Positron annihilation study of capillary condensation of nitrogen gas in a mesoporous solid

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Abstract
Positronium annihilation spectroscopy has been used to study capillary condensation of nitrogen adsorbed in mesoporous VYCOR glass. The temperature and pressure dependence of the relative change in the 3γ:2γ annihilation ratio are reported. Both isobars and isotherms exhibit features which are associated with a shifted gas-liquid transition. The isotherm data suggest that pore filling occurs via progressive layer formation at the pore surface followed by an abrupt gas-liquid phase transition at some pressure below the bulk saturated vapor pressure. Such a picture is consistent with the theoretical treatment in individual pores.