Unusual Interfacial Freezing Phenomena in Hexacontane/Silica Composites.

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 13C nuclear magnetic resonance (solid-state 13C 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 13C 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 silica surface.