Statistical mechanics in the framework of generalized van der Waals theory is used to clarify the mechanisms at work in surface complexation. As a first step this article will focus on a description of ion interactions in solution. A simple semiempirical theoretical framework is proposed on the basis of an extension of the Debye-Huckel theory of the screening mechanism. Ion size effects are accounted for within a restricted primitive model of the electrolyte leading to a shift in the Debye screening length. Remarkably the shift is found to be due only to hole correction of the electrostatic potential energy while the excluded volume effects vanish in the linear approximation. The treatment of diffuse screening is tested by application of the corrected Debye-Huckel theory to the calculation of bulk thermodynamic properties, such as the internal energy, the osmotic coefficient, and mean ionic activity coefficients. Calculations and comparisons with experimental data have been performed for 1: 1, 2: 1, 3: 1, and 2:2 salt solutions. The accuracy is good indicating that the theory is suitable for the intended application in the study of surface complexation. Finally our theory is applied to electrolyte solutions in the solvent primitive model where the solvent is represented by neutral hard spheres. Our analysis indicates that, within the limits of our basic assumptions, the solvent hard spheres only enter the background contributions and leave the screening mechanism unaffected. The simulation results available indicate that this is largely, if not entirely, correct.