Confined Phase Diagram of Binary n-Alkane Mixtures within Three

Oct 3, 2014 - hexadecane/octadecane mixtures (abbreviated as m-C16/C18) have been ... The surface freezing is significantly enhanced after C16/C18 ...
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Confined Phase Diagram of Binary n‑Alkane Mixtures within ThreeDimensional Microcapsules Xia Gao, Dongsheng Fu, Baoquan Xie, Yunlan Su,* and Dujin Wang Beijing National Laboratory for Molecular Sciences, Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ABSTRACT: The confined phase behaviors of microencapsulated normal hexadecane/octadecane mixtures (abbreviated as m-C16/C18) have been investigated by combination of differential scanning calorimetry and in situ wide-angle X-ray scattering. The binary alkane mixtures confined in threedimensional geometrical space demonstrate two novel crystallization features. The surface freezing is significantly enhanced after C16/C18 mixtures being encapsulated, and the surface monolayer formed is proved to be an ideal solid solution composed by C16 and C18. Furthermore, m-C16/C18 mixtures are trapped into a stabilized rotator phase below the crystallization temperatures, whereas C16/C18 mixtures with certain compositions form the low-temperature crystalline structure directly. These confined crystallization features originate from the jointed effects of spatial confinement and chain mixing of the components. Moreover, the phase diagram of the confined binary alkane mixtures (m-C16/C18) is successfully established for the first time, which enlightens the crystallization features of other spatially confined soft-matter binary systems.

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INTRODUCTION Normal alkanes (n-alkanes, CnH2n+2, abbreviated as Cn), as a basic building block unit in systems such as liquid crystals, oils, lipids, and polyolefin, possess plenty of metastable phase transition behaviors, which are often used as the simple representative model for probing the physical nature of the polymeric material system. On the other hand, the long-chain alkanes are widely used as the energy storage materials due to their large phase change potential energy. In these regards, the crystallization behavior investigation of n-alkanes has attracted essential attention both from the scientific and industrial senses.1−3 n-Alkanes with chain length ranging from 16 to 50 have been paid much attention in the past decades with focus on the surface freezing phenomenon, rotator phase transition, and their odd−even effect. Above the bulk crystallization temperature about 3 °C, the surface freezing of liquid n-alkanes occurs with the formation of a surface monolayer.4−9 The rotator phases of alkanes exhibit three-dimensional crystalline positional orders but lack of long-range order in the rotational degree of freedom of the molecules about their long axes.10−12 Even n-alkanes with n ≤ 24 crystallize into the triclinic phase, whereas all odd n-alkanes form the orthorhombic phase through metastable rotator phases, namely the famous odd− even effect.13,14 Also, there has been a great deal of previous work on binary alkanes mixture systems, which involves diverse phase diagrams showing the regions of single phase and multiphase coexistence.15−17 Mixtures generally form an orthorhombic crystal structure, even if the pure components exhibit triclinic phase. Moreover, as components with different chain lengths mix, the range of stability of the rotator phase is significantly affected.18 The hexagonal rotator phase RII is © XXXX American Chemical Society

favored over the distorted RI phase in alkane mixtures, since the spread in chain length results in the weakened coupling between layers. Various studies suggest that the miscibility and the phase transition behavior of mixtures are determined by the carbon number difference (Δn) and the average chain length (n̅ = φn1 + (1−φ)n2, where n1 and n2 are the carbon numbers of components and φ is the molar ratio of the component with n1 carbon number). Binary mixtures typically form a miscible solid solution if the Δn is small,19,20 whereas they tend to exhibit phase separation with Δn larger than 4.21 The n̅ is the other important factor which affects the crystal structure and phase sequence of alkane mixtures.22 The sensitivity to a given carbon number difference Δn should be different for long and short chains because of the different relative importance of the finite number of chain-end conformations. Therefore, for binary mixtures with Δn = 2 and various n̅, the phase diagrams of Cn/ Cn+2 become more and more complex with chain length increasing, and the regions of multiphase coexistence significantly shrink.15 In confined environment, the phase transition behaviors of pure n-alkane systems have been found to be different from free bulk counterparts due to the competition between the geometry confined effect and the chain length affecting the phase structure.23−26 The investigation on the confined mixture system is more relevant with the real polymeric system.27 However, up to now, only few works on the three-dimensional confined binary n-alkanes mixtures are reported due to the Received: July 13, 2014 Revised: October 2, 2014

