Two-Step Freezing in Alkane Monolayers on Colloidal Silica

Article pubs.acs.org/JPCB

Two-Step Freezing in Alkane Monolayers on Colloidal Silica Nanoparticles: From a Stretched-Liquid to an Interface-Frozen State Xia Gao,† Patrick Huber,‡ Yunlan Su,*,† Weiwei Zhao,† and Dujin Wang*,† †

Beijing National Laboratory for Molecular Sciences, Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Institute of Materials Physics and Technology, Hamburg University of Technology (TUHH), Hamburg-Harburg, Eißendorfer Strasse 42, D-21073, Germany S Supporting Information *

ABSTRACT: The crystallization behavior of an archetypical soft/hard hybrid nanocomposite, that is, an n-octadecane C18/SiO2-nanoparticle composite, was investigated by a combination of differential scanning calorimetry (DSC) and variable-temperature solid-state 13C nuclear magnetic resonance (VT solid-state 13C NMR) as a function of silica nanoparticles loading. Two latent heat peaks prior to bulk freezing, observed for composites with high silica loading, indicate that a sizable fraction of C18 molecules involve two phase transitions unknown from the bulk C18. Combined with the NMR measurements as well as experiments on alkanes and alkanols at planar amorphous silica surfaces reported in the literature, this phase behavior can be attributed to a transition toward a 2D liquid-like monolayer and subsequently a disorder-to-order transition upon cooling. The second transition results in the formation of a interfacefrozen monolayer of alkane molecules with their molecular long axis parallel to the nanoparticles’ surface normal. Upon heating, the inverse phase sequence was observed, however, with a sizable thermal hysteresis in accord with the characteristics of the firstorder phase transition. A thermodynamic model considering a balance of interfacial bonding, chain stretching elasticity, and entropic effects quantitatively accounts for the observed behavior. Complementary synchrotron-based wide-angle X-ray diffraction (WAXD) experiments allow us to document the strong influence of this peculiar interfacial freezing behavior on the surrounding alkane melts and in particular the nucleation of a rotator phase absent in the bulk C18.

■

ture.13−16 As a result, the surface ordered monolayer can be favored at the air/liquid interface. Moreover, surface freezing of n-alkanes also occurs at air/ water17,18 and air/solid19−21 interfaces, though the interfacial interactions are essentially different from that at the free surface of pure alkane melts. Induced by surfactants (for example, CTAB), a monolayer exhibiting a hexagonally packed 2D crystal with extended, surface-normal alkane molecules, was found to form at the surface of water.17,18 For n-alkane adsorbed on solid substrates (such as SiO2-coated Si(100) wafers19−23 or mica24), a structure consisting of a monolayer with surface-normal molecular orientation and/or a bilayer with the molecules lying horizontal on the substrate is revealed by high-resolution ellipsometry and synchrotron X-ray scattering measurements.22,23,25−27 In the case of substrates with high surface energy (such as metal substrates28 and graphitic surfaces29) molecules nearby the substrate tend to orient parallel to the substrate.

INTRODUCTION

For most condensed matter systems, surfaces melt at a lower temperature than their underlying bulk, because the molecules in the surface are less constrained than those in the bulk phase.1−4 However, for the surface of hydrocarbon melts a much less common phenomenon, the so-called “surface freezing” phenomenon, is observed by X-ray and surface tension measurements.5−7 It is characterized by the formation of an ordered surface monolayer at the liquid/air interface temperatures ∼3 °C above the freezing temperature of the bulk melt. The molecules in the surface-frozen monolayer are hexagonally (or quasi-hexgaonally) packed with surface-normal orientation. Upon now, this effect has been observed in nalkanes,5 1-alchohols,6,7 diols,8 as well as their mixtures9 and side-chain polymers.10,11 This strongly indicates that this rare phenomenon is related to the chain-like molecular geometry or to the methyl −CH3 tails of these systems. For a surface consisting of terminal −CH3 groups, the surface energy γsl (∼20 mN/m) is much smaller than that of the surface consisting of methylene −CH2 groups (∼30 mN/m).12 In addition, the lateral van der Waals interaction between molecules, tens of kJ/mol, dictates the monolayer’s struc© XXXX American Chemical Society

Received: January 6, 2016 Revised: July 7, 2016

A

DOI: 10.1021/acs.jpcb.6b00119 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Figure 1. SEM images of silica nanoparticles with 300 nm in diameters prepared by the Stöber method (A) and of a C18/SiO2 composite with φC18 = 8% (B).

