O Gel-Emulsions and Their Application in the

May 31, 2012 - thin films of the continuous phase.3,4 Gel-emulsions are also referred to .... GE. aG = turbid gel, S = solution, GE = gel-emulsion. La...
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Preparation of Novel W/O Gel-Emulsions and Their Application in the Preparation of Low-Density Materials Xiangli Chen, Kaiqiang Liu, Panli He, Helan Zhang, and Yu Fang* Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education), School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an, 710062, China S Supporting Information *

ABSTRACT: A series of novel and stable water in oil (W/O) gel-emulsions was created by utilizing a new cholesteryl derivative, a low-molecular mass gelator (LMMGs), as a stabilizer. In the emulsions, n-heptane, n-octane, n-nonane, ndecane, tertiary butyl methacrylate (t-BMA), methyl methacrylate (MMA), or styrene can be used as a continuous phase, water as a dispersed phase, and the stabilizer in the continuous phase is only 2% (w/v). Importantly, the gel-emulsions could be prepared by simple agitation of the mixtures at room temperature, while heating, cooling, and addition of a cosolvent or other additional component are unnecessary. SEM and optical microscopy studies revealed the foam-like structures of the gel-emulsions. Rheological measurements demonstrated that the gelemulsions are mechanically stable and exhibit typical viscoelastic properties. Surprisingly, the storage modulus, G′, and the yield stress of the gel-emulsions with the alkanes as continuous phase decrease along with increasing the volume ratio of the dispersed phase, water, a property different from those of conventional gel-emulsions reported in the literature. From the viewpoint of application, the gel-emulsions as prepared are superior to others due to their simplicity in preparation, less amount of stabilizer needed, and the nonionic nature of the stabilizer, which must benefit practical applications. Furthermore, porous polymer monoliths could be prepared by polymerizing gel-emulsions with organic monomers as a continuous phase.

1. INTRODUCTION Gel-emulsions are two-phase systems, of which one is the internal or dispersed phase, and another is the continuous phase. Unlike routine emulsions, gel-emulsions possess typical rheological properties of physical gels. Generally speaking, for gel-emulsions, the volume fraction of the internalphase is exceeding 0.74,1,2 geometrically a critical value for a container to be fully filled by nondeformed balls, resulting in deformation of the droplets of the dispersed phase, which are separated by thin films of the continuous phase.3,4 Gel-emulsions are also referred to as high internal phase emulsions (HIPEs), highly concentrated emulsions, biliquid foams, and adhesive emulsions, etc. Practically, gel-emulsions have been widely used in food,5,6 cosmetics,7,8 medicine,9,10 chemical industry,11,12 and as templates for the preparation of various materials with porous structures and so on.13−18 Because of the fascinating properties and the widespread uses, creation of new gel-emulsions and extension of their applications have become a hot point of soft matter research. Basically, gel-emulsions can be classified into two categories: oil-in-water (O/W) and water-in-oil (W/O). Stabilizers used for the stabilization of the gel-emulsions could be surfactants, micro-/nanoparticles, and possibly low-molecular mass gelators (LMMGs), in particular, cholesteryl derivatives as reported by our group.19 As stabilizers, surfactants are commonly used but they are not very efficient, and 5−50% (w/v) of the volume of the continuous phase is a normal practice. As for micro-/ © 2012 American Chemical Society

nanoparticles, gel-emulsions stabilized by them may suffer from phase inversion when the volume fraction of the dispersed phase reaches 0.65−0.70. In contrast, cholesteryl derivatives are much more efficient, and 2% (w/v) or even less than that is enough to produce a gel-emulsion with good quality. Additionally, gel-emulsions created by them possess superior rheological properties,19 which may lay the foundation for their real-life applications. As an important class of LMMGs, cholesteryl derivatives have been extensively studied for more than two decades, and even today they still remain as an actively investigated class of compounds.20−22 For example, recently, we reported a few gel systems, which form at room temperature, with cholesteryl derivatives as gelators, and, furthermore, some of the gelators gel oil from oil−water mixtures selectively and efficiently.23−25 It is to be noted that gel-emulsions created by using LMMGs as stabilizers may be formed and present in a way different from routine stabilizers, such as surfactants, micro-/nanoparticles, etc. In the LMMGs-containing gel-emulsions, the continuous phase may exist in a gel state, and the dispersed phase may be physically trapped in the gel. However, not all of the organic gels can be employed as continuous phases, and only those meeting at least the following requirements, first the organic Received: March 1, 2012 Revised: April 20, 2012 Published: May 31, 2012 9275

