Unusual Interfacial Freezing Phenomena in Hexacontane/Silica Composites

The crystallization behaviors of <i>n-</i>hexacontane (C<sub>60</sub>H<sub>122</sub>)/Stöber silica (SiO<sub>2</sub>) composites with various compositions were investigated by a combination of differential scanning calorimetry (DSC), solid-state <sup>13</sup>C nuclear magnetic resonance (solid-state <sup>13</sup>C NMR), and proton NMR relaxation experiments. By means of DSC, C<sub>60</sub>H<sub>122</sub> molecules in C<sub>60</sub>H<sub>122</sub>/silica composites were observed to be involved in the interfacial freezing not present in the free bulk C<sub>60</sub>H<sub>122</sub>. The orientation of C<sub>60</sub>H<sub>122</sub> molecules, being preferentially normal to silica surface, was confirmed by grazing incidence X-ray diffraction experiments on thin <i>n-</i>hexacontane film adsorbed on the silicon wafer with a native SiO<sub>2</sub> layer. Inferred from the solid <sup>13</sup>C NMR data, the interfacial monolayer is in orthorhombic phase with certain chain disorders. It is speculated that the “interfacial freezing” of C<sub>60</sub>H<sub>122</sub> 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.