dissolved or dispersed in n-hexane and bath-sonicated for 15 min and then mixed to the desired volume ratio to obtain composites with certain compositions. After stirring for 48 h at room temperature, the samples were exposed to air in order to evaporate the solvent and then dried in a vacuum oven for 48 h at 25 °C to remove residual solvent. The morphologies of silica spheres and C18/SiO2 composites samples were examined by the JEOL-JSM-6700F scanning electron microscope fitted with a field emission source and operated at an accelerating voltage of 10 kV. Differential scanning calorimetry (DSC) measurements were performed with a Q2000 (TA Instruments) at a rate of 2 °C/min. All measurements were conducted in the temperature ranges of 10−40 °C under a nitrogen atmosphere. The transition temperatures and heat capacities were calculated via the TA Universal Analysis 2000 software. In situ wide-angle X-ray diffraction 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 X-rays was 1.24 Å. WAXD patterns were collected by a Mar CCD with a resolution of 3072 × 3072 pixels (pixel size: 73 × 73 mm2). Image acquisition time was 30 s. The sample to detector distance was 368 mm. The variable-temperature solid 13C NMR experiments were performed on a 400 MHz Bruker Avance III fitted with a BVT 3000 digital thermal controller at 13C Larmor frequency of 100.38 MHz. Samples were contained in a cylindrical 4 mm rotor made of zirconia at room temperature (all samples crystallize into their low-temperature stable crystal structures). The pulse program CPTOSS was used to obtain the NMR spectra of adsorbed molecules on the silica surface after samples were equilibrated at a certain temperatures for 30 min. The contact time was 3 ms.

The surface freezing phenomenon has attracted much attention from both scientific and technological considerations because the surface-frozen monolayer can act as the nucleation site for the rotator phase, a plastic crystalline state of alkanes with orientational disorder. Consequently, the surface monolayer is found to precede bulk crystallization and results in the absence of supercooling in n-alkanes.19−21,30 Moreover, surface freezing causes changes of macroscopic properties, such as capillary wave damping.31,32 Our previous works have focused on how the phase transition behaviors of alkanes are affected by geometric confinement in mesopores33−36 and nanoparticle composites.37,38 An enhanced surface freezing phenomenon has been found in both C16/SiO2 and C19/SiO2 composites. This was explained in terms of the significantly enlarged contribution of interfacial molecules induced by the huge specific area of silica nanoparticles. For odd alkanes, the rotator phase exists both in the bulk and in their composite counterparts, whereas for C16 this phase does exist neither in the bulk nor in the C16/ SiO2 composites. As a result, the question whether the enhanced surface freezing influences the stability of the rotator phase could not be addressed so far. Moreover, the thermodynamic mechanism underlying the enhanced surface freezing has not been fully understood yet. In order to assess the relationship between the interfacial-ordered monolayer and bulk crystallization in the n-alkane/SiO2 composites and to further scrutinize the surface freezing mechanism, n-octadecane (C18) is chosen in this work. It exhibits surface freezing, a transient rotator phase, and more importantly the stability of the rotator phase can be affected by geometrical confinement.34,39 Homogeneous C18/SiO2 composites with monodisperse silica spheres (300 nm in diameters) as the main composition were prepared by the solution mixing method. DSC, VT solid-state 13C NMR measurements, and in situ WAXD measurements were performed to detect the phase transition behaviors of the composites. In addition, a thermodynamic model reported in the literature40 is used to quantitatively analyze the phenomenon observed.

■

RESULTS AND DISCUSSION As shown in Figure 1A, silica particles prepared by the Stöber method exhibit a narrow size distribution. The C18/SiO2 composite with mass ratio of C18 φC18 ≤ 10% is expected to be homogeneous with almost all n-alkane molecules adsorbed on the silica surface and no hints of bulk C18 observable in Figure 1B. Our previous results have shown that the Stöber silica particles can provide efficient surface confinement effects on alkanes at a high SiO2 mass fraction.37,38 Alkane/SiO2 composites exhibit enhanced surface freezing phenomenon, which was believed to be induced by the huge specific area of silica nanoparticles. To explore the underlying thermodynamic mechanism, the phase transitions for C18/SiO2 composites are investigated systematically.

■

EXPERIMENTAL SECTION Normal octadecane (C18H38, or C18) with purity of 99% was purchased from Sigma-Aldrich Co. and was used as received. The spherical silica particles with a dispersion of less than 5% were synthesized following the Stöber method.41 The size of the silica particles used here was 300 nm in diameters. Taking into account the complexity of in situ polymerization, which introduces the chemical bond and grafting density, the solution mixing is used here to ensure the homogeneity of the composites. The C18 and silica particles were separately B


Article


Figure 2. (A) DSC curves for pure C18 (dash dot) and C18/SiO2 composite (blue solid line) both during cooling and heating scans, where hinterfacial freezing denotes the surface phase transition at the higher temperature, and l-interfacial freezing denotes the phase transition at lower temperature; (B) In-situ wide-angle X-ray diffraction patterns of a C18/SiO2 composite with φC18 = 8% upon cooling; (C) DSC curves for a C18/SiO2 composite with φC18 = 8% at different heating/cooling rates.