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Figure 1. SEM images of microcapsules containing C16/C18 mixtures (A) prepared by in situ polymerization of melamine−formaldehyde and the DSC curves of microencapsulated C16/C18 (B) and bulk C16/C18 (C) mixtures with different components during cooling process (only part of DSC curves is shown here).

ranges from 0 to 100%. The corresponding encapsulated samples were prepared by in situ polymerization using melamine−formaldehyde (M-F resin) as the shell material and n-alkane mixtures as the core material according to the literature.32,33 This method provides us with nearly monodispersed and highly heat-resistant microcapsules. The particle size and surface morphology of the prepared microcapsules were examined by a JEOL-JSM-6700F scanning electron microscope, fitted with a field emission source and operated at an accelerating voltage of 5 kV. The melting and crystallization behaviors of all mixtures were examined with a differential scanning calorimeter (TA, Q2000) at a cooling/heating rate of 2 °C/min. The instrument was calibrated with indium before measurements. Specimens were heated from room temperature to 40 °C and then cooled down to −30 °C, followed by heating again to 40 °C. The first cooling and second heating thermograms were recorded. In situ wide-angle X-ray scattering (WAXS) measurements were carried out at the beamline 14 B in the Shanghai Synchrotron Radiation Facility (SSRF). A Linkam thermal stage was used for temperature control. The wavelength of the radiation source was 1.24 Å. WAXS patterns were collected by a Mar CCD with a resolution of 3072 × 3072 pixels (pixel size: 73 × 73 mm2). Image acquisition time was 60 s. The sample-todetector distance was 368 mm.

challenge of the synthesis of confined space with narrow size distribution.28,29 In our previous work, a well-confined crystallization system based on microencapsulated C18/C20 mixtures and C18/C19 mixtures with certain concentrations (10/90 or 90/10) was successfully developed and well investigated, which demonstrated that the confinement effect of microcapsules can endow novel crystallization features to binary mixtures and suppress phase separation exhibiting in bulk state.30,31 To further extend the knowledge on the confined crystallization, here, a new microencapsulated alkane mixture system with a smaller average chain length and thereby stronger repulsion energy between components, m-C16/C18, was developed. In this work, we focus on the confined crystallization behaviors of microencapsulated C16/C18 mixtures by means of thermal analysis (DSC) and in situ wide-angle Xray scattering (WAXS). The m-C16/C18 mixtures are found to exhibit unique crystallization behaviors compared to that in the bulk state. To reveal these crystallization features clearly, the phase diagram of m-C16/C18 (transition temperature as a function of the concentration) is accomplished for the first time. The microencapsulated alkane mixture system is demonstrated to be a creative model system for investigating on confined crystallization, which provides an efficient pathway to understand the complex confined phase behavior in a real polymeric material system.

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RESULTS AND DISCUSSION The microcapsules containing C16/C18 mixtures, shown in Figure 1A, have a mean diameter of ca. 3.5 μm with a narrow size distribution and porous morphology. Early results showed that the microcapsules can provide an efficient confinement effect on the phase transition of pure alkanes or binary

MATERIALS AND METHODS n-C16H34 (C16) and n-C18H38 (C18) with purity >99% were purchased from Sigma-Aldrich Company, and all reagents were used as received. The bulk mixtures were melt-mixed with designated composition; the mass ratio of C18 in the mixtures B

