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

Abstract

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.

Molecular probe dynamics and free volume heterogeneities in n-propanol confined in a regular MCM-41 matrix by ESR and PALS

“…[7][8][9] Among the smallest ones belong quasi-atomic sized ortho-positronium (o-Ps) 10 and stable molecular free radicals, the so-called spin probes of nitroxide type, 11 while the rather large ones are representented by uorescence probes. 12 The rst probe provides the information about the free volume microstructure and its changes with varying internal and external parameters within various organic media in their bulk state 10 as well as in various conned ones through its annihilation behavior [13][14][15][16][17] using positron annihilation lifetime spectroscopy (PALS). PALS response, i.e., o-Ps lifetime, s 3 , as a function of temperature, exhibits several regions of different thermal behavior dening the characteristic PALS temperatures within solid (ordered crystal or disordered glass) and disordered liquid state, such as T G bi , T PALS g and T L bi .…”

A hybrid model for estimation of pore size from ortho-positronium lifetimes in porous materials

“…( 6) and ( 7) produce similar results if λ T is relatively small compared to λ SE and λ T E . four decades [5,11,19,25,27,28,29,30,31,32,33]. In Fig.…”

“…However, once the pore size is increased, its radius moves toward the semiclassical region [18], hence the o-Ps energy is in the range of kT at room temperature with k being the Boltzmann constant. As a result, the o-Ps with lower energy will exist with a longer lifetime, leading consequently to the larger pore size or the higher de-trapping probability of o-Ps [24,25,26]. Therefore, instead of using a spherically symmetric infinite potential well (U goes from 0 to infinity) as in the TE model ( Fig.…”

Nano-Porous-Silicon Powder as an Environmental Friend

“…All of them are widely used in modern materials science. Several studies, in particular on solids and porous systems, have included NPS and nonporous SiO 2 via the PALS technique [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 ].…”