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concentration of 2% (w/v) were tested, and the positive results are shown in Table 1.

solvent is immiscible with water, second the gelator is insoluble in water, and third the interface energy must be low, may form gel-emulsions with water. Clearly, these gel-emulsions are stabilized due to gelation of the continuous phases, rather than crowding of the beads of dispersed phases. It is for this reason that the volume fraction of the dispersed phase in a LMMGsbased gel-emulsion may not necessarily exceed 74%, which extends the fraction range of gel-emulsions. No doubt, this extension must broaden the applications of the gel-emulsions. Two years ago, we reported for the first time that a cholesteryl derivative, butane-1,4-dicarboxamide of dicholesteryl L-alaninate, gels n-heptane, n-octane, n-nonane, n-decane, kerosene, diesel, and petrol spontaneously at room temperature, and introduction of water into the systems results in gelemulsions.19 Recently, to create molecular gels of redoxresponsive properties, we designed another series of dicholesteryl derivatives, of which ferrocene (Fc) was intentionally introduced as a pendant group of the linkers of the cholesteryl derivatives. It is our fortune that during the studies, we found accidentally that one of the dicholesteryl derivatives functions as a very efficient stabilizer of some W/O gelemulsions of n-alkanes with a suitable length of carbon chain (7 ≤ n ≤ 10), a phenomenon similar to that reported before. However, unlike the cholesteryl derivative reported before, the present one can be also used as an efficient stabilizer to create some water/monomer gel-emulsions. This Article reports the preparation, the properties of the LMMGs-based gel-emulsions, and their template application in the preparation of low-density polymeric materials.

Table 1. Gelation Properties of Compound 1 in Various Solvents (2%, w/v)a

a

solvents

results

mixture of solvents (v/v)

results

n-heptane n-octane n-nonane n-decane t-BMA MMA styrene

G G G G S S S

n-heptane:H2O = 2:8 n-octane:H2O = 2:8 n-nonane:H2O = 2:8 n-decane:H2O = 2:8 t-BMA:H2O = 2:8 MMA:H2O = 2:8 styrene:H2O = 2:8

GE GE GE GE GE GE GE

G = turbid gel, S = solution, GE = gel-emulsion.

