ratio plays a critical role in stability and structure of W/O/W emulsion, accordingly influencing the internal porous structures of HIMC-vinyl organosilica microspheres, such as size, size distribution, morphology of pores. Herein, effect of W/O ratio on structures of HIMC-vinyl organosilica microspheres is 12


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discussed by varying the volume fraction of water from 80% to 95% under the conditions of pH=1 and 15 % stabilizer. Figure 3 shows SEM images of HIMC-vinyl SMs originated from the starting W/O/W emulsions with different water contents. As can be seen from Figure 3, whatever the water content is, the mean size of organosilica microspheres is about 2 µm (Nano Measurer 1.2, Figure S2) despite the representation of large differences in the surface morphologies of HIMC-vinyl SM. It is noted that the amount of pore cavities on the surface of HIMC-vinyl SM shows a dramatic increase with increasing water content from 80% to 95%. The higher water content results in an increase in the small water droplets in the big oil droplets due to an increase of osmotic pressure between the internal and external phases.44,45 Meanwhile the oil/water interfacial area was increased, under the given stabilizer concentration in the starting W/O/W emulsions, which means a progressive reduction of stabilizer concentration on unit area. This is easier to cause the instability of oil/water interface, and later the emulsions experience the phase separation at some micro zones of oil/water interface as reaction of oil phase proceeded, which finally creates a lot of pore cavities.46 Furthermore, when the water content is further increased to 95%, the organosilica particles exhibit non-spherical shape structure, no observation of significant increase in size, as shown in Figure 3c. Actually, it is regular rule to the remarkable increase in size of particles or pores compared with the size of starting emulsion droplets. This phenomenon is attributed to coalescence among emulsion droplets in the emulsion-templated technique for synthesizing materials47,48 because of inherently metastable property of emulsions.49,50 Surprisingly, no increase in size of microspheres in this work was observed in corresponding system with 95% water content compared with ones with 80% and 90% water content. While the appearance of non-spherical particles implied the occurrence of the very limited flocculation and non-full coalescence of the internal oil droplets with each other in multi-emulsions. This is reasoned by the utilization of a specially designed stabilizer, which is a derivative of cholesterol. It is known that cholesteryl derivatives stand for a typical class of LMMGs because the cholesteryl moiety has a strong tendency to form 13


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ordered aggregation via inter-molecular van der Waals forces.51,52 Therefore, it is believed that the molecules of Chol-OH stayed at the oil/water interface region self-assemble into a strongly interacted interface film. The resulting interface possess the super-stable property, efficiently resisting fully and fast coalescence of droplets. Subsequently, the solidification of oil droplets can be realized by completing of the sol-gel process, therefore irregular particles as shown in Figure 3c are fabricated. In brief, adjustment of the stability of oil/water interface by designing a special stabilizer is an effective method for controlling the morphology and internal structure of porous materials. Based on the above results, the following investigation is focused on the W/O/W emulsions with 90% water content. The pH value could affect the ionization equilibrium of quaternary amine, and thus influence the emulsifying ability of Chol-OH. It is found that the stable W/O/W emulsions can be formed at the pH value varied from 1 to 3 under the same water content (90%) and concentration of stabilizer (15%). It was also noted that the morphology and size of HIMC-vinyl SM depended greatly on the pH value as shown in Figure 4. Clearly, the degree of protonation of Chol-OH will affect stability and morphology of the starting W/O/W emulsions and further affect the porous structures of the resulting organosilica microspheres. Specially, the size of microspheres

Figure 4. SEM images of HIMC-vinyl organosilica microspheres with different pH value of aqueous phase under the same 90% of water and 15% of stabilizer: (a) pH=1, (b) pH=2, (c) pH=3.

increases from about 2 µm to 4 µm (Nano Measurer 1.2, Figure S3), with increasing pH value from 1 to 2. When the pH value is equal to 1, the Chol-OH is fully acidified, and it prefers to stay at oil/water interface than in oil phase, causing more interfacial 14


