Critical depletion of pure fluids in colloidal solids: Results of experiments on EURECA and grand canonical Monte Carlo simulations

Abstract

A microgravity experiment on the EURECA-1¹ mission of ESA was performed to study the adsorption of a near-critical fluid (SF₆) on a finely dispersed graphitic adsorbent (Vulcan 3-G graphitized carbon). The experimental set-up was housed in a three-stage high-precision thermostat (HPT) which allows for a temperature control within 1 mK over periods of days. The adsorption (surface excess amount) Γ was measured as a function of temperature T along near-critical isochores using a volumetric technique., Five independent runs, either at the critical density (ϱ/ϱc=1.01) or a slightly higher density (ϱ/ϱc=1.04), were performed. The EURECA experiment confirmed a novel critical sorption phenomenon, which is due to the colloidal state of the adsorbent. At temperatures well above the critical temperature T _c the adsorption excess amountΓ increases with decreasing temperature, but closer to the critical temperature T _c exhibits a maximum and then decreases sharply for T→T _c. The phenomenon was also observed for near-critical isochores of SF₆ in a mesoporous glass material. Grand canonical Monte Carlo simulations for a fluid in a slit-pore suggest that the negative critical adsorption effect is caused by depletion in the core region of the pore as T approaches T _c. This effect, which we call critical depletion, is believed to be driven by the proximity of the bulk fluid to its critical point. It may be of significance whenever fluids in contact with mesoporous or colloidal materials approach their critical point.