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 fl uids into pores is proposed. Since equality of pressure is not a necessary condition for full thermodynamic equilibrium between the bulk fluid and tha t confined in a narrow cavity, previous studies have been performed with th e Volume of both simulation cells fixed. More naturally, the pressure of th e bulk fluid should be constrained and the volume (and hence density) allow ed to attain its equilibrium value. Thus we propose a scheme in which volum e fluctuations within the bulk box are permitted; the volume of the pore re mains constant. The pressure is an input parameter and the amount of adsorp tion, as a function of applied pressure, is obtained directly. We demonstra te that the novel moves obey microscopic reversibility. Such a method is mo st useful when the equation of state for the model fluid is unknown. Thus, we illustrate the method by simulating an associating model of water adsorb ed into a slit-shaped carbon pore with activated sites. Good qualitative co rrespondence with experiment is obtained; further refinement of our model i s expected to yield quantitative agreement.