Crystallization Behaviors of n-Octadecane in Confined Space

Sep 25, 2008 - In this paper, the confined crystallization and phase transition behaviors of n-octadecane in microcapsules with a diameter of about 3 ...
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J. Phys. Chem. B 2008, 112, 13310–13315

Crystallization Behaviors of n-Octadecane in Confined Space: Crossover of Rotator Phase from Transient to Metastable Induced by Surface Freezing Baoquan Xie,†,§ Guoming Liu,†,§ Shichun Jiang,‡ Ying Zhao,† and Dujin Wang*,† Beijing National Laboratory for Molecular Sciences, Key Laboratory of Engineering Plastics, Joint Laboratory for Polymer Science and Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China, State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China, and Graduate School of Chinese Academy of Sciences, Beijing 100190, China ReceiVed: December 30, 2007; ReVised Manuscript ReceiVed: August 17, 2008

In this paper, the confined crystallization and phase transition behaviors of n-octadecane in microcapsules with a diameter of about 3 µm were studied with the combination of differential scanning calorimetry (DSC), temperature dependent Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The main discovery is that the microencapsulated n-octadecane crystallizes into a stable triclinic phase via a mestastable rotator phase (RI), which emerges as a transient state for the bulk n-octadecane and is difficult to be detected by the commonly used characterization methods. As evident from the DSC measurement, a surface freezing monolayer, which is formed at the interface between the microcapsule inner wall and n-octadecane, induces the crossover of the RI from transient to metastable. We argue that the existence of the surface freezing monolayer decreases the nucleating potential barrier of the RI phase, and consequently the lower relative nucleation barrier in the confined geometry turns the transient RI phase into a metastable one. 1. Introduction Crystallization is a widely studied phenomenon which imparts important physical properties to polymeric materials. Polymers often crystallize into a metastable state when cooling from the melt.1 There is a broad class of metastable states in polymers and other materials, and the metastability is dictated by their microscopic phase size.2 Since it is very important to clarify the size effect of metastable states in polymers, in recent years, considerable attention has been paid to research the confined crystallization of polymers, which usually occurs in block copolymers, graft polymers, and polymer blends.3-5 However, it is difficult to propose a quantitative formula to describe the transition dynamics of metastable states in polymers, due to the existence of multiple hierarchical structure and various domain sizes in these states. Compared to the complex polymer materials, normal alkanes [CnH2n+2, n-alkanes, abbreviated Cn], consisting of linear chains of saturated hydrocarbons as the most fundamental organic series, can provide well-defined model systems for studying the complex crystallization behaviors of polymer materials, surfactants, lipids, and liquid crystals.6 Therefore, in-depth investigation of the normal alkane phase behavior is beneficial to better understand the crystalline phase transition and ultimate properties of many polymer materials. The crystallization behavior of n-alkanes has been extensively studied, and a plethora of phases have been identified, which occur between the isotropic liquid and full-crystallization states.7-10 It is generally recognized that the bulk n-alkanes exhibit two unique features of phase transitions, the first of which is the so-called “rotator phase”, exhibiting long-range * Author to whom correspondence should be addressed. E-mail address: [email protected]. † Beijing National Laboratory for Molecular Sciences. § Graduate School of Chinese Academy of Sciences. ‡ State Key Laboratory of Polymer Physics and Chemistry.

order in the molecular axis orientation and the center-of-mass position but lacking long-range order in the rotational degree of freedom of the molecules around their long axis. Odd and even n-alkanes show different crystalline forms of the rotator phase. The face-centered orthorhombic modification called RI usually occurs in even alkanes Cn (for n g 20) as well in odd alkanes C11 through C25.11-13 The stability of the rotator phases is strongly affected by the carbon number of the n-alkanes. For example, the RI phase is a stable rotator phase for C22, and a metastable rotator phase for C20, but a transient rotator phase for both C16 and C18.11,14,15 The transient rotator phases, which cannot stay at a definite temperature for a long time, are not only intermediate forms on the path to the final state, but also play a key role for understanding the crystallization process. The second unique feature is the surface freezing phenomenon occurring for chain lengths ranging from C15 to C50, which has been widely investigated by X-ray reflectivity, grazing incidence X-ray diffraction, surface tension measurements, and moleculardynamics simulations.16-20 A surface crystalline monolayer is formed at the temperature of up to ∼3 °C above the bulk crystallization temperature, and it is stacked into a planar hexagonal rotator phase. As this surface crystallization is intimately related to the formation of the bulk rotator phase,21 the correlation of these two phenomena is crucial for in-depth understanding of the crystallization behavior of n-alkanes. The research of crystallization behaviors of n-alkanes in a confined space, in which the amount of surface alkane molecules is dramatically increased with the decreasing of size of confined space, can help to detect and understand the precise mechanism of crystallization of n-alkanes and the effect of surface freezing on the bulk rotator phase. One kind of confined spaces is the two-dimension hard confined systems such as mesoporous glass22 and silicon,23 which have been used to alter the crystallization kinetics and crystal morphology of n-alkanes from the bulk state. The even alkanes C14 and C16 confined in