As is well-known, even n-alkanes with n ≤ 20 and n > 40 do not exhibit an equilibrium rotator phase.39 Therefore, below the crystallization temperature TPL/S (26 °C, P is for pure), pure C18 exhibits a liquid-to-solid (L/S) phase transition and forms a low-temperature triclinic crystal in accord with the single latent peak in the DSC curve (the black dash dot in Figure 2A). Interestingly, for the C18/SiO2 composite with high silica loading (i.e., φC18 = 8%), multiple peaks and hence indications for peculiar phase transitions are observed in the DSC traces both during cooling and heating scans (see the blue solid line in Figure 2A). To further explore the phase sequence of the C18/ SiO2 composite, in situ WAXD measurements were performed. The results are shown in Figure 2B. With temperature decreasing to 25 °C, two intensity peaks, that is, the (110) and (200) Bragg peaks of an orthorhombic rotator phase I (RI), occur. Upon further cooling the composite to 21 °C, the (010), (011), (100) and (111) peaks of the triclinic phase emerge. In other words, C18/SiO2 composites transform into a metastable rotator phase from the isotropic liquid and subsequently crystallize to the stable triclinic phase with further decreasing temperature. This explains the two latent peaks below TCL/R (TCL/R ≈ 26 °C, C denotes composite) as shown in Figure 2A. Obviously, the stability of the rotator phase RI indicates a crossover from a transient character in bulk to a metastable one in the composite due to the presence of the silica particles. The underlying mechanism will be explored in further detail below. Noticeably, there are two novel small exothermic peaks appearing above the TCL/R of C18/SiO2 composites. One of them is located at 29 °C (3 °C higher than TCL/R), the other one with a smaller exothermic intensity appears at 2 °C above TCL/R. Correspondingly, upon heating the inverse phase sequence is observed above the melting temperature of the composites,

however with a sizable thermal hysteresis. In addition, the DSC signals, especially for the exothermic/endothermic peaks which are assigned to interfacial freezing, are independent of the scanning rates shown in Figure 2C. Thus, it can be safely concluded that the smaller exothermic peak is not a shoulder peak of the peak appearing at 29 °C, indicating that these two novel small exothermic peaks demonstrate a first-order phase transition character. For n-alkanes either in free bulk or adsorbed on the solid substrates, the surface freezing phenomenon is observed to occur at ∼3 °C above the crystallization temperature.13−16 Considering the similar phase transition temperature as well as the first-order phase transition characteristics as that of surface freezing, two novel peaks in the C18/SiO2 composites are likely to arise from interfacial molecules in the C18/SiO2 composites, as we argue in more detail below. For convenience, the two small peaks are hereafter referred to as “h-interfacial freezing” and “l-interfacial freezing”, respectively. Because the surface of Stöber silica particles studied here is chemically similar to the native oxide layer typical of SiO2 substrates, we investigated the structure of C18 films adsorbed on silicon wafer using grazing-incident-angle X-ray diffraction (see Supporting Information). As inferred from these X-ray measurements, the long-axis of the alkane molecules is oriented normal to the adsorbing SiO2 surface. This is consistent with the results reported in the literatures,19−21 where using ellipsometry, an ordered monolayer with molecular orientation normal to the planar silica or mica substrates coexists with droplets of liquid n-alkanes a few degrees above the bulk melting/crystallization temperature. Though the air is replaced by the solid substrate, the orientation of n-alkane molecules normal to the surface is preferable unless the interaction between n-alkanes molecules and the substrate is stronger than C


Article


Figure 3. (A) Variation of phase transition temperatures of the C18/SiO2 composite with the concentration of C18, where the black square denotes the h-interfacial freezing, the red empty square denotes the l-interfacial freezing, the blue diamond represents the liquid-rotator transition temperature, and the pink circle represents the rotator-crystal transition temperature. (B) Absolute enthalpy of h-interfacial freezing and l-interfacial SF,l SF,h SF,l freezing in the composites both during heating (ΔHSF,h m , ΔHm ),and cooling (ΔHc , ΔHc ) processes. m denotes the melting process, c denotes the cooling process.

at the chain-ends). This expression suggests that the amount of molecules in the surface monolayer decrease linearly with increasing the mass of C18 in the composite. In other words, the latent heat for surface freezing should vary linearly with the content of C18 in the composites. This is consistent with the experimental result where a linear relationship between ΔHSF,h c and φC18 is demonstrated in Figure 3B. Therefore, h-surface freezing is further evidenced to arise from the enhanced surface freezing phenomenon by the silica surface. The difference between the coefficients of expression 1 (−0.0153) and the experimental slope (−0.057) may originate from the combined contribution from the not well-specified latent heat of surface freezing for pure n-alkanes,17,18 as well as the inhomogeneities of the composite specimens. As shown in Figures 2A and 3A, below the h-interfacial freezing temperature another novel exothermic peak referred to as l-interfacial freezing, is observed for C18/SiO2 composites with φC18 ≤ 10%. It vanishes gradually with φC18 increasing to 15%. Meanwhile, the transition temperatures exhibit a strong dependence on φC18. For normal alkanes adsorbed on the solid substrate with submonolayer coverage, the amount of n-alkanes cannot form a continuous and complete monolayer with surface-normal molecular orientation.42−44 In the case of C30 deposited on silica substrate with submonolayer coverage, an interfacial alkane monolayer with a molecular orientation preferentially normal to the interface but no positional order is observed immediately after cooling below the surface freezing temperature Tsf ≈ 70 °C. Further cooling to a lower temperature (67.5 °C), the positional molecular ordering with hexagonal symmetry is observed in the surface monolayer by X-ray diffraction.45 Interestingly, Volkmann et al. also reported a crystalline-to-plastic transition in the monolayer phase of C32 on SiO2 substrate with submonolayer coverage.46 They speculate that this kind of disorder-to-order transition arises from the enhanced translational disorder induced by the imperfect substrate surface and also the efficient heat flux through the substrate.13−16 Because the silica sphere’s surface in this work is amorphous and imperfect, it is most likely that the l-surface freezing phase transition with tiny enthalpy can also be attributed to such a disorder-to-order transition. To provide complementary information on the characteristics of the phase transition in the surface monolayer near the silica surface, especially on the orign of the l-interfacial freezing, variable-temperature solid-state 13C NMR measurements of

