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Unusual Interfacial Freezing Phenomena in Hexacontane/Silica Composites Xia Gao, Yunlan Su, Weiwei Zhao, Qingyun Qian, Xin Chen, Robert Wittenbrink, and Dujin Wang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b00603 • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 26, 2017

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The Journal of Physical Chemistry

Unusual Interfacial Freezing Phenomena in Hexacontane/Silica Composites Xia Gao1,2, Yunlan Su1,2*,Weiwei Zhao1,2, Qingyun Qian3, Xin Chen3, Robert Wittenbrink3, Dujin Wang1,2 1. Beijing National Laboratory for Molecular Sciences, Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China 2. University of Chinese Academy of Sciences, Beijing 100049, China 3. ExxonMobil Asia Pacific Research & Development Co., Ltd, 1099 Zixing Road, Minhang district, 200241 Shanghai, P.R. China

Corresponding authors: Dr. Yunlan Su Beijing National Laboratory for Molecular Sciences, Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China Phone: +86-10-82618533 Fax: +86-10-82618533 *Emails: [email protected]

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Abstract: The crystallization behaviors of n-hexacontane (C60H122)/Stöber silica (SiO2) composites with various compositions were investigated by a combination of differential scanning calorimetry (DSC), solid-state (solid-state

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C nuclear magnetic resonance

C NMR) and Proton NMR relaxation experiments. By means of DSC,

C60H122 molecules in C60H122/silica composites were observed to be involved in the interfacial freezing not present in the free bulk C60H122. The orientation of C60H122 molecules, being preferentially normal to silica surface, was confirmed by grazing incidence X-ray diffraction experiments on thin n-hexacontane film adsorbed on the silicon wafer with a native SiO2 layer. Inferred from the solid

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C NMR data, the

interfacial monolayer is in orthorhombic phase with certain chain disorders. It is speculated that the “interfacial freezing” of C60H122 formed in the presence of silica particles is driven by the combination of the strong attraction between the molecules and the enhanced number of interfacial molecules on the amorphous and rough silica surface. Keywords: Interfacial freezing, n-hexacontane, silica nanoparticle, confined crystallization

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1. INTRODUCTION Normal alkanes (CH3-(CH2)n-2-CH3, abbreviated as CnH2n+2) are attracting many attentions as prototypes of polymers as well as the main constituents of biologically important molecules such as lipids and commercial lubricants.1 They are also of fundamental scientific interest, due to the simplicity in their chemical structures but the abundance in the phase transitions.2-13 Especially, a peculiar surface freezing phenomenon has been discovered for alkanes with carbon number n ranging from 16~50.6-11 Different from the surface melting observed in most condensed matter systems,14,15 surface freezing is a phenomenon where a crystalline monolayer is formed at the surface of the liquid alkane at a temperature up to ~3 °C above the bulk melting point Tm. In particular, the surface-frozen monolayer can act as the nucleation site forthe rotator phase which is 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.2-5 So far, the surface freezing phenomenon has attracted much attention from both scientific and technological considerations. A number of theoretical approaches16-20 ranging from simple surface energy considerations,11,16 entropic stabilization by surface-normal molecular fluctuations,17 to a lowering of the molecule’s internal energy by chain-end conformational disorder18 have been used to explain the surface freezing phenomenon. With the carbon number n increasing (i. e. n ≥ 50), neither surface freezing nor rotator phase occurs in the phase sequence of n-alkanes. Instead, long-chain n-alkanes exhibit a simple melting/crystallization phase transition. Moreover, a premelting 3