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mixtures.29−31 The confined crystallization behaviors of microencapsulated C16/C18 mixtures with different compositions were first investigated by DSC (Figure 1B). Comparing with the bulk samples (Figure 1C), a new exothermic peak located about 3 °C above the bulk crystallization temperature, emerges for the m-C16/C18 mixtures during the cooling process, which is assigned to the surface freezing phenomenon.29−31 Interestingly, the crystallization temperatures for m-C16/C18 samples are nearly the same as those of their free bulk counterparts, whereas the transition temperatures from the rotator phase to the low-temperature crystal phase are much lower than those of bulk counterparts (Figure 1B,C). The crystallization features of confined n-alkane mixtures and underlying mechanism are explored and discussed in detail below. Enhanced Surface Freezing. As reported by Maeda et al., for alkane molecules adsorbed on solid substrates, the interplay between the lateral monolayer cohesion and its cohesion to the solid substrates is very important to the stability of surface freezing.34 And they found that alkanes on solid substrates would not exhibit surface freezing and the surface monolayer with normal molecular orientation collapse under the strong force filed of substrates (≥100 mN/m).35,36 Since the shells of microcapsules are made of melamine resin, the weak van der Waals force dominates the interaction between amorphous resin and nonpolar alkanes molecules in the interface. Therefore, surface freezing can still occur with alkane molecules “standing” on the shells of microcapsules. Since the n-alkane mixtures are confined in a three-dimensional environment with 3.5 μm in diameters, the percentage of the surface monolayer molecules inside a microcapsule is calculated to be ca. 1% based on the melting enthalpy of m-C16/C18 obtained from DSC measurements. Such a result means that 1% of the total alkane molecules stand at the interface between the microcapsule shell and the liquid alkane mixture, which is significantly enhanced so that the surface freezing can be directly detected by the normal DSC method. Moreover, the relationship between surface freezing temperatures with concentrations is investigated for the first time, and a proportional relationship obeying Vergard’s law of ideal solution is demonstrated in Figure 2. When the surface monolayer forms at the shell/liquid alkanes interface, the free energy of surface monolayer is Fs = Hs + kBT[(1 − φs) ln(1 − φs) + φs ln φs] + ωφs(1 − φs), where φs is the molar ratio of C18 in the surface monolayer and ω is the repulsive energy terms between C16 and C18.22 From the equation above, the phase behavior of monolayer is determined

by the balance between the mixing entropy and the repulsive interchange. Molecules in the surface monolayer arrange in the hexagonal structure and the in-plane subcell parameter ratio a/ b equals √3.6,10 This means that surface monolayer exhibits a larger distance between the molecules than rotator phase and crystal phase, which results in the repulsive interchange ω ≈ 0.18kBT.22 When the repulsive energy term ω ≪ 2kBT, the mixing entropy is dominant, inducing a homogeneous mixing of two components at all φ. Equating the surface and bulk chemical potentials, the surface concentration is nearly equal to bulk concentration due to the negligible repulsive energy in the surface monolayer. Consequently, the surface monolayer acts as an ideal solid solution, and the surface freezing temperatures of m-C16/C18 increase linearly with the concentrations. Crossover of RI from Transient to Metastable. Based on whether or not a rotator phase is stable existence both during heating and cooling process, it can be clarified into stable rotator phase (stable existence in both cooling and heating process), metastable rotator phase (only stable existence in cooling process), and transient rotator phase (cannot existence in either cooling or heating process). For binary alkane mixtures, the stability of orthotropic rotator phase RI strongly depends on the composition and carbon number difference.15,17,20 Bulk C16/C18 mixtures with the concentrations of C18 in the range of 20−90% crystallize into RI, while others are trapped into their low-temperature ordered structures (triclinic phase) directly.15 However, in the m-C16/C18 samples with all concentrations, rotator phase RI is detected during crystallization from melt, which is confirmed by the coexistence of two characteristic diffraction peaks of (110) and (200) at 2θ = 17° and 18.4°, as shown in Figure 3. Obviously, the RI becomes a stable rotator phase in all concentrations after C16/C18 mixtures being confined in the microcapsules. Since alkanes form anisotropic crystals, the nucleation barrier ΔG = 8πa4d2σsl2γsl/(ΔSΔT)2 is obtained, where a2 is the area/ molecule, d is the molecule length, γsl is the surface energy/area of the solid−liquid interface at the molecular ends, and σsl is the energy/area along the molecular axis.14 ΔS is the entropy of transition/molecule, and ΔT is the undercooling below the equilibrium melting temperature. Form the equation, a larger surface energy γsl increases the nucleation barrier. In microcapsules studied here, the surface freezing is significantly enhanced as described above and demonstrates the similar inplanar structure as rotator phase. If RI forms during crystallization from melt, no new interface need be created to grow additional layers on the enhanced surface phase, which can extraordinarily decrease the nucleation barrier. Therefore, it is observed that the crossover of RI from transient rotator phase in the bulk state to stable rotator phase in the confinement environment. Furthermore, demonstrated by the nonlinear line (illustrating the relationship between liquid-rotator transition temperature with concentration) below the theoretical Vegard’s law line in Figure 4A, RI behaves as a regular solid solution rather than an ideal one. As reported in the literature, the alkane molecules in RI exhibit a rectangular (distorted hexagonal) lattice and pack more closely with bilayers stacking sequence.10,12 And the subcell parameter ratio a/b of RI is smaller than √3 as shown in Figure 4B; furthermore, the distance between molecules in RI decreases as the temperature decreases. Since the molecules get close enough, the repulsion energy in RI lattice dominates over the mixing entropy. Consequently, RI behaves as a regular solution.