the lateral intermolecular interaction in the surface monolayer.21 The interaction between amorphous silica surface and alkanes molecules, which will be calculated from a thermodynamic model suggested by Ocko et al. in ref 40, is on the order of tens mN/m2. Furthermore, the amorphous and imperfect silica surface endows molecules near the silica surface with enhanced translational disorder and conformational defects at the chain ends. This can essentially stabilize the surface freezing monolayer against the entropy loss and thus thermodynamically stabilize the surface monolayer.13−16,19−21 In this regard, surface freezing prefers to occur in the C18/SiO2 composites with the longitudinal axis of alkane molecules normal to the silica surface. As shown in Figure 3A, the h-interfacial freezing peaks appear for C18/SiO2 composites with various compositions and the peak temperatures are located at 29 °C, coinciding with the surface freezing temperature for pure C18 and independent of the compositions. Therefore, the latent heat peak indicated by h-interfacial freezing can be traced to the surface freezing monolayer of C18 formed on the surface of silica particles. Although surface freezing is reported to be a first-order phase transition, this phenomenon has not been detected by DSC for pure n-alkane melts so far. In the C18/SiO2 composites, the latent heat for surface freezing is enlarged dramatically due to the huge surface-to-volume ratio of SiO2 particles and thus an enhanced number of interfacial molecules is involved in surface freezing. This allows for detecting surface freezing for C18/SiO2 composites by normal DSC methods as shown in Figure 2A. In this regard, the latent heat involved in surface freezing should be proportional to the amount of molecules “standing” on the silica surface. Assuming an ideal homogeneous composite and a continuous surface monolayer on the silica surface, the mass of surface freezing molecules can be calculated as follows msf =

mSiO2 4πr 2 4

ρSiO 3 πr 2

l ρ = 3 C18 C 18

3lC18ρC

18

r ·ρSiO

(1g − mC18)

2

= A(1g − mC18) = 0.0153(1g − mC18)

(1)

where mSiO2 and mC18 is the mass of SiO2 and C18 in the composites respectively, and mSiO2 + mC18 = 1g, ρ is the density of each components, r is the radius of silica particles, and lC18 is the extended chain length of C18 molecule with all-trans conformation (actually, there are some conformational defects D


Article

The Journal of Physical Chemistry B C18/SiO2 composite with φC18 = 8% were performed. For the composite with φC18 = 8%, most of C18 molecules are near the silica surface and then solidify in a nonequilibrium monolayer just below the h-interfacial freezing temperature, just as discussed above. As shown in Figure 4, C18 in the composite

cooling this adsorbed monolayer transforms into a 2D surfacefrozen state with densely packed, surface-normal molecules. Following the argumentation of Ocko et al.40 the disordering temperature, T*, is related to the alkane’s methyl/SiO2 interaction, ECH3, through a simple thermodynamical model balancing interfacial bonding, chain stretching elasticity, and entropic effects:40,50 ⎛ ECH3 ⎞⎡ ⎛ λ ⎞ 8αKECH3 ⎤ ⎥ T * = TB + ⎜ ⎟ ⎢1 − ⎜ ⎟ ⎝ 9 ⎠ (18RT ) ⎥⎦ ⎝ ΔS ⎠⎢⎣ −1

(2)

