Grand Potential,Helmholtz Free Energy,and Entropy Calculation in Heterogeneous Cylindrical Pores by the Grand Canonical Monte Carlo Simulation Method

摘要

The adsorption of fluids in porous media is still an open area of research,since no model is able to explain all experimental features.The difficulties rise from the complexity of the real porous materials which present surface heterogeneities,large pore size distributions,and complex networks of interconnected pores.In parallel to experimental efforts trying to produce more ordered porous materials,theoreticians try to introduce more disorder in their models,with the help of molecular simulation for instance.This grand canonical Monte Carlo simulation study concentrates on the adsorption of a simple Lennard-Jones fluid in three porous substrates,to compare the effect of purely geometric heterogeneity (spatial deformation of the external potential) as opposed to purely chemical heterogeneity (amplitude variations of the external potential).This separation is unrealistic,since geometric fluctuations of a real pore diameter along its axis generally induce variations in the amplitude of the external potential created by the pore.However it enables one to compare both effects.In this paper,a thermodynamic integration scheme is applied to a complete set of adsorption/desorption isotherms.The grand potential,free energy,and entropy are calculated,which allows one to discuss the features of the phase diagrams.It is shown that a purely geometric deformation (undulation) of the external potential does not affect the thermodynamic characteristics of the confined fluid.On the other hand,amplitude modulation of the external potential (chemical heterogeneity) strongly distorts the phase diagram.This heterogeneity is actually able to stabilize a "bridgelike" phase which corresponds to an accumulation of molecules in the most attractive region of the pore.