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Crystallization Behaviors of n-Octadecane mesopores show an intermediate RI phase, which does not occur in the bulk state of these molecular solids. Analogous observations have been made for medium length n-alkanes spatially confined in emulsified n-alkane microdroplets,24 a three-dimension soft confined system, in which a structure change from the triclinic in the bulk to an orthorhombic structure in the soft small droplets was detected. These investigations enlighten a new strategy to design and understand the specific phase behaviors of n-alkanes in a two-dimensional hard confined or soft microencapsulated environment. Until now, however, seldom work has been done on the crystallization behavior of n-alkanes confined in a three-dimensional hard space, for example, in hard shell microcapsules. The hard shell of microcapsules can afford a stable crystallization environment, in which the nucleation and crystal growth of n-alkanes are easily investigated. In previous reports,25 the crystallization behaviors of the odd n-alkane (n-nonadecane) confined by a hard shell microcapsule (d ) 5 µm) have been studied. During the crystallization process of microencapsulated n-nonadecane, the surface freezing was directly observed by DSC measurement and a metastable hexagonal rotator phase RII emerged with the inducement of surface freezing. However, no confined crystallization investigation has been reported for the even n-alkanes confined in a hard three-dimensional geometry. Therefore, it is the aim of our present work to explore the specific crystallization and melting behaviors of microencapsulated even n-alkanes. In this paper, we prepared microcapsules containing an even alkane (C18) with controllable size (d ) 3 µm) and morphology (multiporous surface) and investigated the crystallization behaviors of C18 confined in such a small hard microcapsule, aiming to understand in-depth the complexity of rotator phase stability and phase transition of even alkanes. We presented a new DSC evidence for the surface freezing in microencapsulated n-octadecane, and more interestingly, it was found that the surface crystalline monolayer induced the formation of a new bulk rotator phase (RI), and to our best knowledge, this phenomenon has not been reported elsewhere for the noctadecane system. 2. Experimental Section n-Octadecane was purchased from the Aldrich Company (purity >99%) and used as received. Using melamineformaldehyde resin as shell material and n-octadecane as core material, microcapsules were prepared by in situ polymerization.26 This polymerization method and subsequent sample treatment provided us with nearly monodispersed and high heatresistant microcapsules, in which the crystallization and melting of n-octadecane have been investigated. The particle size and surface morphology of the microcapsules were examined on a Hitachi S-4300 scanning electron microscope (SEM), fitted with a field emission source and operated at an accelerating voltage of 15 kV. The differential scanning calorimetry (DSC) measurements were carried out on a Mettler DSC822e calorimeter at different cooling/heating rates of 1, 2, 5, 10 °C/min, respectively. Specimens were heated from 0 to 50 °C and then cooled down to 0 °C, followed by heating again to 50 °C. The first cooling and the second heating thermograms were recorded. Temperature dependent Fourier transform infrared spectroscopy (FTIR) measurements of the bulk and microencapsulated n-octadecane were performed on a Bruker EOUINOX 55 spectrometer equipped with a temperature variable cell, and the obtained spectra were processed by the Bruker OPUS program. The temperature-variable cell was kept in vacuum, and liquid

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Figure 1. SEM micrographs of the microcapsules containing noctadecane prepared by in situ polymerization of melamine and formaldehyde, with 4-(1,1,3,3-tetramethylbutyl)phenyl-polyethylene glycol (Triton X-100) as emulsifier. n-Octadecane is microencapsulated inside the microcapsules and occupies 16 wt% of the total weight of microcapsules (DSC).

