| Abstract
| - The morphology of many porous materials is sponge-like. For describing fluid adsorption in such materials,we propose here a quite general sponge model which is built by digging spherical cavities in a continuum.In contrast to the hard-sponge model proposed recently by Zhao et al. [Zhao, S. L.; Dong, W.; Liu, Q. H. J.Chem. Phys.2006, 125, 244703], the continuum in the present soft-sponge model is permeable to fluid particles.Although the general expression of the fluid−matrix interaction potential is not pair additive, we were ableto extend some statistical-mechanics formalism of liquid state to deal with this model. We derived thediagrammatic expansions of various correlation functions and Ornstein−Zernike equations. Usually, one wouldnot expect that the thermodynamic quantities (e.g., internal energy) of a system with a nonpair-additiveinteraction can be completely determined from the structural information at the two-body level. We found aremarkable result that for the soft-sponge model considered here, the internal potential energy can be determinedfrom only two-body correlation functions. In the particular case of a soft-sponge model with nonoverlappingcavities, we show that the fluid−matrix interaction can be also described by a pair-additive potential. In thiscase, the Madden−Glandt formalism applies. The Ornstein−Zernike equations obtained by using the pair-additive fluid−matrix interaction potential look to be quite different from those obtained by starting with thenonpair-additive potential. We found the relationship between the two descriptions and show how the twosets of Ornstein−Zernike equations can be transformed from each other for a soft-sponge model withnonoverlapping cavities.
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