Statistical-mechanical treatment of an absorbed monolayer with local order

A statistical-mechanical treatment of a monolayer of monomeric solvent molecules and of polymeric neutral solute molecules adsorbed in an unspecified number of orientations is provided in which local order is accounted for on the bases of the quasi-chemical approximation. This treatment is more general than that reported previously (R. Guidelli, J. Electroanal. Chem., 123 (1981) 59), in that it avoids certain unnecessary simplifying assumptions and may account for polymeric solute molecules consisting of segments with different properties. Such a treatment is applied to a square lattice of adsorbed water molecules interacting laterally between themselves both via dipole-dipole forces and via H-bonds. The resulting model accounts satisfactorily for the shape of the inner-layer capacity curves at interphases between water and sp metals of different hydrophilicity, for the temperature dependence of the inner-layer capacity at the Hg-water interphase, and for the differences between the differential capacity curves at the HgH 2 and HgD 2 O interphases. Reasonable explanations based on the above model are also provided for the high capacity hump at the interphases between water and Ag or Au single crystals, and for the non-unitary slopes of the Parsons-Zobel plots at interphases between water and strongly hydrophilic sp metals.