nitrogen was used as the coolant. A resolution of 2 cm-1 was chosen, and 64 scans were accumulated. The IR spectra were recorded at temperatures ranging between 50 and -15 °C during cooling process. At every temperature point, the samples were equilibrated for about 3-5 min before measurements. Temperature dependent XRD experiments were performed on a Rigaku D/max-2500 X-ray diffractometer over a temperature range of 0 to 35 °C, using Cu KR radiation (1.54 Å), power of 200 mA/40 kV, and rotating angle 2θ ) 5-40°. The samples with thickness of about 1 mm were enclosed in aluminum foil, first heated from 0 to 35 °C, and kept for 3 min, followed by cooling to 0 °C. The heating and cooling rates were all 1 °C/min, and at every temperature point, the samples were equilibrated for about 5 min before measurements. 3. Results and Discussion The n-octadecane containing microcapsules with d ) 3 µm and porous surface structure and narrow size distribution were used in this paper as research objects (Figure 1). Taking the core-shell material with the same density,27 we can obtain the relationship between the radius of the core material and the ratio of the weight of the core material to that of the total: r1 ) [Wc/ (Ww + Wc)]1/3r2, where r1 and r2 correspond to the radius of the core and the microcapsule, Wc and Ww represent the weight of the core and wall layer, respectively. With Wc/(Ww + Wc) ) 0.16 (measured by DSC) and r2 ) 1.5 µm, r1 is calculated to be 0.81 µm. n-Octadecane has 17 C-C single bonds with an average length of 1.25 Å in all-trans conformation, so the volume percentage of surface monolayer molecules inside microcapsules can be calculated as follows: 17 × 1.25 Å × 4πr12/(πr13 × 4/3) ≈ 0.8%, where r1 takes the unit of Å. Such a result means that about 0.8% of the alkane molecules stand on the core-shell interface of the microcapsules and consequently exert significant influence on the crystallization behavior of n-octadecane. The crystallization behaviors of the bulk and microencapsulated n-octadecane were first studied by DSC. For the bulk n-octadecane, there is only one phase transition emerging during the cooling or heating process (Figure 2a), which is related to the phase transition between a triclinic phase and melt. For the microencapsulated n-octadecane (Figure 2b), the phase change behavior in the heating process is similar to that of bulk n-octadecane. The phase change behavior of cooling process, however, is quite different, in which three exothermic peaks

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Figure 2. DSC traces of n-octadecane during the heating and cooling processes: (a) bulk n-octadecane; (b) microencapsulated n-octadecane. Specimens were heated to 40 °C at a rate of 2 °C/min and then cooled down to 0 °C at the same rate, followed by heating again to 50 °C. The first cooling and second heating thermograms were recorded. The insertion in Figure 2b is the magnification of the surfacing freezing peak.

Figure 3. DSC traces of microencapsulated n-octadecane at different cooling rates. Except the cooling rate, all the other measuring conditions are the same as described in Figure 2. The inserted figure on the right up corner is the magnification of the surfacing freezing peaks at different cooling rate.

appear. Above the bulk freezing temperature about 3 °C, a small exothermic peak with normal enthalpy (∆Hs) of 0.21 J/g emerges, as shown in the insertion of Figure 2b, corresponding to the surface freezing of n-octadecane. This small peak is assigned to a thermodynamic first-order phase transition, taking the similar result of n-hexadecane in emulsified droplets as a reference.15 In the previously reported microcapsulated noctadecane systems,25 the surface freezing of n-alkanes has also been directly detected in hard microcapsules by thermal calorimetry, which induces the formation of a rotator phase II (RII). According to Sirota and Ocko,13,19 the surface monolayer of n-alkanes tends to pack into a two-dimensional hexagonal crystal, which can be an ideal nucleation site for the bulk crystallization and subsequently induce the formation of bulk rotator phase. As shown in Figure 2b, except the small surface freezing peak, there are two big exothermic peaks emerging during the cooling process of the microencapsulated n-octadcane. The exothermic peak at 25.4 °C with small supercooling is assigned to the bulk crystallization, the normal enthalpy (∆Hc) of which is 21.4 J/g, lower than that of the melt peak (∆Hm ) 27.5 J/g). The enthalpy difference may be attributed to the emerging of the new broad peak at 21.2 °C in the cooling process, which does not exist in the heating process. The enthalpy sum of the new broad peak (∆HR*) and the bulk crystallization peak (∆Hc) is approximately equal to the normal melting enthalpy (∆Hm). The DSC results suggest that the microencapsulated n-octadecane is first trapped into a new metastable rotator phase from isotropic liquid rather than directly into the triclinic phase, as is the case for pure n-octadecane. With temperature further decreasing, the metastable rotator phase converts to the stable triclinic phase at about 21.2 °C. To evaluate the crystallization behaviors of microencapsulated n-octadecane more clearly, the DSC measurements were performed at different cooling rates, 1 °C/min, 2 °C/min, 5 °C/ min and 10 °C/min, respectively. As shown in Figure 3, the surface freezing and two exothermic peaks were detected during all the cooling processes, corresponding to the isotropic liquid to a new metastable phase and the new metastable phase to triclinic phase, respectively. A very small relative supercooling (onset temperature difference) appeared as about 0.3 °C between the 1 °C/min and 10 °C/min cooling processes, which may be related with the heterogeneous nucleation process induced by the surface freezing. In order to confirm the phase change detected by DSC, the temperature dependent FTIR measurements on bulk and microencapsulated n-octadecane were carried out. For alkanes, the

