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

Abstract

Positronium annihilation spectroscopy has been used to study capillary condensation o^ nitrogen adsorbed in mesoporous VYCOR glass. The temperature and pressure dependences of the relative change in the 2y:2y 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 of adsorption in individual pores. PACS numbers: 64.7aFx, 36.10.Dr, 78.70.Bj Although the adsorption of gases on porous substrates has been the subject of investigation [l] since the beginning of this century, it is only in recent years that any fundamental understanding of the underlying physical processes has emerged. One striking phenomenon that is observed for a wide variety of substrates is the capillary condensation of the adsorbed gas to a dense liquidlike state filling the pores at a pressure which is less than the bulk saturated vapor pressure (Psat). Theory suggests that this phenomenon corresponds to a shift of the bulk condensation transition arising from finite-size effects. Adsorption isotherm measurements [1,2] (adsorbed mass versus pressure) register the transition as a rapid increase in the mass adsorbed, with accompanying pronounced hysteresis. Neutron scattering studies of water vapor in porous VYCOR glass [3] exhibit hysteresis which has also been associated with a shifted gas-liquid phase transition. However, the precise nature of capillary condensation and the interpretation of the associated hysteresis in real porous materials remain rather controversial issues [2]. Here we present the first data which demonstrate definitively that positron and/or positronium annihilation spectroscopy [4] can be used to study capillary condensation in mesoporous materials and provide direct information about the pore environment.Under favorable circumstances [4(a)], i.e., in nonmetallic materials such as glasses and polymers and many liquids and gases, a positron will form positronium (Ps). Statistically, three-quarters of the Ps will be ortho-Ps (o-Ps) and one-quarter para-Ps (/7-Ps). In a vacuum, the o-Ps decays via the emission of 3/ photons and has a lifetime of ^140 ns. p-Ps has a lifetime of 125 ps and annihilates into 2/ photons. In anything other than vacuum (< IxlO"^ Pa) o-Ps has, because of its relatively long lifetime, a finite probability of interacting [4(b),4(c)] with the surrounding medium. The consequence of such an interaction is to facilitate 2/-photon decay, a process called quenching [4(b),4(c)], and to shorten the o-Ps lifetime. The extent to which a surrounding medium facilitates <9-Ps quenching will obviously depend on the density of the medium. The probability of such quenching can be monitored by measuring the relative changes in the 3y:2y annihilation ratio.A fraction of pos...

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