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process is reported to occur in the lamellar surface of long chain molecules C50H102 and C60H122.21,22 During heating process, the regular packing of the end-group sequence of long chain molecules is disrupted due to thermal energy, as a result the chain ends develop into a disordered state at 30-40 oC below the melting point of long chain alkanes. When n-alkanes are adsorbed on solid substrates (for example, mica and silica substrates), the peculiar crystalline monolayer can also been observed, i. e. the interfacial freezing. It is worth noting that C50H102 and C60H122 films adsorbed on Si (111) surface are revealed to demonstrate interfacial freezing with n-alkane chain being closely aligned to the substrate normal and exhibiting a high degree of in-plane surface order, which is not observed in their free bulk specimens. In other words, the carbon number n of n-alkanes, forming a crystalline monolayer above the bulk melting point Tm, ranges from 14 to 60 when n-alkanes are adsorbed on solid substrates.24-28 Maeda et al. indicated that the interplay between the lateral monolayer cohesion and its adhesion to the surrounding solid substrate is very important for proper assessment of interfacial freezing phenomena.25 In other words, unless the van der Waals adhesion of the monolayer to the substrate is stronger than the lateral monolayer cohesion, the orientation of n-alkane molecules normal to the surface is preferable, both in the state of adsorbed layers at the solid surface and at the liquid–vapor interface. Though, there is no theoretical explanation or model to fully illustrate the interfacial freezing phenomena of n-alkanes, especially those occurred in long-chain n-alkanes. 4


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At present, our group has prepared a series of alkane/SiO2 composites (such as C16H34/SiO2, C18H38/SiO2, C19H40/SiO2).29-31 In these composites, a crystalline monolayer similar to those occurred in the free liquid/vapor interface of their bulk counterparts is observed through conventional DSC method. It is proved that the interaction between amorphous silica surface and alkane molecules, which is calculated to be 0.3 kJ/mol and much smaller than the chain-chain van der Waals interaction, favors the normal orientation of alkanes molecules in the interfacial region.30 In this way, the interfacial freezing is enhanced in the n-alkane/SiO2 composites, which provides further experimental evidences for the theoretical explanation about the interfacial freezing on the solid substrates proposed by Maeda et al.25 Also, the interfacial monolayer in the composites is found to stabilize the rotator phase to some extent. Obviously, n-alkanes/SiO2 composite system is a useful medium to illustrate the underlying mechanism and the role of interfacial freezing in the phase transition of n-alkanes. In this work, we are curious that in the presence of silica spheres whether the interfacial freezing occurs for those n-alkanes which do not exhibit surface freezing in their bulk state. In this regard, C60H122 is chosen in this work since neither surface freezing nor rotator phase occurs in phase transition of bulk specimen as mentioned above. Moreover, C60H122 seems to be in a pivotal position in the homologous series of alkanes and polymer with incipient chain folding and the premelting process in its crystal structure.11,32 Thus, the phase transition behavior of C60H122 mixed with inorganic filler is worthy to be explored. In this work, homogeneous C60H122/SiO2 5



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composites with silica spheres as the main composition were prepared by the solution mixing method. DSC and solid-state

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C NMR measurements were performed to

detect the phase transition behaviors of the composites. Meanwhile, C60H122 films adsorbed on the silion wafer with native SiO2 layer (which is chemically same with the silica surface) was also studied by means of grazing incident X-ray diffraction to provide structural information for n-alkanes on the silica surface. A unique interfacial freezing phenomenon is explored for the C60H122/SiO2 composites and the underlying mechanism is discussed in the text below. 2. EXPERIMENTAL SECTION 2-1. Materials. Hexacontane (C60H122) with purity of 98% was purchased from Sigma-Aldrich Co. and used as received. The spherical silica particles with size-dispersion less than 5% were synthesized according to the modified Stöber method.33 2-2. Preparation of the C60H122/SiO2 composites. To ensure the homogeneity, the C60H122/SiO2 composites were obtained by means of solution mixing. The C60H122 and silica particles were separately dissolved or dispersed in toluene and then mixed with desired volume ratio. After stirring the mixture for 48 h at 70 °C, the samples were obtained by evaporating solvent at atmospheric environment and then dried in a vacuum oven for 48 h at 70 °C to remove the residual solvent. 2-3. Preparation of the C60H122/SiO2 solid substrate. Thin films of C60H122were prepared by self-assembly of alkane molecules from solution (a concentration of 2 mg/ml in n-heptane) onto polished Si(100) wafers (Semiconductor Processing Co., 6