−1

where ΔS = 0.14 kJ·mol ·K is the bulk melting entropy of an 18-carbon alkane,41 TB = 27 °C is the alkane’s bulk melting temperature, λ = ρl/ρs = 0.85 is the alkane’s liquid/solid mass density ratio, R and αK = 7 are the gas constant and number of carbons per Kuhn segment, respectively,51 and ECH3 is the excess free energy of the alkane’s interface bound terminal CH3 group over one in the melt. Our T* = 29 °C yields ECH3 = 0.3 kJ/mol, approximately 1/30 of the value of C18OH on planar amorphous silica.31 This value appears reasonable, because the interaction here is nearly the same as the surface energy change of the free alkane melt 3 mJ/m2 = 0.46 kJ/mol and solely van der Waals like,50 whereas in the case of the C18OH/SiO2 system the interaction is determined by the much stronger hydrogen bonding. Thus, the thermodynamic model provides further evidence for the identity of h-interfacial freezing and linterfacial freezing discussed above. In Figure 3A, the less the mass ratio of C18 is, the lower the crystallization temperature TCL/R of C18/SiO2 composites is. As mentioned above, the surface monolayer is enhanced by the silica surface in the composites. Calculated by the formula 1 − ΔHm/[φC18 × 240.7 J/g] (240.7 J/g is the melting enthalpy of pure C18), about 64% of alkane molecules are involved in surface freezing for C18/SiO2 composites with φC18 = 5%. According to the literature,19−21,30 the surface monolayer can act as the ideal nucleation site for the rotator phase and thus reduce the nucleation barrier. As a result, the stability of rotator RI phase exhibits the crossover from a transient one in bulk C18 to the metastable one in the C18/SiO2 composites. In this regard, the nucleation is probably not the reason for the lower C crystallization temperature TL/R of C18/SiO2 composites compared to that of pure C18. For the composite with φC18 = 5%, the remaining C18 molecules are minor and confined separately into a plethora of the randomly connected interspaces of the silica network. As a result, after forming a complete solid monolayer on the silica surface, the composites have only a limited reservoir of molecules in every interspace for the subsequent bulk crystallization. Thus, the liquid molecules have to be transported over large distances to deposit on the solidification front, which effectively increases the undercooling and eventually results in the fact that the composites with φC18 ≤ 10% demonstrating significantly lower crystallization temperatures compared to pure C18. With φC18 increasing up to 20%, there are sufficient molecules in the composite for bulk crystallization, therefore the hysteresis induced by the molecular transport is reduced and eventually disappears, resulting in the gradual increase of TCL/R shown in Figure 3A. For n-alkane/SiO2 composites with hydrocarbon chain lengths ranging from 16 to 19, the enhanced surface freezing phenomenon can be observed independent of the chain length. As discussed above, the van der Waals interaction between amorphous silica surface and alkane molecules allows for the

Figure 4. Variable-temperature solid-state 13C NMR spectra of C18/ SiO2 composite with φC18 = 8% at selected temperatures indicated in the figure during cooling.

is in the isotropic liquid state at above 31 °C, as confirmed by the peak at 29.0 ppm typical of the chemical shift for interior methylenes (int-CH2) of liquid pure n-alkanes.38,39 Decreasing the temperature to 29 °C, h-interfacial freezing occurs with an identical NMR spectrum to that of liquid alkanes, meaning that the surface monolayer formed is in an amorphous state. Further decreasing the temperature to 27 °C, l-interfacial freezing occurs, which results in a downward-shift of the int-CH2 by 0.2 ppm. For n-alkane molecules, the different molecular packing in different crystal phases results in the shift for the resonances of the int-CH2 groups.47,48 Generally, the more molecular order the crystal phase exhibits, the stronger the intermolecular interactions are, which is accompanied by the downfield shift of the resonances of int-CH2 groups. In this regard, a surface ordered monolayer forms just below the l-interfacial freezing temperature, since the chemical shift of int-CH2 at 27 °C is between that of the liquid state and the rotator phase of nalkanes.47,48 Hence, the NMR measurements provide additional experimental evidence that the l-interfacial freezing transition is related to a disorder-to-order transition in the surface monolayer. The disorder-to-order transition in the interface monolayer is further evidenced by the fact that the latent heat for l-interfacial freezing is tiny and shows no dependence on the amount of interfacial molecules, as shown in Figure 3B. As the temperature is further decreased to 21 °C, the C18/SiO2 composite is trapped into the triclinic phase, indicated by 3 ppm downfield shift of the int-CH2 resonances compared to that in the liquid phase. Unfortunately, we have no direct microscopic information on the evolution in the interfacial structure. However, recently such microscopic insights were provided for n-alcohols at planar sapphire49 and more importantly planar amorphous silica surfaces.40 Interestingly, it was reported that n-alcohols exhibit a peculiar interfacial phase behavior at amorphous solid surfaces similar to that found here. Upon cooling the alcohols melt, a 2D liquid-like monolayer forms at the amorphous sapphire substrate. This was inferred from surface-sensitive Xray scattering experiments which indicated that the alcohol molecules align loosely along the surface normal with their OHgroups directed toward the silica substrate. Upon further E



