- 2:39 PM

Imbibition Transition and the "Gate Effect" In Adsorption

Angela D. Lueking, Energy and Mineral Engineering, The Penn State University, 120 Hosler Bldg, University Park, State College, PA 16802 and Milton W. Cole, Physics, The Pennsylvania State University, University Park, PA 16802.

Adsorption in some regular nanoporous materials has been interpreted in terms of a “gate effect” model, in which the low pressure (P) uptake is limited by an initial blocking of access until a sufficiently high pressure opens a “gate”. A simple model (an imbibition transition) is discussed in which a key element in the model is the ability of the pore to expand (or contract, depending on the system) in response to the adsorbate. At low P, the density of the gas is low, so the pore does not respond significantly to the gas's presence. There exists a free energy barrier to pore expansion, since it is coupled to the gas density. At sufficiently high P, this barrier height becomes small; eventually, the system becomes thermodynamically unstable above a critical value of P. At very high P, after the transition occurs, the equilibrium coverage is high because the system's free energy is significantly reduced by the gas-pore coupling. Key parameters in the model are the derivative of the adsorption potential with respect to pore diameter and the force constant for breathing mode-like fluctuations of the pore. Relevant experimental data will be discussed.