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Boston, MA) with dimension of approximately 10 mm× 10 mm. The wafers have a native silicon-oxide film with about 12-25 Å in thickness. The C60H122 films were prepared via spin-coating from n-heptane solutions on these silicon wafers (ambient conditions, RT, 3000 rpm). The nonvolatile alkanes remain on the substrate after heptane evaporation with a coverage that scales linearly with the concentration. 2-4. Characterization. The morphology of C60H122/SiO2 composite samples were examined by a JEOL-JSM-6700F scanning electron microscope (SEM) 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) in the temperature range of 40~110 °C under nitrogen atmosphere. The transition temperatures and heat capacity were calculated via TA Universal Analysis 2000 software. In order to ensure the reproducibility, two or three measurements were recorded for each sample. Solid-state 13C NMR experiments were performed on a 400 MHz Bruker Avance III at

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C Larmor frequency of 100.38 MHz. Samples were contained in a cylindrical

4mm 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. The contact time was 3 ms. The film thickness of C60H122 spin-coated on the silicon wafer was measured by an ellipsometer (M-2000 V, J. A. Woollam Co., Lincoln, NE) at a fixed incident angle of 70°. The measurements were conducted at room temperature. 7



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The grazing incidence X-ray diffraction (GIXD) measurements of the final film structure were carried out on a Xeuss SAXS/WAXS system (Xenocs SA, France) at room temperature. A multilayer focused Cu Kα X-ray source (GeniX3D Cu ULD), generated at 50 kV and 0.6 mA, was utilized. The wavelength of the X-ray radiation was 0.15418 nm. A semiconductor detector (Pilatus 300 K, DECTRIS, Swiss) with a resolution of 487×195 pixels (pixel size = 172×172 m2) was applied to collect the scattering signals. Each GIXD pattern was collected with an exposure time of 600 seconds. The incident beam made an angle of ∼ 0.1o with the SiO2 surface, which is slightly less than or comparable to the critical angle for C60H122 (0.15o) and SiO2 (0.11o), respectively. 3. RESULTS AND DISCUSSIONS As shown in Figure 1, it is obvious that the silica obtained by means of modified Stöber method33 is spherical in shape. The average size of the particles is calculated to be 350 ± 1.3 nm, indicating nearly monodispersion of the silica particles in our work. Moreover, the C60H122/SiO2 composites with 10 wt % C60H122 are homogenous as no bulk-like C60H122 particles are observed. This is essential for the study of the effect of silica surface on the phase transition behaviors of C60H122.34,35

Figure 1 SEM images of Stöber silica with 350 ± 1.3 nm in diameter (left) and C60H122/SiO2 8


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composite with φC60H122 = 10 wt % (right).

Firstly, the crystallization behaviors of pure C60H122 and C60H122/SiO2 composites with various compositions (3-20 wt % C60H122) were studied by DSC method, shown in Figure 2. According to references, C60H122 has a sufficiently long chain that does not display rotator phase.23,36 Instead, pure C60H122 exhibits a unique premelting process at about 30-40 oC below the melting temperature, accompanied with a gradual increase in the conformational mobility of segments near the crystal surface with increasing temperature.21,22 As shown in Figure 2A and 2B, pure C60H122 shows one sharp exothermic/endothermic peak in the temperature range of 80~110 oC, representing the complete crystallization/melting of n-alkanes crystal. The crystallization temperature (TP,c) and melting temperature (TP,m) of pure C60H122 is 99.5 oC and 100.3 oC, respectively. Besides the melting/crystallization peaks, a weak and broad endothermic peak is observed on the thermograms of pure C60H122 (at~60 o

