PORE CONDENSATION AND CRITICAL-POINT SHIFT OF A FLUID IN CONTROLLED-PORE GLASS

The phase behavior of a pure fluid (SF6) in controlled-pore glass (CPG) materials with mean pore diameters 24 and 31 nm has been mapped in a range of temperatures from T - T-c = 15 K to T - T-c = -7 K and reduced densities rho/rho(c) from 0.5 to 0.8. Sorption isochores were measured by a volumetric technique. The isochores exhibit a transition from pore condensation and hysteresis at low rho/rho(c) to a continuous reversible mode of pore filling at high rho/rho(c). From these temperature scans the coexistence curve of the pore fluid was derived. It is found that the critical temperature T-cp of the pore fluid is shifted downward relative to the bulk critical temperature T-c. The critical-point shift Delta T-c = T-c - T-cp amounts to 0.92 +/- 0.24 K for the glass with 24 nm mean pore width and 0.48 +/- 0.23 K for the glass with 31 nm mean pore width. In both glasses the critical density and the entire coexistence curve of the pore fluid is shifted to higher densities compared with the bulk fluid. These findings are in qualitative agreement with theoretical predictions of confined geometry effects on the criticality of fluids in narrow pores, which predict that Delta T-c will vary with the pore width D as a power law Delta T-c proportional to D--y where y approximate to 1.6 (for slit pores). The present results suggest a value y greater than or equal to 2 for SF6 in CPG materials.