Figure 2. Relationship between surface freezing temperatures and the molar ratio of C18 for m-C16/C18; the inset reveals the in-plane packing of molecules in surface monolayer. C

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Figure 3. Phase transition procedures of m-C16/C18 with different compositions (A, 95/5; B, 27/73; C, 32/68; D, 5.6/94.4) during cooling by in situ WAXS. WAXS results of other mixtures are not shown here, and part C is obtained by the temperature-dependent XRD and the wavelength is 1.54 Å.

Figure 4. (A) Peak temperatures of liquid−rotator transition for m-C16/C18; the inset is the in-plane packing of RI. (B) Tendency of the subcell parameter ratio a/b with temperature; the parameters a and b are calculated according to the WAXS results of m-C16/C18 = 5.6/94.4 in Figure 3D.

Enhanced Stability of RI. Further decreasing temperature, the solid−solid transition occurs, and the alkane mixtures tend to stack into a more ordered crystal structure. Pure C16 or C18 exhibits the triclinic phase, whereas their mixtures with different compositions show different low-temperature crystal structures from each other.15 For example, C16/C18 mixtures exhibit orthorhombic phase when the molar ratio of C18 ranges from ∼30% to ∼70% because the chain mixing reduces the stack efficiency of terminal methyl−methyl and favors the formation of the orthorhombic phase.29 With the compositions varying, m-C16/C18 samples also demonstrate different low-temperature crystal structures from each other (see Figure 3). For m-C16/ C18 mixture with 68% of C18, the orthorhombic phase forms, indicated by the existence of two characteristic diffraction peaks (110) and (200) in Figure 3C. For another sample with 94.4% of C18, four characteristic peaks corresponding to triclinic phase exist after the solid−solid transition (Figure 3D). Moreover,

when the concentration of C18 is 5% or 73%, the microencapsulated mixture demonstrates the coexistence of triclinic and orthorhombic phases, which means that phase separation occurs in these confined mixtures after the solid−solid transition, as shown in Figure 3A,B. However, the concentration range for m-C16/C18 mixtures exhibiting phase separation is narrower than that of bulk mixture (well illustrated by the narrow two-phase coexistence regions in Figure 6).15 In our previous works, it has been observed that phase separation can be suppressed after normal alkane mixtures are encapsulated in microcapsules with ca. 3.5 μm in diameter.30,31 Through suppressing the longitudinal chain diffusion of alkane molecules, the confined environment leads to an enhanced kinetic lifetime of RI and then better mixing of the two components, thereby contributing to the suppression of the solid−solid phase separation of binary n-alkane system. In this regard, the temperature region of RI stable existence for m-C16/ D