C, see Figure 2C), representing the premelting phenomenon reported in references 21

and 22. It has been proved by our previous work that the interfacial effect of silica particles exhibits essential influence on the phase transition behavior of n-alkanes or

n-alcohols.29-31 As expected, in the presence of silica particles the phase transitions of C60H122 are significantly different from that of pure specimens. When φC60H122 = 10, 15, 20 wt %, C60H122/SiO2 composites display two phase transition peaks in the range of 80~110 oC (see Figure 2A and B), but do not exhibit the premelting phenomenon as shown in Figure 2C. Interestingly, upon cooling, one of the phase transitions occurs at a temperature 2 oC below the crystallization temperature TP,c of pure C60H122, and the

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other is at a temperature 2 oC above TP,c. Correspondingly, upon heating the inverse phase sequence is observed for the composites around the melting temperature TP,m of pure C60H122. Moreover, the DSC signals are independent of the scanning rates (Figure 2D). This means the novel peak appearing above TP,c is not a shoulder peak of the one at lower temperature and demonstrates a first-order phase transition characteristics. For n-alkanes either in free bulk or adsorbed on the solid substrates, only the surface/interfacial freezing phenomenon is observed to occur at ~3 oC above the crystallization temperature.6-11 In this regard, the peaks at a temperature 2 oC above TP,c are inferred to arise from the interfacial freezing in the C60H122/SiO2 composites, as we discuss in more detail below.

Figure 2. DSC traces of C60H122/SiO2 composites with various compositions during cooling (A) and heating (B) processes at constant cooling/heating rate (2 oC/min). (C) DSC heating traces of 10


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C60H122/SiO2 composites in the temperature range of 40~110 °C. For clarity, the heat flow of pure C60H122 was multiplied by a factor. (D) DSC curves of C60H122/SiO2 composite with φC60H122 = 10 wt % at different heating/cooling rates.

Since surface/interfacial freezing is a first-order phase transition,10 the latent heat for surface/interfacial freezing is proportional to the amount of molecules in the monolayer. For the C60H122/SiO2 composites, due to the large specific surface area of SiO2 particles, the interfacial interactions and surface energies become extremely important. The amount of alkane molecules standing on the interface between the SiO2 surface and the C60H122 is much larger than that on the vapor–liquid interface of bulk sample. As a result, an enhanced number of interfacial molecules are involved in the interfacial freezing of C60H122/SiO2 composites. This makes it possible that the enhanced enthalpy assigned to interfacial freezing is detected by conventional DSC methods as shown in Figure 2A, which is similar to our previous studies on C16H34/SiO2, C18H38/SiO2 and C19H40/SiO2 nanocomposites.29-31Assuming an ideal homogenous composite and a continuous interfacial monolayer on the silica surface, the mass of interfacial freezing molecules can be calculated as follows: mSiO2 ⋅ 4πr 2 3lC ρC m sf = ∗ l ∗ ρC60 = 60 60 mSiO2 4 3 C60 r ⋅ ρ SiO2 ρ SiO2 ⋅ πr 3

(1),

where mSiO2 and mC 60 is the mass of SiO2 and C60H122 in the composites respectively, ρ is the density of each components, r is the radius of silica particles, lC60 is the extended chain length of C60H122 molecule with all-trans conformation (actually, there are some conformational defects at the chain-ends). Thus the ratio of the mass for interfacial freezing molecules to the mass of C60H122 molecules in the composites is as 11



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follows: (1 − ϕ C60 ) m sf 3lC60 ρ C60 mSiO2 3lC60 ρ C60 (1 − ϕ C60 ) = = = 0.055 mC60 rρ SiO2 mC60 rρ SiO2 ϕ C60 ϕ C60