■

formation of a surface freezing monolayer in the interfacial region and further the huge surface-to-volume ratio of SiO2 particles induces an enhanced number of interfacial molecules involved in surface freezing. As already mentioned in the Introduction, the in-plane structure of the surface monolayer is similar to the metastable rotator RI phase (orthorhombic crystal).30 Hence, it can act as nucleation site for the structural compatible orthorhombic rotator phase. However, the relationship between surface freezing and the stability of the rotator phase cannot be illustrated based on our previous experimental data from C19/ SiO2 and C16/SiO2. For C19, the rotator phase exists both in the bulk and in their composite counterparts whereas for C16, the chain length is so short that the rotator phase occurs in neither the bulk nor the C16/SiO2 composites. Interestingly, it could indeed be proven here that the enhanced surface freezing affects significantly the stability of the rotator phase in the C18/ SiO2 composites. In bulk C18, the rotator phase does not exist, whereas in its composite counterparts, the rotator phase occurs during the cooling process accompanying the enhanced surface freezing.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-10-62556180. Fax: +8610-82362045. *E-mail: [email protected]. Phone: +86-10-82618533. Fax: +86-10-82612857. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We acknowledge the financial support by National Natural Science Foundation of China (21474120). We gratefully appreciate the staff and scientists at beamline 14B in Shanghai Synchrotron Radiation Facility (SSRF) for beam time and technical assistance. The German research foundation (DFG) contributed to this research within the collaborative research initiative “Tailor-made Multi-Scale Materials Systems” (project B7), Hamburg.

■

■

REFERENCES

(1) Alcoutlabi, M.; McKenna, G. B. Effects of Confinement on Material Behaviour at the Nanometer Size Scale. J. Phys.: Condens. Matter 2005, 17, R461−R524. (2) Frenken, J. W. M.; van der Veen, J. F. Observation of Surface Melting. Phys. Rev. Lett. 1985, 54, 134−137. (3) Wei, X.; Miranda, P. B.; Shen, Y. R. Surface Vibrational Spectroscopic Study of Surface Melting of Ice. Phys. Rev. Lett. 2001, 86, 1554−1557. (4) Lied, A.; Dosch, H.; Bilgram, J. H. Surface Melting of Ice Ih Single-Crystals Revealed by Glancing Angle X-ray Scattering. Phys. Rev. Lett. 1994, 72, 3554−3557. (5) 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: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1997, 55, 3164−3182. (6) Gang, O.; Ocko, B. M.; Wu, X. Z.; Sirota, E. B.; Deutsch, M. Surface Freezing in Hydrated Alcohol Melts. Phys. Rev. Lett. 1998, 80, 1264−1267. (7) Gang, O.; Wu, X. Z.; Ocko, B. M.; Sirota, E. B.; Deutsch, M. Surface Freezing in Chain Molecules. II. Neat and Hydrated Alcohols. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1998, 58, 6086−6100. (8) Gang, O.; Ocko, B. M.; Wu, X. Z.; Sirota, E. B.; Deutsch, M. Tunable Surface Phases in Alcohol-Diol Melts. Phys. Rev. Lett. 1999, 82, 588−591. (9) Wu, X. Z.; Ocko, B. M.; Deutsch, M.; Sirota, E. B.; Sinha, S. K. Surface Freezing and Surface-Phase Behaviors in Binary Mixtures of Alkanes. Phys. B 1996, 221, 261−266. (10) Gautam, K. S.; Dhinojwala, A. Melting at Alkyl Side Chain Comb Polymer Interfaces. Phys. Rev. Lett. 2002, 88, 145501. (11) Gautam, K. S.; Kumar, S.; Wermeille, D.; Robinson, D.; Dhinojwala, A. Observation of Novel Liquid-Crystalline Phase above the Bulk-Melting Temperature. Phys. Rev. Lett. 2003, 90, 215501. (12) Sirota, E. B.; Wu, X. Z.; Ocko, B. M.; Deutsch, M. What Drives the Surface Freezing in Alkanes? Phys. Rev. Lett. 1997, 79, 531. (13) Tkachenko, A. V.; Rabin, Y. Fluctuation-Stabilized Surface Freezing of Chain Molecules. Phys. Rev. Lett. 1996, 76, 2527−2530. (14) Zhang, Y.; Ou-Yang, Z. C.; Iwamoto, M. Surface Freezing in Normal Alkanes: A Statistical Physics Approach. J. Chem. Phys. 2006, 124, 214906. (15) Colussi, A. J.; Hoffmann, M. R.; Tang, Y. Conformational Disorder Binds n-Alkanes into Surface Monolayers above the Normal Freezing Point. Langmuir 2000, 16, 5213−5217. (16) Li, H. Z.; Yamamoto, T. The Surface-Ordered Phase of nNonadecane: A Molecular Dynamics Simulation. J. Chem. Phys. 2001, 114, 5774−5780.

