The results of a comprehensive positronium annihilation study of the phase behaviour of carbon dioxide in porous Vycor glass are presented. Isobaric measurements of the 3 gamma :2 gamma annihilation ratio show that on cooling, the gas-liquid phase boundary is raised by ~5 K while the liquid-solid transition is depressed by ~12 K relative to the bulk. The resulting phase diagram suggests the existence of a `triple point` of the confined fluid at a temperature ~10 K and a pressure ~2 bar below the bulk triple point.
The hysteresis behaviour of capillary confined CO2 gas is studied through the positron/positronium annihilation technique. The arrangement of the confining medium (VYCOR glass) allowed simultaneous measurement of the bulk and pore confined phase behaviour. Isobaric temperature cycling showed marked hysteresis at the gas-liquid and liquid-solid phase boundaries for the confined gas. Observations at the gas-liquid transition are in agreement with capillary condensation theories and simple ideas are proposed to explain the liquid-solid transition behaviour.
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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