(2), where φC60H122 is the mass ratio of C60H122 in the composites. From equation (2), it can be calculated that all C60H122 molecules participate in interfacial freezing when φC60H122 ≈ 5.2 ± 0.03 wt %. This well explains the DSC results of C60H122/SiO2 composites when φC60H122 ≤ 5 wt %. Since all C60H122 molecules participate in interfacial freezing for composites with φC60H122 ≤ 5 wt %, these composites display only one phase transition peak on the thermograms and tend to melt above TP,m, see Figure 2B. However, the peak position of the melting transition is exactly the same in the 3% sample and in the pure C60H122. Here, we believe that the coincidence of Tm between 3% sample and pure C60H122 is just accidental, which arises from confinement effect of the silica network in the composite as discussed below. Once the inference about the interfacial freezing of C60H122 is rational, the ratio of interfacial freezing molecules in the composites can also be deduced from melting enthalpies as follows: ∆H bulk , m m sf = (1 ) ⋅ 100% mC60 ∆H pure, m ⋅ ϕC 60

Where ∆Hpure,

m

and ∆Hbulk,

m

(3)

are the melting enthalpies of pure C60H122 and the

bulk-like C60H122 in the composites, respectively. The ratios of interfacial freezing molecules in the composites calculated by Equation 2 and Equation 3 are listed in Table 1. It is obvious that the assumption about interfacial freezing of C60H122 in the composites is well supported, since C60H122 molecules involved in interfacial freezing 12


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in the C60H122/SiO2 composites deduced by Equation 2 are well consistent with that acquired by Equation 3 (Table 1). In other words, in the specimen with 10% C60H122, nearly 50% of molecules are deposited in the interface and involved in interfacial freezing phenomenon, thus melting at a temperature higher than TP,m. For those bulk-like molecules, they tend to crystalline into an orthorhombic phase with disordered chain conformation due to the existence of silica particles (based on the solid

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C NMR results in Figure 4 and XRD results in Figure S1), which induce a

lower melting temperature than TP,m. Altogether, in the presence of silica spheres, C60H122 demonstrates interfacial freezing phenomenon above the melting temperature and thus the DSC curves in Figure 2 are substantially different from that of pure bulk C60H122. Table 1 Comparison of msf/mC60H122 based on Equation 2 and Equation 3

φC60H122

msf/mC60H122

msf/mC60H122

(calculated by Equation 2)

(calculated by Equation 3)

∆Hbulk(J/g)

10%

13.0±0.4

49.5%

48.0±0.6%

15%

26.0±0.4

31.0%

30.7±1.0%

20%

40.9±0.1

22.1%

18.5±2.3%

Note: msf/mC60H122 means the percentage of C60H122 molecules in the C60H122/SiO2 composites involved in interfacial freezing.

For homologues with chain length n in the range of 16~50, the most surprising phenomenon is that n-alkanes show surface freezing, whereby a single crystalline monolayer is formed at the surface of the isotropic liquid bulk at temperatures up to ~3 oC above the bulk freezing temperature.10 Moreover, the effect of temperature and 13



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chain length on surface freezing is explored qualitatively based on different theoretical descriptions.10,11,17-20 Tkachenko and Rabin proposed that the surface freezing in normal alkanes may be stabilized by the strong fluctuations along the molecular axis of the uniaxially ordered stretched chains in a solid monolayer floating at a liquid phase, which provides sufficient entropy to stabilize the surface monolayer against the formation of bulk rotator phase.17 The theoretical results for chain length and the temperature range over which the surface freezing monolayer is stable, is in good agreement with the experimental data on alkanes.10 According to this theory, when the carbon number of normal alkanes is higher than 60, the energy penalty arising from the interchain ethylene-ethylene mismatch would become too large to exceed the free energy gain due to the formation of surface freezing, which undermines the stability of surface freezing monolayer. Therefore, bulk C60H122 exhibits no surface freezing. For normal alkane/Stöber silica composites, our previous studies29-31 tend to support the interfacial freezing theory that the molecular orientation on the silica surface strongly depends on the molecule/silica surface interaction proposed by Maeda et al25 and entropic stabilization by surface-normal molecular fluctuations.17 In our work, the interaction between amorphous silica surface and alkane molecules, which is calculated to be 0.3 kJ/mol, is in the order of van der Waals interaction and therefore favors the normal orientation of normal alkanes in the interfacial region.30 In addition, the large specific surface area of SiO2 spheres induces an enhanced number of molecules deposited in the interfacial region. Altogether, CnH2n+2/SiO2 composites 14