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Figure 5. (A) Temperature region (TL/R−TS/S) of RI stable existence for m-C16/C18 samples (black diamond) and bulk C16/C18 samples (red circle). (B) Rotator−crystal transition temperatures (TR/C) both for bulk (black squares) and m-C16/C18 samples (pink circles).

terminal methyl−methyl interaction for the C16/C18 mixtures with φ > 40% is significantly disturbed,23 leading to weakened interlayer coupling. All of these favor the enhanced stability of RI phase for m-C16/C18. Moreover, the geometric space exerts strong confinement effect on the longer chain molecules and can suppress the longitudinal chain diffusion effectively. Therefore, microencapsulated mixtures C16/C18 with φ > 40% demonstrate larger temperature hysteresis in terms of the rotator−crystal transition. Phase Diagram of m-C16/C18. With combination of the data from DSC and WAXS, the phase diagram of m-C16/C18 has been established successfully, as shown in Figure 6.

C18 and C16/C18 is plotted as a function of the concentrations of C18 (Figure 5A). With the concentrations of C18 in the range of 25−70%, RI for m-C16/C18 mixtures shows enhanced lifetime as wide as 35 °C, rather than the narrower temperature region in bulk state. Single alkanes in two- and three-dimensional confined geometry often show the enhanced stability of rotator phases since the wall of confinement systems can act as sources of random strain fields that stabilize the disordered rotator phase.24,25 When encapsulated in microcapsules, there exists a hierarchy of uncorrelated adsorption sites for C16/C18 samples on the rough wall of the amorphous shells, which leads to an enhanced disorder along the longitudinal direction of molecules next to the wall. This kind of longitudinal disorder can be easily transferred to other molecules as RI nucleate on the enhanced surface monolayer as described above. Moreover, the threedimensional geometric space further maintains the kind of disorder by suppressing the longitudinal diffusion of alkane molecules.29 As a result, the lamellar ordering in bulk crystalline structures is greatly disturbed in the confinement systems, which can be proved by the disappearance of a series of (00l) reflections at lower Bragg angles 2θ related to the lamellar arrangement of the molecules (shown in Figure 3). The weakened interlayer coupling favors the stability of RI for the mC16/C18 mixtures.37 In addition, mixing different components also effectively decreases the interlayer coupling induced by the spread of the chain length. Therefore, compared to their bulk counterparts, all m-C16/C18 samples demonstrate enhanced stability of RI with a wide temperature region. With the enhanced kinetic lifetime of RI and thereby better mixing of two components, phase separation occurring in bulk state can be suppressed under confinement, leading to the narrower concentration regions for m-C16/C18 exhibiting phase separation in Figure 6. As the temperature region of RI stable existence is widened, the microencapsulated mixtures show lower rotator−crystal transition temperatures than their bulk counterparts, as is shown in Figure 5B. However, what should be emphasized is that the temperature hysteresis for the rotator−crystal transition becomes more evident when the concentration φ of C18 reaches 40% or more, with the largest variation of 16.8 °C for 80%. For other microencapsulated mixtures, the confinement effect on the rotator−crystal transition seems to be weaker with the temperature depression of less than 5 °C. With C18 being the major component, the average chain length becomes larger and the interchain interaction, which favors the stability of RI, becomes stronger.23 In addition, since chain-end gauche conformation occurs easily for long chain,22 the

Figure 6. Phase diagram of microencapsulated C16/C18 mixtures: L = liquid, SF = surface freezing monolayer, R = rotator phase, T = triclinic phase, and O = orthorhombic phase. The cooling procedures are traced to finish the phase diagram in order to study the rotator−crystal transition in detail, and more mixtures with various concentrations are difficult to be studied due to the restriction of preparation method; thus, the two-phase regions are not be specifically determined.