CONCLUSIONS As explored by calorimetry, X-ray diffraction, and solid-state 13 C NMR measurements, C18/silica composites exhibit a peculiar phase transition phenomenology significantly deviating from the bulk C18 behavior: an enhanced surface freezing effect is accompanied by a crossover of the rotator phase from transient to metastable. Moreover, in contrast to our previous results on C16/SiO2 and C19/SiO2 composites two-step transitions are involved in the interface freezing of the C18/ silica composites at a high SiO2 mass fraction. On the basis of the experimental data and a thermodynamic model, the alkane’s methyl/SiO2 interaction is calculated to be van der Waals like, contributing to the formation of interfacial ordered molecules and accounting for the observed behavior quantitatively. Moreover, this peculiar interfacial freezing behavior is found to affect significantly the crystallization behaviors of the surrounding alkane melts, in particular the formation of a rotator phase absent in the bulk state of C18. Interestingly, the finding in this work is in accord with those reported for long-chain alkanes on planar silica surfaces and also corroborates the two-step phase transitions reported in alkanols on planar silica in the literature.40,49 It highlights the intimate relationship between surface freezing and interfacial interactions and also illustrates well the relationship between the interfacial-ordered monolayer and bulk crystallization in the n-alkane/SiO2 composites. For the future, we suggest studies on the chain length dependence of this phenomenology and on the influence of the surface chemistry, for example, by surface functionalization of the nanoparticles. We are also optimistic that molecular dynamics simulations could provide important insights in the reported phenomenology, similar to that for the free surface of molten alkanes.14

■

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b00119. Structure of C18 films adsorbed on silicon wafer (PDF) F


Article

The Journal of Physical Chemistry B (17) Lei, Q. F.; Bain, C. D. Surfactant-Induced Surface Freezing at the Alkane-Water Interface. Phys. Rev. Lett. 2004, 92, 176103. (18) Yefet, S.; Sloutskin, E.; Tamam, L.; Sapir, Z.; Deutsch, M.; Ocko, B. M. Surfactant-Induced Phases in Water-Supported Alkane Monolayers: II. Structure. Langmuir 2014, 30, 8010−8019. (19) Merkl, C.; Pfohl, T.; Riegler, H. Influence of the Molecular Ordering on the Wetting of SiO2/Air Interfaces by Alkanes. Phys. Rev. Lett. 1997, 79, 4625−4628. (20) Holzwarth, A.; Leporatti, S.; Riegler, H. Molecular Ordering and Domain Morphology of Molecularly Thin Triacontane Films at SiO2/ Air Interfaces. Europhys. Lett. 2000, 52, 653−659. (21) Maeda, N.; Yaminsky, V. V. Experimental Observations of Surface Freezing. Int. J. Mod. Phys. B 2001, 15, 3055−3077. (22) Pithan, L.; Meister, E.; Jin, C.; Weber, C.; Zykov, A.; Sauer, K.; Brütting, W.; Riegler, H.; Opitz, A.; Kowarik, S. Thermally Driven Smoothening of Molecular Thin Films: Structural Transitions in nAlkane Layers Studied in Real-Time. J. Chem. Phys. 2015, 143, 164707. (23) Weber, C.; Frank, C.; Bommel, S.; Rukat, T.; Leitenberger, W.; Schäfer, P.; Schreiber, F.; Kowarik, S. Chain-Length Dependent Growth Dynamics of n-Alkanes on Silica Investigated By EnergyDispersive X-Ray Reflectivity in situ and in Real-Time. J. Chem. Phys. 2012, 136, 204709. (24) Maeda, N.; Kohonen, M. M.; Christenson, H. K. Phase Transition of n-Alkane Layers Adsorbed on Mica. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2000, 61, 7239−7242. (25) Volkmann, U. G.; Pino, M.; Altamirano, L. A.; Taub, H.; Hansen, F. Y. High-Resolution Ellipsometric Study of an n-Alkane Film, Dotriacontane, Adsorbed on a SiO2 Surface. J. Chem. Phys. 2002, 116, 2107−2115. (26) Mo, H.; Taub, H.; Volkmann, U. G.; Pino, M.; Ehrlich, S. N.; Hansen, F. Y.; Lu, E.; Miceli, P. A Novel Growth Mode of Alkane Films on a SiO2 Surface. Chem. Phys. Lett. 2003, 377, 99−105. (27) Mo, H.; Trogisch, S.; Taub, H.; Ehrlich, S. N.; Volkmann, U. G.; Hansen, F. Y.; Pino, M. Studies of the Structure and Growth Mode of Dotriacontane Films by Synchrotron X-Ray Scattering and Molecular Dynamics Simulations. J. Phys.: Condens. Matter 2004, 16, S2905− S2910. (28) Yamamoto, M.; Sakurai, Y.; Hosoi, Y.; Ishii, H.; Kajikawa, K.; Ouchi, Y.; Seki, K. Structures of a Long-Chain n-Alkane, n-C44H90, on a Au(111) Surface: An Infrared Reflection Absorption Spectroscopic Study. J. Phys. Chem. B 2000, 104, 7363−7369. (29) Masnadi, M.; Urquhart, S. G. Effect of Substrate Temperature on the Epitaxial Growth of Oriented n-Alkane Thin Films on Graphite. Langmuir 2012, 28, 12493−12501. (30) Sirota, E. B. Supercooling, Nucleation, Rotator Phases, and Surface Crystallization of n-Alkane Melts. Langmuir 1998, 14, 3133− 3136. (31) Gang, H.; Patel, J.; Wu, X. Z.; Deutsch, M.; Gang, O.; Ocko, B. M.; Sirota, E. B. Macroscopic Manifestations of Surface Crystallization. Europhys. Lett. 1998, 43, 314−319. (32) Huber, P.; Soprunyuk, V. P.; Embs, J. P.; Wagner, C.; Deutsch, M.; Kumar, S. Faraday Instability in a Surface-Frozen Liquid. Phys. Rev. Lett. 2005, 94, 184504. (33) Huber, P.; Wallacher, D.; Albers, J.; Knorr, K. Quenching of Lamellar Ordering in an n-Alkane Embedded in Nanopores. EPL 2004, 65, 351−357. (34) Huber, P.; Soprunyuk, V.; Knorr, K. Structural Transformations of Even-Numbered n-Alkanes Confined in Mesopores. Phys. Rev. E 2006, 74, 031610. (35) Henschel, A.; Hofmann, T.; Huber, P.; Knorr, K. Preferred Orientations and Stability of Medium Length n-Alkanes Solidified in Mesoporous Silicon. Phys. Rev. E 2007, 75, 021607. (36) Huber, P. Soft Matter in Hard Confinement: Phase Transition Thermodynamics, Structure, Texture, Diffusion and Flow in Nanoporous Media. J. Phys.: Condens. Matter 2015, 27, 103102. (37) Jiang, K.; Xie, B. Q.; Fu, D. S.; Luo, F. L.; Liu, G. M.; Su, Y. L.; Wang, D. J. Solid−Solid Phase Transition of n-Alkanes in Multiple Nanoscale Confinement. J. Phys. Chem. B 2010, 114, 1388−1392.