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exhibit enhanced interfacial freezing, which can be detected by means of common DSC methods. In this regard, hydrophobic modifications of silica surface for example with 3-Aminopropyltriethoxysilane (APTES), should influence the interfacial interaction between C60H122 and SiO2 and thus can provide important information on the underlying mechanism for interfacial freezing of C60H122 adsorbed on the silica surface. Therefore DSC experiments of composites with modified silica were also performed and the results were shown in Figure S2. Both in cooling and heating process, a single phase transition peak is observed with various compositions. Moreover, the phase transition temperatures of composites are slightly lower than that of pure C60H122 and independent of composition. This indicates that the interfacial freezing phenomena is absent in C60H122/modified SiO2 composites. In fact, the behavior of the melting points reported in Figure S2 is perfectly analogous to the behavior of the melting point of the “bulk-like” component in Figure 2B, which is induced by the confinement effect of the silica network. With the surface modification of silica spheres by APTES, the surface free energy decreases to 5.7 mJ/m2 compared to untreated silica 12.6 mJ/m2,37 indicating the attractive interaction between alkane and NH2-CH2-CH2-CH2- group on the surface of modified SiO2 is larger than the weak interaction between alkane and bare SiO2(0.3 kJ/mol).30 As Maeda et al. show,25 the interplay between the lateral monolayer cohesion and its adhesion to the surrounding solid substrate is very important for proper assessment of interfacial freezing phenomena. Because the van der Waals adhesion of the monolayer to the substrate is stronger than the lateral monolayer cohesion, orientation of n-alkane 15



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molecules normal to the APTES-coated SiO2 surface is not preferable and interfacial freezing of C60H122 no more exists. This provides further evidence for the interfacial freezing theory proposed by Maeda et al.25 and validated by our previous works. In this work, the chain length of extended C60H122 molecule (7.8 nm) is far smaller than the radius of silica particles (175 nm), thus molecules could be regarded as residing at a nearly flat silica surface. Moreover, the surface of Stöber silica particles studied here is chemically similar to the native oxide layer typical of silicon wafer, C60H122 molecules at these two interfaces should exhibit the same molecular orientation. In this regard, we investigated the structure of C60H122 films adsorbed on silicon wafer using grazing-incident-angle X-ray diffraction (GIXD) to provide structural information for C60H122/SiO2 composites. The thickness of the C60H122 film on silicon wafer is about 49 nm measured by ellipsometry. Figure 3 shows the out-of-plane and in-plane GIXD profiles of a C60H122 films adsorbed on the silicon substrate at room temperature. The crystal structure of C60H122 has been reported to be orthorhombic unit cell with a = 4.923 nm, b = 7.361 nm, c = 154.684 nm.12 The in-plane GIXD result shows the characteristic reflections of (110) and (200) plane at 2θ = 21o and 23o, respectively, which are major reflections of the orthorhombic crystal. This agrees with the XRD result of C60H122/SiO2 composites (see Figure S1) and the broad Bragg peaks indicates many small crystals formed on the silicon wafer. Whereas, the out-of-plane profiles show quite different diffraction peaks from the in-plane ones. Obviously, the out-of-plane pattern is dominated by a series of (00l) diffraction peaks with high intensity, which suggests an orthorhombic phase grows 16


