Sorption strain as a packing phenomenon

We employ Monte Carlo simulations in a semi-grand canonical ensemble to analyze the relation between sorption strains and the thermodynamic state of a confined fluid composed of "simple" fluid molecules that possess only translational degrees of freedom. Fluid molecules are confined to a slit-pore whose walls are composed of individual atoms distributed across the plane of each substrate according to the (100) structure of the face-centered cubic lattice. The substrates can be deformed to a certain extent on account of their own thermal energy and due to the interaction with the fluid molecules. We determine the phase diagram in both the bulk and in confinement for both rigid and deformable solid substrates. By using finite-size scaling concepts the location of the critical point is determined accurately. Our results indicate for the first time that the previously observed variation of sorption strains with the amount of adsorbed fluid material [G. Günther et al., Phys. Rev. Lett., 2008, 101, 086104] is caused by packing effects (i.e. stratification of the confined fluid) but is largely independent of the precise nature of the thermodynamic state considered.