IR spectra of methylene rocking vibrational bands can be regard as a characteristic band for different crystalline states. It is wellknown that the doublet at 720-730 cm-1 of methylene rocking band corresponds to an orthorhombic crystal form in n-alkanes and linear polyethylene, while a single peak at 717.5 cm-1 corresponds to triclinic form.28 A weak band splitting has been detected in the disordered phase of some n-alkanes, accompanied with the alteration of peak position and absorption intensity. The IR spectrum of alkane liquid phase is a wide and low peak with a frequency of 721 cm-1, as shown in Figure 4 (45 °C). In the region of the rocking vibrations, the IR spectra of the low temperature orthorhombic crystal have a clear-cut doublet of intense bands with frequencies of 720 cm-1 and 730 cm-1. For the orthorhombic rotator phase, one can observe two bands, one band with a frequency of 721 cm-1 and the other broadband at high frequency, the overlapping of which is often observed. The splitting of the two bands is less than 10 cm-1 and slightly changes with temperature, as shown in Figure 4b (24 °C). For alkanes with a triclinic subcell, there is one intense band with a frequency of 717 cm-1, as shown in Figure 4 (5 °C). To separate the overlapping of the rocking vibration bands of methylene chain, we used the standard program Origin 7.5 to fit each spectrum by Lorentzian function, through which the peak position and peak intensity were obtained automatically, as shown in Figure 5. Figure 6 shows the peak intensity and peak position variation of methylene rocking band with temperature during the cooling process. For the bulk n-octadecane (Figure 6a), the single peak of methylene rocking band shifts abruptly from 721.2 cm-1 to 717.5 cm-1 in a short temperature interval of about 2 °C (from 30 to 28 °C), and simultaneously, the absorption intensity increases quickly. As the temperature further decreases, the changes of both peak position and absorption intensity occur gently. This indicates that there is only one phase transition in the cooling process of bulk n-octadecane, corresponding to the bulk crystallization from melt to triclinic. It should be pointed out that the phase transition temperature detected by FTIR is a little bit higher than that measured by DSC, which can be attributed to the different temperature controlling style and systematic error of the two characterization methods. The microencapsulated n-octadecane shows quite different variation trend of IR spectra. As shown in Figure 6b, with temperature decreasing from 50 to -15 °C, two transition regions can be detected. The first transition of peak intensity occurs from 28 to 25 °C and remains stable from 25 to 22 °C, while the peak position remains unchanged from 28 to 22 °C. The peak position of the second transition starting from 22 to 15 °C, however, decreases abruptly from 721.2 cm-1 to 717.5 cm-1, and the peak

Crystallization Behaviors of n-Octadecane

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Figure 4. Temperature dependent FT-IR spectra of bulk (a) and microencapsulated (b) n-octadecane during the cooling process.

Figure 5. Curve fitting of the bands in the region of the rocking vibrations (24 °C), peak 1 with a frequency of 720 cm-1 and peak 2 with a frequency of 726 cm-1.