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preferentially with their ab-planes parallel to the silica films, i. e. C60H122 bulk-like particles oriented with the long axis of the molecules perpendicular to the surface. In addition, the interplanar spacing of the ordered structure along the normal direction of wafer (d=2π/Qz) is calculated to be 68.5 Å based on the out-of-plane diffraction profile, which is slightly smaller than the length of C60H122 molecules with all-trans conformation (78.7 Å) and the long period for the orthorhombic structure of pure sample (78.0 Å22). The temperature-dependent grazing-incidence diffraction (Figure S3) was carried out to probe the interfacial structure at high temperature. It shows that with temperature increasing the interfacial structure is dominated by a series of weaker (00l) diffraction peaks in the out-of-plane profiles, and no diffraction signal is observed in the in-plane profiles. This suggests that C60H122 molecules oriented with the long axis of the molecules perpendicular to the wafer surface even without ordered in-plane structure when temperature is high. Considering the same chemical structure as the SiO2 layer on the silicon wafer, it is reasonably concluded that C60H122 molecules also orient with their longitudinal axis perpendicular to the surface of Stöber silica particles when temperature is higher than 99 oC. However, there is still possibility that the first few C60H122 layers adjacent to the silica surface comprise surface-parallel molecules.27 Mo et al. postulated that the interfacial ordered monolayer cannot be in direct contact with the SiO2 substrate and instead one or two alkanes layers adsorb on the SiO2 surface with the their long-axis oriented parallel to the interface followed by a alkane monolayer with the long-axis perpendicular to it.27 Altogether, GIXD experiments provide further structural evidence for the assumption 17



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that C60H122/SiO2 composites exhibit interfacial freezing phenomenon rather than premetling.

Figure 3.Out-of-plane and in-plane GIXD profiles of a C60H122 thin film adsorbed on the silicon wafer covered by native SiO2 at room temperature (incident angle = 0.1o)

To provide molecular conformations and microscopic structure of the interfacial monolayer formed in the composites, solid-state

13

C NMR measurements have been

performed (Figure 4). The chemical shifts for resonance peaks of specimens are summarized in Table 2. For crystalline C60H122, there are three characteristic resonance peaks at 13.8 ppm for terminal methyl group (CH3), 23.9 ppm for α-ethylene CH2 group and 32 ppm for interior ethylene groups (int-CH2). They indicate that the molecules possess all-trans conformation in the orthorhombic unit cell. For composites with φC60H122 = 5 wt %, the resonance of int-CH2 carbons in the composite is nearly the same as that of crystalline C60H122, whereas the chemical shift of α-CH2 and CH3 carbons moves upfield and downfield, respectively. For aliphatic molecules, it is well known that the change of a carbon-carbon bond conformation from trans to gauche results in a significant upfield shift for the resonances of the 18


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carbons in α- and β- position to the rotated bond.38-40 This indicates that the solid structure that 5% sample formed is an orthorhombic phase with certain chain disorders. Combined the fact that nearly all molecules in the 5% specimen participate in interfacial freezing, it can be postulated that the interfacial monolayer structure formed in the composite is orthorhombic phase. Interestingly, this is consistent with the variation tendency of surface monolayer structure with chain length, where the surface monolayer is in a rotator phase for n44.10

Figure 4. Solid 13C NMR spectra of C60H122/SiO2 composites with various compositions at room temperature, the percentage inset is for φC60H122. Table 2 Comparison of chemical shifts of C60H122/SiO2 composites with that of crystalline C60H122 based on reference.22

Sample

Chemical shift (ppm) Int-CH2

α-CH2

CH3

Composite-3wt%

29.5

_

_

Composite-5wt%

32.0

23.8

14.2

Composite-10wt%

32.0

23.8

14.2

Pure C60H122(crystal)

32.0

23.9

13.8

19



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PureC60H122(liquid)