It is to be noted that shaking of the two-phase system results in gelemulsion, but phase-selective gelation is seen when the system is left alone. Furthermore, conversion of phase-selective gelation to gelemulsion or gel-emulsion to phase-selective gelation can be realized via heating plus agitation or heating plus cooling, as shown in Figure 1. It is also worthy of mention that the volume fraction of the dispersed phase could be as high as 98% (v/v). 2.3. SEM Observation. SEM images of the xerogels of the gelemulsions were taken on a Quanta 200 scanning electron microscopy spectrometer (Philips-FEI). The accelerating voltage was 20 kV, and the emission was 10 mA. The xerogels were prepared by freezing the gel-emulsions in liquid nitrogen, and then evaporated by a vacuum pump for 12−24 h. Before examination, the xerogels were attached to a copper holder by using conductive adhesive tape, and then coated with a thin layer of gold. 2.4. Optical Microscopy Observation. Optical microscopic images were taken on a Lecia-DMRX optical microscopy spectrometer. The samples used for this measurement were the same as those used for the preparation of the xerogels, which were employed for the SEM measurement. 2.5. Confocal Laser Scanning Fluorescence Microscopy Observation. Confocal laser scanning fluorescence microscopic pictures of the gel emulsions were taken on a TCS SP5 laser scanning confocal microscopy spectrometer. The fluorescent probe with the same structure as that shown in the Supporting Information (cf., Scheme S1) was employed for the measurements, and the probe was synthesized in our group. The excitation and emission wavelengths employed are 459 and 525 nm, respectively. 2.6. Rheological Measurements. Rheological measurements were carried out with a stress-controlled rheometer (TA Instruments AR-G2) equipped with steel-coated parallel-plate geometry (20 mm diameter). The gap distance was fixed at 1000 μm. A solvent-trapping device was placed above the plate to avoid evaporation. All measurements were conducted at ambient temperature (25 °C). First, a stress sweep measurement at fixed frequency was conducted, which provides information about the mechanical strength of the gel sample. Second, a frequency sweep was conducted from 0.0628 to 628 rad/s at a constant stress of 1.0 Pa that results in small strain, well within the linear regime. 2.7. X-ray Diffraction (XRD) Measurements. The XRD measurements of the xerogel of the gel-emulsion were conducted on a Japan Rigaku D/max-III diffractometer with Cu Kα X-ray generated (λ = 1.5418 Å) under a voltage of 40 kV and a current of 40 mA. The scan rate was 1°/min. The xerogel was prepared by letting a wet gel on a glass slide evaporate spontaneously for 2 weeks.

2. MATERIALS AND METHODS 2.1. Materials. The stabilizer (1), which is a dicholesteryl derivative with a ferrocenyl structure appended to its linker (cf.,

Scheme 1. Molecular Structure of the Stabilizer (1)

Scheme 1), was prepared and characterized in the way as reported before.26 Other reagents were obtained from Sinopharm Chemical Reagent Co. Ltd.. All solvents used for preparation of the gel-emulsion and for its further application were of analytical grade and were used without further purification. Water used throughout was doubly distilled. 2.2. Preparation of the Gel-Emulsion. In a typical preparation, 0.0040 g of the compound was added to a mixture of 0.2 mL of one of the alkyl solvents (n-heptane, n-octane, n-nonane, n-decane) or one of the monomers (tertiary butyl methacrylate (t-BMA), methyl methacrylate (MMA), styrene), and 0.8 mL of water at room temperature, then the resulting mixture was stirred and slightly heated if necessary to make sure that the compound is dissolved completely, and then the mixture was vigorously shaken by hand for 2 min; finally, a viscous and light yellow emulsion was produced. Formation of gelemulsion was confirmed by inverting the test tube to observe if the mixture inside could still flow. A positive result is obtained when the mixture inside shows no fluidity. In this way, the gelation behaviors of the compound in some alkyl solvents and organic monomers at a

3. RESULTS AND DISCUSSION 3.1. Gelation Behaviors of the Compound. Examination of the results from the gelation test shown in Table 1 reveals that the compound gels the four alkyl solvents tested, but cannot gel any of the monomers. However, the compound gels 9276

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Figure 1. Schematic representation of the preparation of the gel-emulsion of 1/water/n-decane and comparison with selective gelation.

Figure 2. Phase behavior of the system of 1/water/n-decane with different water content.

gel the monomers tested at room temperature; (2) but it gels mixtures of t-BMA and water provided the water content in the mixture exceeds 74% (v/v), a typical and conventional gelemulsion. From the differences in the phase behavior of the two systems, it should not be difficult to make a tentative conclusion that the formation mechanism of the two gelemulsions is different. For 1/water/n-decane, the continuous phase should be the organic solvent, and it would have been gelled throughout the whole composition range. It can be imagined that it is the gel that might physically encompass the immiscible solvent, water, the dispersed phase, and makes the system lose fluidity. However, for 1/water/t-BMA, it may belong to a conventional gel-emulsion as it forms only when the volume fraction of the dispersed phase exceeds the critical value of 74%, and the continuous phase, the monomer, cannot be gelled by the compound. Of course, the structures and formation mechanisms of the gel-emulsions need to be confirmed. 3.2. Microscopy Studies of the Gel-Emulsions. The gelemulsions were used directly for optical microscopic observations. Some typical results are shown in Figure 4. With reference to the images, it is seen that each of the two images is significantly composed of two phases, of which one is continuous, the continuous phase, and the other is discontinuous, the dispersed phase, indicating that both systems possess characteristics of gel-emulsions. From further interrogation of the images, it is revealed that the sizes of the compartments of the dispersed phase vary from less than 200 μm to a few hundred micrometers in diameter, a result similar to other gel-emulsions reported in the literature.27−31 This observation is further confirmed by fluorescence confocal microscopy measurements (cf., Figure S1), and the fluorescence from the probe shows clearly and definitely that it is the