intensity increases correspondingly. Combining the DSC data and FTIR results, we come to a temporary conclusion that with temperature decreasing, the microencapsulated n-octadecane is first trapped into a metastable rotator phase from isotropic liquid and then changed to the stable triclinic phase. In order to further confirm the phase change behaviors of microencapsulated n-octadecane, the temperature dependent XRD measurements were carried out. In the heating process (Figure 7a), four characteristic peaks of triclinic (010), (011), (100), and (111) emerge from 10 to 29 °C. With temperature further increasing, the characteristic diffraction peaks of triclinic phase disappear abruptly at 31 °C and only a wide halo is left. This indicates that the microencapsulated n-octadecane directly changes from triclinic crystal to isotropic liquid during the heating process. This result is in good agreement with DSC data. In the cooling process (Figure 7b), two diffraction peaks (110) at 2θ ) 21.1° and (200) at 2θ ) 22.9° appear first at 27 °C, which are the characteristic peaks of orthorhombic rotator phase RI.12,22 As temperature decreases to 25 °C, the triclinic peaks of (010), (011), (100), and (111) emerge. At the same time, the (110) and (200) diffraction peaks still exist, meaning that RI and triclinic crystal form coexist in this temperature range. With further decrease in temperature, the rotator phase RI peaks (110) and (200) disappear, indicating that the phase transition has been finished. The XRD results are consistent with those of FTIR and DSC measurements in the crystallization process. Therefore, a conclusion can be drawn that although the surface freezing could not be detected by the XRD method, the orthorhombic metastable rotator phase RI does exist for the microencapsulated n-octadecane, which is an intermediate state from isotropic melt to triclinic stable phase. The X-ray data of the bulk n-octadecane are given in Figure 8. During the heating and cooling processes of the bulk

Figure 6. The peak intensity (square) and peak position (triangle) of the methylene rocking vibrational band as a function of temperature for both (a) bulk n-octadecane and (b) microencapsulated n-octadecane. Using the Origin 7.5 software, the rocking vibration band of each spectrum was fitted by Lorentzian function, through which the peak position and peak intensity were obtained.

n-octadecane, only one phase transition between low temperature triclinic phase and melt phase emerged, and no rotator phase appeared during the cooling process. For even n-alkanes, it is known that the phase diagram contains a crossover from a stable rotator phase (for n g 24) to a metastable one (for n ) 20, 22) and then to a transient one (for n e 18).11,14 In the crystallization process of free bulk n-alkanes, the heterogeneous nucleation of the triclinic phase from the liquid is dominant due to impurities, whereas the nucleation of the rotator phase from the liquid is tiny for the number of surface molecules is relatively small compared to the huge number of bulk molecules. Figure 9 describes the free energy (per molecule or per unit volume) of the triclinic and liquid phases relative to that of the rotator phase, as a function

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Figure 7. Temperature dependent XRD results of microencapsulated n-octadecane: (a) heating process and (b) cooling process. The heating and cooling rates were 1 °C/min.

Figure 8. Temperature dependent XRD results of bulk n-octadecane: (a) heating process and (b) cooling process. The heating and cooling rates were 1 °C/min.

but being inaccessible on heating. Conclusively, the decrease of the nucleating barrier caused by the surface freezing induces the occurrence of RI phase, which makes the crossover of transient rotator phase RI to metastable rotator phase RI for the microencapsulated n-octadecane (Figure 9b). 4. Conclusions

Figure 9. Schematic illustration of the phase change behaviors during cooling and heating processes for (a) bulk n-octadecane and (b) microencapsulated n-octadecane. The thin line indicates the heating process, and the broad line corresponds to the cooling process.

of temperature. The potential barrier of heterogeneous nucleation for the rotator phase RI from the bulk is so high that RI is just a transient phase for the bulk n-octadecane, which cannot sustain a certain temperature region (Figure 9a). The crystallization behaviors and rotator phase stability can be strongly affected by the confined geometry and the space size; for example, the metastable RI has been observed during the cooling process of the mesopore-confined C14H30 in Vycor glass.22 For the confined n-alkanes, the amount of surface molecules becomes much more bigger compared with the bulk n-alkanes, thus endowing the heterogeneous nucleation effect of the surface monolayers. However, as the impurities are limited to the separated microcapsules, the heterogeneous nucleation of the triclinic phase is limited. In our microencapsulated n-octadecane system, the metastable RI phase, which appears only upon cooling process, has been detected by three independent methods, i.e., DSC, FTIR, and XRD, indicating that microcapsule-confined noctadecane shows the same phase sequence of bulk n-eicosane (C20), with the rotator phase becoming metastable on cooling