(38) Fu, D. S.; Su, Y. L.; Gao, X.; Liu, Y. F.; Wang, D. J. Confined Crystallization of n-Hexadecane Located inside Microcapsules or outside Submicrometer Silica Nanospheres: A Comparison Study. J. Phys. Chem. B 2013, 117, 6323−6329. (39) 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. (40) Haddad, J.; Steinrück, H. G.; Hlaing, H.; Kewalramani, S.; Pontoni, D.; Reichert, H.; Murphy, B. M.; Festersen, S.; Runge, B.; Magnussen, O. M.; et al. Order and Melting in Self-Assembled Alkanol Monolayers on Amorphous SiO2. J. Phys. Chem. C 2015, 119, 17648− 17654. (41) Stö ber, N.; Fink, A.; Bohn, E. Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Colloid Interface Sci. 1968, 26, 62−69. (42) Basu, S.; Satija, S. K. In-situ X-Ray Reflectivity Study of Alkane Films Grown from the Vapor Phase. Langmuir 2007, 23, 8331−8335. (43) Sefler, G. A.; Du, Q.; Miranda, P. B.; Shen, Y. R. Surface Crystallization of Liquid n-Alkanes and Alcohol Monolayers Studied by Surface Vibrational Spectroscopy. Chem. Phys. Lett. 1995, 235, 347−354. (44) Corrales, T. P.; Bai, M. J.; del Campo, V.; Homm, P.; Ferrari, P.; Diama, A.; Wagner, C.; Taub, H.; Knorr, K.; Deutsch, M.; et al. Spontaneous Formation of Nanopatterns in Velocity-Dependent DipCoated Organic Films: From Dragonflies to Stripes. ACS Nano 2014, 8, 9954−9963. (45) Xie, B. Q.; Liu, G. M.; Jiang, S. C.; Zhao, Y.; Wang, D. J. Crystallization Behaviors of n-Octadecane in Confined Space: Crossover of Rotator Phase from Transient to Metastable Induced by Surface Freezing. J. Phys. Chem. B 2008, 112, 13310−13315. (46) Cisternas, E. A.; Corrales, T. P.; del Campo, V.; Soza, P. A.; Volkmann, U. G.; Bai, M.; Taub, H.; Hansen, F. Y. Crystalline-toPlastic Phase Transitions in Molecularly Thin n-Dotriacontane Films Adsorbed on Solid Surfaces. J. Chem. Phys. 2009, 131, 114705. (47) Möller, M.; Cantow, H. J.; Drotloff, H.; Emeis, D.; Lee, K. S.; Wegner, G. Phase Transitions and Defect Structures in the Lamellar Surface of Polyethylene and n-Alkane Crystallites. Makromol. Chem. 1986, 187, 1237−1252. (48) Vanderhart, D. L. Influence of Molecular Packing on Solid-State 13 C Chemical Shifts: The n-Alkanes. J. Magn. Reson. 1981, 44, 117− 125. (49) Ocko, B. M.; Hlaing, H.; Jepsen, P. N.; Kewalramani, S.; Tkachenko, A.; Pontoni, D. Unifying Interfacial Self-Assembly and Surface Freezing. Phys. Rev. Lett. 2011, 106, 137801. (50) The Physical Chemistry of Lipids: From Alkanes to Phospholipids; Small, D. M., Ed.; Plenum Press: New York, 1986. (51) Flory, P. J. Statistical Mechanics of Chain Molecules; Interscience Publishers: New York, 1969.

G


Two-Step Freezing in Alkane Monolayers on Colloidal Silica

Recommend Documents