A fundamental-measure theory for inhomogeneous associating fluids

The fundamental-measure theory (FMT) of Rosenfeld for hard spheres is extended to inhomogeneous associating fluids on the basis of Wertheim’s first-order thermodynamic perturbation theory (TPT1). The excess intrinsic Helmholtz energy, which includes contributions from hard-sphere repulsion and from intermolecular bonding, is represented as a functional of three weighted densities that are related to the geometry of spherical particles. In the absence of association, this theory is the same as the original FMT, and at bulk conditions it reduces to TPT1. In comparison with Monte Carlo simulation results, the extended fundamental-measure theory provides good descriptions of the density profiles and adsorption isotherms of associating hard spheres near a hard wall. Calculated results indicate that the critical temperatures for the vapor–liquid equilibria of associating fluids in hard slit pores are suppressed compared with that for the bulk fluid and the confinement has more significant impact on the liquid side than the vapor side of the coexistence curve. Unlike nonpolar fluids at similar conditions, saturated associating liquids in hard slit pores do not exhibit strong layering near the solid surface.