Constant pressure Gibbs ensemble Monte Carlo simulations of adsorption into narrow pores

An extension to the Gibbs ensemble method for the study of adsorption of fluids into pores is proposed. Since equality of pressure is not a necessary condition for full thermodynamic equilibrium between the bulk fluid and that confined in a narrow cavity, previous studies have been performed with the volume of both simulation cells fixed. More naturally, the pressure of the bulk fluid should be constrained and the volume (and hence density) allowed to attain its equilibrium value. Thus we propose a scheme in which volume fluctuations within the bulk box are permitted; the volume of the pore remains constant. The pressure is an input parameter and the amount of adsorption, as a function of applied pressure, is obtained directly. We demonstrate that the novel moves obey microscopic reversibility. Such a method is most useful when the equation of state for the model fluid is unknown. Thus, we illustrate the method by simulating an associating model of water adsorbed into a slit-shaped carbon pore with activated sites. Good qualitative correspondence with experiment is obtained; further refinement of our model is expected to yield quantitative agreement.