Phase transitions of CO2confined in nanometer pores as revealed by positronium annihilation

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.