Unusual Interfacial Freezing Phenomena in Hexacontane/Silica Composites

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.