Considering the narrow exothermic peak caused from the fast surface freezing transition (temperature interval only about 0.3 °C), the boundary between liquid (L) and liquid + surface freezing (L + SF) phases is determined by the peak temperature of surface freezing transition according to the DSC results. The liquidus and solidus are determined from the onset and end temperatures of the liquid−rotator transition (corresponding to black squares and pink squares in Figure 6, respectively). The upper phase boundary of solid−solid transition is the onset temperature of the rotator−crystal transition at DSC curves, while the lower phase boundary is based on the combined results of WAXS and DSC. It is obvious E

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that phase sequence of m-C16/C18 mixtures is liquid (L)− surface freezing (SF) + liquid (L)−rotator phase (R)−crystal phase (C) with temperature decreasing. With the confinement effect of microcapsules, the phase diagram of m-C16/C18 is significantly different with that of bulk C16/C18 as reported by Mondieig et al.15 First, the rotator phase occupies a large part of the phase diagram, indicating the enhancement of the stability of rotator phase. Second, the surface freezing transition is an assignable phase change in the m-C16/C18 phase diagram. What is more, the regions for two-phase coexistence (T + O) in the m-C16/C18 system are narrower than that in bulk. The unique features demonstrated by the confined phase diagram sum up the confined phase transition behavior of m-C16/C18 so well. As mentioned above, though the shorter average chain length of C16/C18 mixtures than C18/C19 or C18/C20, the microcapsule with 3.5 μm in diameter still exerts a significant confinement effect on the crystallization behaviors of the binary mixtures, leading to the phase diagram of m-C16/C18 being different from that of the free bulk state. It is also evident that the shorter the average chain length of mixtures is, the weaker the confinement effect on them is. Furthermore, the present work on the crystallization behaviors of the confined n-alkane mixtures can be helpful to better understand the mechanism of phase change materials and shed some new light on the designation and preparation of such kinds of materials.

in Shanghai Synchrotron Radiation Facility (SSRF) for beam time and technical assistance.

