CRITICAL DEPLETION OF A PURE FLUID IN CONTROLLED-PORE GLASS - EXPERIMENTAL RESULTS AND GRAND-CANONICAL ENSEMBLE MONTE-CARLO SIMULATION

The effect of confined geometry on the critical-adsorption of a fluid has been studied by measuring the physisorption of sulfur hexafluoride (SF6) in a mesoporous controlled-pore glass (CPG, mean pore width 31 nm) along an isochoric path at the critical density rho(c) in the one-phase region above the critical temperature of the fluid (15 K greater than or equal to T - T-c greater than or equal to 0.3 K). Whereas the surface excess amount of a fluid against a single planar adsorbing surface is expected to increase in a monotonic way along rho = rho(c) for T --> T-c, it is found for the present system that the excess amount of fluid in the pore reaches a maximum at T - T-c approximate to 1.5 K and then decreases sharply on further approaching T-c along the critical isochore. Close to the critical point (T - T-c < 0.3 K) the mean density of the pore fluid even tends to values lower than the density of the bulk fluid. A grand canonical ensemble (GCE) Monte Carlo simulation of a fluid between two parallel walls (wall-to-wall distance 20 molecular diameters) in equilibrium with a bulk reservoir at the critical density suggests that this effect can be attributed to a depletion in the core region of tbe pore as T approaches T-c.