the mixture of water and anyone of them at room temperature provided the volume ratios of them are reasonable. Considering the volume ratio of water in the mixture solvents exceeds 74%,1,2 a critical value for gel-emulsion, and the fact that water is immiscible with the solvents under study, the mixture gels formed should be gel-emulsions. To investigate the effect of water content on the phase behavior of the gel-emulsions, 1/ water/n-decane and 1/water/t-BMA were adopted as example systems. Some of the results are shown in Figures 2 and 3, respectively.

Figure 3. Phase behavior of the system of 1/water/t-BMA with different water content.

Reference to the data and the results shown in Figure 2 reveals: (1) The compound gels n-decane straightforward at room temperature. (2) For mixture solvents of water content from 30% to 60% (v/v), the compound still gels them, but macroscopically the gels are not homogeneous. The gel becomes one phase when the water content in the mixture reaches 90% as evidenced by the picture shown in the figure. (3) It is to be noted that the compound gels any of the mixtures of water/n-decane provided the water content does not exceed 98% (v/v). (4) As mentioned already, the compound is insoluble in pure water. Similarly, reference to the data and the results shown in Figure 3 reveals: (1) 1 cannot 9277

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of which the water content is 80% (v/v). It is clearly seen that both traces are characterized by three reflection peaks. The obtained spacings for the pure gel are 4.39, 2.18, and 1.45 nm, respectively. Within a tolerable error range, the results exactly follow the ratio of 1:1/2:1/3 (cf., Figure 6 a), and the first spacing is very close to the stretched length of 1 as modeled by molecular dynamics (MD) simulation, indicating that the structures of gelator networks existing in the gel or gelemulsions are results of aggregation of this fundamental structure. In comparison, the spacings, the d values, for the xerogel of the 1/water/n-decane gel-emulsion are 4.77, 2.33, and 1.57 nm, respectively, of which the ratio among them is very close to 1:1/2:1/3, indicating that in this case the compound assembled in a way similar to that in the pure gel system. Again, the interlayer distance within the aggregates possessing lamellar structure is close to the length of the quasi dicholesterol structure (Figure 6b). No doubt, this is in support of the result from SEM measurements. 3.4. Rheological Studies. As it is well-known, within a conventional gel-emulsion, the dispersed phase exists in atactic polyhedrons rather than perfect spheres, which may result in more specific rheological properties. Therefore, rheological measurements at stress sweep and frequency sweep mode were conducted on samples of different water to n-decane ratios. It is to be noted that for all of the samples under study, the concentration of the gelator was kept at 2.0% (w/v) of the oil phase of the gel-emulsions. The stress sweep measurements were conducted by measuring storage modulus (G′) and loss storage (G″) of the gel-emulsions as functions of the oscillating shear stress, and the results are shown in Figure 7a. It can be seen from the figure that for all of the gel-emulsions under study, G′ is greater than its corresponding G″ at low stress values, indicating that the gel-emulsions exhibit viscoelastic property dominated by elasticity, like typical molecular gels. It is also seen that for a given sample both G′ and G″ remain constant below a critical stress value referring to the dynamic yield stress of the gelemulsion. However, both G′ and G″ decrease abruptly for each of the three samples once the stress exceeds the corresponding critical value. The sharp decrease in the G′ and G″ values is an indication of breakup of the gel network structure. Further reference to the figure reveals that G′ increases from ∼160.0 to ∼2650.0 Pa along with decreasing water content from 95% to 75% (v/v), and, correspondingly, the yield stress increases from ∼25.0 to ∼50.0 Pa, suggesting that both the stability of the network and the elasticity of the gel-emulsions well depend upon the water content in them. However, this is a result very different from that reported in the literatures. For conventional gel-emulsions, both G′ and the yield stress of a system increase rather than decrease along with increasing the content of the dispersed phase.32−36 This unusual result may suggest that the mechanical strengths of the gel-emulsions as studied in this research are determined by the mechanical strength of the continuous phase, in support of the assumption that the continuous phase is in a gel state. This observation may be rationalized by considering the fact that with increasing the ratio of the dispersed phase in the system, the bulkheads, which are the gel phase, between the dispersed droplets become thinner and thinner, and thereby their mechanical strength must decrease, resulting in lower storage modules and lower yield stresses. The results from frequency sweep measurements are shown in Figure 7b. G′ remains larger than G″, and there is almost no