The confined phase transition behaviors of n-octadecane in microcapsules have been investigated with the combination of several characterization methods. As evident from the DSC measurements, a surface freezing monolayer is formed, which is attributed to the large specific area inside the microcapsules. A metastable RI phase has been found in the cooling process of microencapsulated n-octadecane. The surface freezing monolayer plays an important role on formation of ideal nucleation sites by decreasing the nucleation barrier of the RI phase, inducing the crossover of rotator phase from transient to metastable. This result is propitious to better understand the effect of surface freezing monolayer on the bulk crystallization process and the relationship between the rotator phase stability and the confined geometry size. Acknowledgment. We thank the National Natural Science Foundation of China (No. 50573086) for financial support. References and Notes (1) Keller, A.; Cheng, S. Z. D. Polymer 1998, 39, 4461. (2) Cheng, S. Z. D.; Zhu, L.; Li, C. Y.; Honigfort, P. S.; Keller, A. Thermochim. Acta 1999, 332, 105. (3) Loo, Y. L.; Register, R. A.; Ryan, A. J.; Dee, G. T. Macromolecules 2001, 34, 8968. (4) Zhu, L.; Cheng, S. Z. D.; Calhoun, B. H.; Ge, Q.; Quirk, R. P.; Thomas, E. L.; Hsiao, B. S.; Yeh, F.; Lotz, B. J. Am. Chem. Soc. 2000, 122, 5957.

Crystallization Behaviors of n-Octadecane (5) Loo, Y. L.; Register, R. A.; Ryan, A. J.; Dee, G. T. Phys. ReV. Lett. 2000, 84, 4120. (6) Small, D. M. The Physical Chemistry of Lipids; Plenum: New York, 1986. (7) Sirota, E. B.; Singer, D. M. J. Chem. Phys. 1993, 101, 10873. (8) Sirota, E. B.; King, H. E.; Hughes, G. J.; Wan, W. K. Phys. ReV. Lett. 1992, 68, 492. (9) Doucet, J.; Denicolo, I.; Craievich, A.; Collet, A. J. Chem. Phys. 1981, 75, 5125. (10) Dirand, M.; Bouroukba, M.; Chevallier, V.; Petitjean, D. J. Chem. Eng. Data 2002, 47, 115. (11) Sirota, E. B.; Herhold, A. B. Science 1999, 283, 529. (12) Sirota, E. B.; King, H. E.; Singer, D. M.; Shao, H. H. J. Chem. Phys. 1993, 98, 5809. (13) Sirota, E. B. Langmuir 1998, 14, 3133. (14) Sirota, E. B.; Herhold, A. B. Polymer 2000, 41, 8781. (15) Shinohara, Y.; Kawasaki, Ueno, N. S.; Kobayashi, I.; Nakajima, M.; Amemiya, Y. Phys. ReV. Lett. 2005, 94, 97801. (16) Wu, X. Z.; Ocko, B. M.; Sirota, E. B.; Sinha, S. K.; Deutsch, M.; Cao, B. H.; Kim, M. W. Science 1993, 261, 1018.

J. Phys. Chem. B, Vol. 112, No. 42, 2008 13315 (17) Wu, X. Z.; Sirota, E. B.; Sinha, S. K.; Ocko, B. M.; Deutsch, M. Phys. ReV. Lett. 1993, 70, 958. (18) Wu, X. Z.; Ocko, B. M.; Tang, H.; Sirota, E. B.; Sinha, S. K.; Deutsch, M. Phys. ReV. Lett. 1995, 75, 1332. (19) Ocko, B. M.; Wu, X. Z.; Sirota, E. B.; Sinha, S. K.; Gang, O.; Deutsch, M. Phys. ReV. E 1997, 55, 3164. (20) Prasad, S.; Dhinojwala, A. Phys. ReV. Lett. 2005, 95, 117801. (21) Sloutskin, E.; Sirota, E. B.; Kraack, H.; Ocko, B. M.; Deutsch, M. Phys. ReV. E 2001, 64, 1063. (22) Huber, P.; Soprunyuk, V. P.; Knor, K. Phys. ReV. E 2006, 74, 031610. (23) Henschel, A.; Hofmann, T.; Huber, P.; Knorr, K. Phys. ReV. E 2007, 75, 021607. (24) Montenegro, R.; Landfester, K. Langmuir 2003, 19, 5996. (25) Xie, B. Q.; Shi, H. F.; Jiang, S. C.; Zhao, Y.; Han, C. C.; Xu, D. F.; Wang, D. J. J. Phys. Chem. B 2006, 110, 14279. (26) Sun, G.; Zhang, Z. J. Microencapsul. 2001, 18, 593. (27) Sliwka, W. Angew. Chem., Int. Ed. Engl. 1975, 14, 539. (28) Ungar, G.; Masˇie´, N. J. Phys. Chem. 1985, 89, 1036. (29) Sear, R. P. Langmuir 2002, 18, 7571.

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