29.5

22.4

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13.6

As discussed above, all alkane molecules in 3% sample participate in the interfacial freezing, in this sense that its interfacial structure should be the same with that formed in the 5% sample. However, compared to that in 5% sample, the chemical shifts of int-CH2 carbons in the 3% sample move upfield by 2.5 ppm (see Figure 4), indicating the appearance of intra-chain conformational disorder and a disordered molecular chain packing (In this way, the onset of rapid molecular-level motion interferes with signal enhancement by cross-polarization and therefore induces the weak intensity of int-CH2 peak for 3% sample in Figure 4. In addition the less content of α-CH2 and CH3 in the C60H122 molecules, α-CH2 and CH3 peaks were nearly absent in the spectrum. Here, the signals assigned for silica nanoparticles at 16.2 and 58.7 ppm appear on the spectrum for composite with φC60H122 = 5 and 3 wt %, arising from the high silica concentration and longer acquisition time.).40 To illustrate the possible underlying reasons, the chain dynamics in the composites was also detected by Proton NMR relaxation experiments (see Figure S4 and Table S1). The chain mobility of C60H122 is in the order of pure C60H122 >> 10% sample > 3% sample. Obviously, the local molecular dynamics of C60H122 is strongly hindered due to the existence of silica nanoparticles.41-43 Moreover, the higher the silica concentration is, the stronger the confinement effect of silica network on C60H122 is. Therefore, the C60H122/SiO2 composites with φC60H122 = 3 wt % demonstrate an interfacial structure with loose molecular packing and thus a lower melt temperature. Since the surface/interfacial ordered monolayer was reported to act as the 20


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nucleation site for the rotator phase and thus affect the stability of rotator phase,2-5, 29-31

it is necessary to study whether rotator phase occurs in the C60H122/SiO2

composites. For the composite with φC60H122 = 10 wt %, the resonance of int-CH2 carbons is nearly the same as that of crystalline C60H122, whereas the chemical shift of α-CH2 and CH3 carbons moves upfield and downfield, respectively (Figure 4). It means that C60H122 tends to crystallize into orthorhombic phase with certain terminal chain disorders, which well explains the DSC results that bulk-like molecules in the composite tend to melt at a temperature lower than TP,m (Figure 2B). Also this demonstrates that no rotator phase occurs in the phase sequence of the composite since molecules in rotator phase rotate about their molecular long axis and thus the chemical shift of the int-CH2 moves upfield in rotator phase with respect to that in crystalline structure. Obviously, rotator phase cannot be mediated by the interfacial monolayer in the C60H122/SiO2 composites. This is substantially different from that reported in our previous work,29-31 whereby interfacial monolayer is proved to induce the crossover of rotator phase from transient one in free bulk to stable one in the n-alkane/SiO2 composites. 4. CONCLUSIONS In summary, C60H122/silica composites exhibit peculiar phase transition behaviors, which significantly deviates from the bulk C60H122, as explored by calorimetry, GIXD and solid-state

13

C NMR measurements. For composites, an enhanced interfacial

freezing effect is observed. The rotator phase, which is not observed in bulk C60H122, cannot be mediated by the interfacial freezing of C60H122/SiO2 composites, which is 21



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different from our previous results on C16/SiO2, C18/SiO2 and C19/SiO2 composites. Comparing the phase transition behaviors of C60H122/SiO2 composites with different silica surface properties, the interfacial interaction between alkane and SiO2 is believed to be the major contributor for the formation of interfacial ordered molecules and ultimately interfacial freezing behavior. This highlights the intimate relationship between interfacial freezing and interfacial interactions and also well illustrates the association between the interfacial-ordered monolayer and bulk-like crystallization of the n-alkane/SiO2 composites. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work is supported by National Natural Science Foundation of China (21474120) and ExxonMobil Asia Pacific Research and Development Company, Ltd. We also appreciate the staff scientists at beamline 14B in Shanghai Synchrotron Radiation Facility (SSRF) for beam time and technical assistance. The authors thank Dr. Guoming Liu and Mr. Yudan Shui for assistance in the GIXD characterization and DSC measurement. Supporting Information Available: The Variable-temperature XRD patterns of C60H122/SiO2 composites, DSC curves of C60H122/modified silica composite, Variable-temperature grazing-incidence diffraction patterns of C60H122 thin film, 1H 22


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TOC:

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Silica

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