Phase behaviors of n-octacosane in nanopores: Role of pore size and morphology

The nanoconfinement effect on the phase behaviors of octacosane ( n -C 28 H 58 , C 28 ) was studied using differential scanning calorimetry (DSC) and X-ray diffraction (XRD). In the nanopores of controlled pore glasses (CPGs), silica gels (SGs), SBA-15, and KIT-6, C 28 showed depressed phase transition temperatures. The depression of phase transition temperatures, degree of supercooling and enthalpies of phase transitions for C 28 confined in the CPGs, SGs and SBA-15 exhibited a linear dependence on reverse pore diameter. The phase transition temperatures also varied with the changing pore geometry, and the depression of melting points of C 28 in SBA-15 is about 2 times larger than that in the CPGs and SGs. The interfacial interactions of the pore walls to alkane seem to have a negligible influence on the phase behaviors. The phase transition sequences of the pore-confined C 28 changed largely with the appearance of new R I and R II rotator phases.