Effect of Confinement on the Phase Transition of Benzene in Nanoporous Silica: A Positron Annihilation Study

We report new results on the solid-liquid phase transition of benzene confined in nanopores of silica using positron annihilation spectroscopy. The pore sizes, ranging from 0.7 to 7.0 nm, were evaluated by positron lifetime and the Brunauer-Emmett-Teller (BET) techniques. Temperature dependent Doppler and lifetime measurements revealed discontinuities at temperatures below the bulk freezing temperature of benzene corresponding to the freezing of benzene confined in pores of different dimensions present in the silica samples. The temperatures corresponding to the freezing of confined liquids were assigned to the pore sizes on the basis of the well-known inverse correlation of depression in the freezing point with pore sizes. However, the magnitude of shift in the freezing point could not be explained on the basis of the classical Gibbs-Thomson relation. On the other hand, the data are seen to be consistent with the molecular cluster theory for microcrystal growth in confined region.