Figure 4. Optical micrographs of the systems of gel-emulsions (80% of water in mixture solvents, v/v): (a) 1/water/n-decane; (b) 1/water/tBMA.

oil that is the continuous phase. From careful examination of Figure 4b, it can be also seen that the continuous phase gradually infiltrates into the dispersed phase as reflected by the pale color of the borders of the dispersed phase. It is to be noted that the microstructures of the gel-emulsions could be adjusted by varying the composition of the system, the preparation process, or the preparation condition (cf., Figures S2−S4). To gain a deeper understanding of the microstructures of the gel-emulsions and the functions of the stabilizer, the structures of the aggregates of the stabilizer in the gel-emulsions were studied. For the system of 1/water/n-decane, gel-emulsions of water contents of 80%, 90%, and 95% (v/v) were specially studied because the emulsions with water contents lower than 80% are not homogeneous. The xerogels from these systems were carefully prepared in a way as described in the experimental section, and adopted for SEM measurements. The stabilizer in the systems is always 2% (w/v) of the volume of the oil phase. Figure 5 depicts some typical results.

Figure 5. SEM images of the gel networks of different water content in n-decane: (a) 80% (v/v), (b) 90% (v/v), (c) 95% (v/v), and (d) 0% (v/v).

In reference to the images, it is seen that morphologically they are similar to each other, strong evidence to support the guess that the organic phase in the gel-emulsion was gelled. This similarity was further confirmed by the result from XRD analysis. 3.3. XRD Analysis. Figure 6 depicts the XRD traces of the xerogel of the pure 1/n-decane gel and that of the gel-emulsion, 9278

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Figure 6. XRD traces of the xerogels of the pure gel (a) and that of the gel-emulsion (b).

Figure 7. Evolution of G′ and G″ as functions of the applied shear stress at different water content of the gel-emulsions of 1/water/n-decane (a), and evolution of G′ and G″ of the gel-emulsion with different water content as functions of frequency (b).

crossover between the storage modulus and the modulus within a range of frequencies from 0.0628 to 300 rad/s, indicating that there are permanent physical junctions (i.e., they have long lifetimes) in the gel emulsions in the frequency range. The dumping factors (DF = G″/G′) of the gel emulsions remain less than 1, further supporting that the networks of the gel emulsions exhibit solid-like viscoelastic properties over the frequency range below 300 rad/s. Furthermore, the G′ values of the gel-emulsions at different water content exhibit weak dependence on the frequencies below 300 rad s−1, suggesting strongly that the gel-emulsions under study show good tolerance to external forces at the frequency range examined. However, the G′ values of the gel emulsions start to flocculate when the frequency reaches to 300 rad s−1, indicating that the elasticity of the gel emulsions is sensitive to rapid movement and the physical junctions in the gel networks start to break at this critical frequency, specifically, following the equation of |η*| = [(G′(ω)2 + G″(ω)2)/ω2]1/2,37 where |η*| stands for the complex viscosity, ω is the angel frequency, and the meanings of other symbols are the same as discussed earlier. Figure 8 shows the dependences of the complex viscosities of the gel emulsions on angle frequencies adopted in the frequency sweep measurements. Examination of the data shown in the figure indicates that the gel emulsions exhibit similar shear thinning behavior, which will bring convenience to real-life applications of the systems. 3.5. Template Application. One of the gel-emulsions, 1/ water/t-BMA, has been used as a template for the preparation of low-density materials. During the preparation, an initiator,