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(1) Turnbull, D.; Cormia, R. L. Kinetics of Crystal Nucleation in Some Normal Alkane Liquids. J. Chem. Phys. 1961, 34, 820−831. (2) Strobl, G.; Ewen, B.; Fischer, E. W.; Piesczek, W. Defect Structure and Molecular Motion in the Four Modifications of nTritriacontane. I. Study of Defect Structure in the Lamellar Interfaces Using Small Angle X-ray Scattering. J. Chem. Phys. 1974, 61, 5257− 5264. (3) Uhlmann, D.; Kritchevsky, G.; Straff, R.; Scherer, G. Crystal Nucleation in Normal Alkane Liquids. J. Chem. Phys. 1975, 62, 4896− 4903. (4) Wu, X. Z.; Sirota, E. B.; Sinha, S. K.; Ocko, B. M.; Deutsch, M. Surface Crystallization of Liquid Normal Alkanes. Phys. Rev. Lett. 1993, 70, 958−961. (5) Tkachenko, A. V.; Rabin, Y. Fluctuation-Stabilized Surface Freezing of Chain Molecules. Phys. Rev. Lett. 1996, 76, 2527−2530. (6) Ocko, B. M.; Wu, X. Z.; Sirota, E. B.; Sinha, S. K.; Gang, O.; Deutsch, M. Surface Freezing in Chain Molecules: Normal Alkanes. Phys. Rev. E 1997, 55, 3164−3182. (7) Gang, O.; Ocko, B. M.; Wu, X. Z.; Sirota, E. B.; Deutsch, M. Surface Freezing in Chain Molecules. Synchrotron Radiat. News 1999, 12, 34−36. (8) Sloutskin, E.; Wu, X. Z.; Peterson, T. B.; Gang, O.; Ocko, B. M.; Sirota, E. B.; Deutsch, M. Surface Freezing in Binary Mixtures of Chain Molecules. I. Alkane Mixtures. Phys. Rev. E 2003, 68, 031605. (9) Ocko, B. M.; Hlaing, H.; Jepsen, P. N.; Kewalramani, S.; Tkachenko, A.; Pontoni, D.; Reichert, H.; Deutsch, M. Unifying Interfacial Self-Assembly and Surface Freezing. Phys. Rev. Lett. 2011, 106, 137801. (10) Ungar, G.; Maŭić, N. Order in the Rotator Phase of n-Alkanes. J. Phys. Chem. 1985, 89, 1036−1042. (11) Sirota, E. B.; King, H. E., Jr.; Hughes, G. J.; Wan, W. K. Novel Phase Behavior in Normal Alkanes. Phys. Rev. Lett. 1992, 68, 492−495. (12) Sirota, E. B.; King, H. E., Jr.; Singer, D. M.; Shao, H. H. Rotator Phases of the Normal Alkanes: An X-ray Scattering Study. J. Chem. Phys. 1993, 98, 5809−5824. (13) Dorset, D. The Crystal Structure of Waxes. Acta Crystallogr., Sect. B 1995, 51, 1021−1028. (14) Sirota, E. B. Supercooling, Nucleation, Rotator Phases, and Surface Crystallization of n-Alkane Melts. Langmuir 1998, 14, 3133− 3136. (15) Mondieig, D.; Rajabalee, F.; Metivaud, V.; Oonk, H. A. J.; Cuevas-Diarte, M. A. Alkane Binary Molecular Alloys. Chem. Mater. 2004, 16, 786−798. (16) Robles, L.; Mondieig, D.; Haget, Y.; Cuevas-Diarte, M. A.; Alcobe, X. Non Isomorphism and Miscibility in the Solid State: Determination of the Equilibrium Phase Diagram n-Octadecane C18H38 + n-Nonadecane C19H40. Mol. Cryst. Liq. Cryst. 1996, 281, 279−290. (17) Oonk, H. A. J.; Mondieig, D.; Haget, Y.; Cuevas-Diarte, M. A. Perfect Families of Mixed Crystals: the Rotator I n-Alkane Case. J. Chem. Phys. 1998, 108, 715−722. (18) Sirota, E. B.; King, H. E., Jr.; Shao, H. H.; Singer, D. M. Rotator Phases in Mixtures of n-Alkanes. J. Phys. Chem. 1995, 99, 798−804. (19) Matheson, R. R., Jr.; Smith, P. A Simple Thermodynamic Analysis of Solid-Solution Formation in Binary Systems of Homologous Extended-Chain Alkanes. Polymer 1985, 26, 288−292. (20) Gilbert, E. P. The Stability of Binary Alkane Blends. Phys. Chem. Chem. Phys. 1999, 1, 1517−1529. (21) Wu, X. Z.; Ocko, B. M.; Tang, H.; Sirota, E. B.; Sinha, S. K.; Deutsch, M. Surface Freezing in Binary Mixtures of Alkanes: New Phases and Phase Transitions. Phys. Rev. Lett. 1995, 75, 1332−1335. (22) Sloutskin, E.; Sirota, E. B.; Gang, O.; Wu, X. Z.; Ocko, B. M.; Deutsch, M. Surface and Bulk Interchange Energy in Binary Mixtures of Chain Molecules. Eur. Phys. J. E 2004, 13, 109−112.

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CONCLUSIONS Although the relative small average chain length for C16/C18 mixtures, three-dimensional microcapsules still exert an excellent confinement effect on them. And the novel features of crystallization behavior of microencapsulated alkane mixtures are well illustrated by the established phase diagram. The linear relationship between surface freezing temperatures and concentrations represents an ideal solid solution behavior of surface monolayer. The wide temperature region of RI phase in the phase diagram indicates that geometric confinement significantly enhances the stability of rotator phase by suppressing the longitudinal chain diffusion and maintaining the weakened interlayer coupling. The microcapsules also help suppress the phase separation occurring in bulk state, inducing the narrower two-phase coexistence regions in the phase diagram. Besides, the temperature hysteresis for the rotator− crystal transition demonstrates that the shorter the average chain length of mixtures is, the weaker the confinement effect on them is. It is believed that this work can contribute to the study of crystallization behaviors of n-alkanes in threedimensional confinement systems, which can also enlighten the spatially confined phase behaviors of other soft matter systems.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone +86-10-82618533; Fax +86-10-82612857; e-mail [email protected] (Y.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (51103166) and China National Funds for Distinguished Young Scientists (50925313) for financial support. We gratefully appreciate the staff and scientists at beamline 14B F

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