Nonfreezing Interfacial Layers of Cyclohexane in Nanoporous Silica

Abstract

We have been able to estimate the thickness of nonfreezing interfacial layers of cyclohexane confined in nanoporous silica. This thickness was estimated from calorimetric measurements and simple geometrical analysis. The nonfreezing layers coexist with molecules that can undergo phase transition when the pore size of the confining medium is larger than 3.5 nm. In agreement with the Gibbs−Thompson equation, the melting and freezing temperatures of the molecules capable of undergoing phase transition scale linearly with the inverse pore size of the silica. Experimentally determined (apparent) ΔH, however, seems to vary with physical size, which contradicts the inherent assumptions in the Gibbs−Thompson equation. Corrections to account for the presence of nonfreezing layers of molecules with thickness ca. 2.14 ± 0.2 nm not only leads to a ΔH invariant with physical size, but it also provides insight into density variation due to confinement.