Figure 8. Complex viscosities of gel emulsions at different water content as functions of angle frequency.

azodiiso-butyronitrile (AIBN), and a cross-linker, divinylbenzene (DVB), were introduced into a mixture of water and the monomer, t-BMA, then the mixture was thoroughly degassed, and then vigorously stirred to prepare gel-emulsion. After that, the system was heated to 40 °C for 4 h to start prepolymerization, and then the temperature was increased to 80 °C for 20 h to finish polymerization. Finally, a polymer monolith with water inside was obtained. The monolith as obtained was thoroughly washed with plenty of methanol, and finally naturally dried in air. Figure 9 shows an example product as obtained, of which the density is lower than 0.2 g/cm3. It is to be noted that this porous monolith can be also used as a template in the preparation of porous inorganic−organic 9279

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Province (2010ZDKG-89) for financial support. This work is also supported by the “Program for Changjiang Scholars and Innovative Research Team in University” of China (IRT1070). We are indebted to Mrs. Bing Cao (School of Life Sciences, Shannxi Normal University) for her assistance with confocal laser scanning fluorescence microscopy measurements.

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composite monolith with superior mechanical strength and gas permeability, a result that will be reported in a separate paper.

4. CONCLUSIONS In summary, 1 is an efficient gelator for n-heptane, n-octane, nnonane, and n-decane, and in addition it is also an efficient stabilizer for gel-emulsions of water/n-decane, water/t-BMA, water/MMA, and water/styrene at room temperature. For the system of 1/water/n-decane, the maximum volume ratio of water (dispersed phase) in the system could be 98% (v/v). Optical microscopy measurements confirmed the foam-like structure of the gel-emulsions. SEM observation and XRD analysis revealed that the gelator assembled in a similar way in its pure n-decane gel and in its water/n-decane gel-emulsion. Rheological measurements demonstrated that both the 1/ water/n-decane gel-emulsion and the 1/n-decane gel possess good mechanical stability and exhibit typical viscoelastic properties. Unlike conventional gel-emulsions, the LMMGsbased gel-emulsions, at least the examples reported in this work, possess a number of superiors. For example, the content of the stabilizer in a gel-emulsion can be notably lower than that when conventional surfactant or micro-/nanoparticles are used as stabilizers, and the preparation process could be very simple. No doubt, these characteristics must favor their practical applications. Furthermore, salt resistance property will be possessed by a gel-emulsion if a neutral LMMGs, as the one reported in this article, is adopted as a stabilizer. This will definitely add additional values to the applications of the gelemulsions. Compound 1 has also been utilized as a stabilizer for the preparation of some gel-emulsions of water in some monomers, and a porous polymer monolith was successfully prepared by polymerization of the gel-emulsion of 1/water/tBMA with divinyl-benzene as a cross-linker.

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AUTHOR INFORMATION

REFERENCES

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Figure 9. Image of a LMMGs-based water in (t-butyl methacrylate) gel-emulsion (a), a low-density polymer monolith (b) prepared by polymerization of the gel-emulsion with divinylbenzene as a crosslinker, and the SEM image of the porous structure of the monolith (c).

S Supporting Information *

Additional figures and scheme. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*Tel.: 0086-29-81530788. Fax: 0086-29-85310097. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Natural Science Foundation of China (20902055, 20927001, and 91027017) and the 13115 Project of Shaanxi 9280

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dx.doi.org/10.1021/la300856h | Langmuir 2012, 28, 9275−9281