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area and finally generating plenty of small multi-emulsion droplets. After the sol-gel processing, the smaller droplets are converted into the smaller HIMC-vinyl SMs, as shown in Figure 4a. When the concentration of hydrochloric acid in aqueous solution is increased to 1 mol/L, which is 10 times the case of pH=1, W/O/W emulsions were destroyed. Therefore the porous microspheres were not observed in SEM image (Figure S4). Whereas, as shown in Figure 4b, the introduction of water at pH=2 into starting emulsion lead to the increase in size of the resulting HIMC-vinyl SMs. The reason is that the less protonated Chol-OH molecules at the oil/water interface cause the decrease of interfacial area, and finally producing the bigger emulsion droplets. When the pH value increases to 3, coexistence of microspheres and monolith can be seen in Figure 4c, clearly suggesting that W/O/W emulsions experience partly phase inversion, and then the trend is greatly increased when the pH value is increased to 4, which is confirmed by the appearance of more monoliths and fewer microspheres as shown in Figure S5. W/O/W emulsions can be formed by simple emulsifying water and VTEO using a single stabilizer Chol-OH, which means that Chol-OH is able to simultaneously stabilize both the W/O and O/W interface. Therefore distribution of Chol-OH at the W/O and O/W interfaces is greatly important for the stability of multi-emulsions, and then concentration of Chol-OH will further inevitably affect the behaviors of W/O/W emulsions. Effects of concentration of Chol-OH on the formation and structures of W/O/W emulsions have been investigated under the constant values of pH=2 and 90% of water content. It has been found that the increase of concentration of Chol-OH resulted in significant increases of droplet size of multi-emulsions and also amount of internal droplets contained in oil droplets (Figure S6). This is because more stabilizer molecules generate more interfacial area stabilized by Chol-OH. Clearly, changes mentioned above in the structures of W/O/W emulsions would subsequently yield the large difference in morphology and structures of the resulting organosilica microspheres. Figure 5a-5d shows the SEM images of as-synthesized organosilica microspheres with the Chol-OH concentration ranging from 0% to 25%. The systems without Chol-OH acting as a control was also investigated, and solid microspheres 15


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Figure 5. SEM images (a-d) of HIMC-vinyl organosilica microspheres with different concentrations of stabilizer under the same water content (90%) and pH value of water (pH=2): (a) 0%, (b) 5%, (c) 15%, (d) 25%; N2 adsorption-desorption isotherms (e-f) and corresponding pore distributions (h-i) of the corresponding HIMC-vinyl organosilica microspheres (b-d): (e, h) 5%, (f, i) 15%, (g, j) 25%.

without any pore cavities on the surface are fabricated (Figure 5a). When 5% Chol-OH was used, a small amount of HIMC-vinyl SMs can be observed as show in Figure 5b, suggesting that Chol-OH is absolutely necessary for the generation of porous structures of organosilica microspheres. It is well believed that the pore cavity on the surface of microspheres is caused by the phase inversion process during the sol-gel reaction of oil phase.46 When Chol-OH concentration is equal to or more than 15%, all the microspheres almost exhibit plenty of pore cavities on the surface, while more Chol-OH resulted in more pore cavities (Figure 5c and 5d). Another marked characteristic is a dramatic increase in mean size of microspheres with range from about 2 µm to 6 µm from Figure 5b to Figure 5d. The increase of amount of pore cavities and size could be attributed to the growing amount of the internal water droplets in the oil droplets due to the more interfacial areas with increasing Chol-OH 16


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concentration.30,36 Further tests were carried out to confirm the effect of stabilizer concentration on porous structure of microspheres. As for two systems at pH=2 with 95% of water content and at pH=1 with 90% of water content, increase of Chol-OH concentration from 5% to 25% caused the significant growth of the amount of the pore cavities on the surface (Figure S7). It is very general that more stabilizer cause more amount of pore cavities. Effects of stabilizer concentration on porous structures of HIMC-vinyl SMs were further studied using N2 adsorption-desorption measurements at 77K. Figure 5e-5g demonstrates the typical nitrogen adsorption/desorption isotherms of HIMC-vinyl SMs by measuring the corresponding samples from Figure 5b-5d. The N2 adsorption-desorption isotherms of three HIMC-vinyl SMs show typical type IV curves with pronounced capillary condensation steps and a hysteresis loop with H3 plus H4 in the middle P/P0 range of 0.5-0.9. These properties reveal the presence of meso-pores with narrow size distribution and slit-shaped pores.53 The pore size with a narrow distribution is about 4 nm, as shown in Figure 5h-5j. No evident adsorption is observed in the isotherm of the solid particles (Figure S8), which indicate the absence of any pores. All isotherms for three HIMC-vinyl SMs show rapid N2 uptake at low relative pressures (P/P0

One-Step Synthesis of Hydrophobic ... - ACS Publications

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