Effects of surface and physical confinement on the phase transitions of cyclohexane in porous silica.

Abstract

We have measured the specific heat (${\mathit{C}}_{\mathit{p}}$) of cyclohexane at 120T300 K when cyclohexane was physically restricted in porous Spherosil (silica) samples of pore radii 4, 7.5, 15, 30, and 62.5 nm. The behaviors of the monoclinic-to-cubic structural transition and of the melting transition of cyclohexane were determined. As expected, both transition temperatures, i.e., solid-solid and melting, inversely scaled with the pore radius (${\mathit{R}}_{\mathit{p}}$). It is argued that the surface heterogeneity, the presence of hydroxyl groups, and the radius of curvature (especially for smaller pores) induce considerable disorder in the adsorbed layers of cyclohexane, thus, resulting in the nucleation of crystalline grains of various sizes rather than in a single crystalline plug of cyclohexane in the porous silica samples. The distribution of the grain boundaries of crystalline cyclohexane produces a specific-heat peak for the melting transition, which almost resembles the \ensuremath{\lambda} shape for ${\mathit{R}}_{\mathit{p}}$\ensuremath{\le}30 nm. Unlike the monoclinic-to-cubic transition, the \ensuremath{\lambda} anomaly of the melting transition does not reflect a logarithmic dependence near the transition temperature. It is also argued, from the comparative observed ${\mathit{C}}_{\mathit{p}}$ behavior of the bulk cyclohexane with the physically restricted cyclohexane, that the cyclohexane liquid is more viscous than the bulk when it is confined in pores of 4 and 7.5 nm.