We model ion solvation in water. We use the MB model of water, a simple two-dimensional statistical mechanical model in which waters are represented as Lennard-Jones disks having Gaussian hydrogen-bonding arms. We introduce a charge dipole into MB waters. We perform (NPT) Monte Carlo simulations to explore how water molecules are organized around ions and around nonpolar solutes in salt solutions. The model gives good qualitative agreement with experiments, including Jones-Dole viscosity B coefficients, Samoilov and Hirata ion hydration activation energies, ion solvation thermodynamics, and Setschenow coefficients for Hofmeister series ions, which describe the salt concentration dependence of the solubilities of hydrophobic solutes. The two main ideas captured here are (1) that charge densities govern the interactions of ions with water, and (2) that a balance of forces determines water structure: electrostatics (water's dipole interacting with ions) and hydrogen bonding (water interacting with neighboring waters). Small ions (kosmotropes) have high charge densities so they cause strong electrostatic ordering of nearby waters, breaking hydrogen bonds. In contrast, large ions (chaotropes) have low charge densities, and surrounding water molecules are largely hydrogen bonded.

The replica Ornstein-Zernike ͑ROZ͒ equations, supplemented by the hypernetted chain and mean spherical closures, were solved for an ionic fluid adsorbed in a disordered charged matrix. To obtain the numerical solution of the ROZ equations we performed renormalization of the initial equations. Both the matrix and adsorbed fluid were modeled as charged hard spheres in a dielectric continuum, i.e., in the so-called restricted primitive model. As a result, the pair distribution functions between fluid ions and for fluid-matrix correlations were obtained. Structural properties were studied as a function of the matrix density, the concentration of adsorbed electrolyte and for different prequenching conditions. The isothermal compressibility, excess internal energy, and the chemical potential were calculated and discussed with respect to of the model parameters. Comparison with the Monte Carlo computer simulations of Bratko and Chakraborty ͓J. Chem. Phys. 104, 7700 ͑1996͔͒ indicates that the theory yields qualitatively correct results for the model system.

The replica Ornstein–Zernike (ROZ) equations for an ionic fluid adsorbed in an electroneutral, disordered matrix of ions were applied to a model where both ionic subsystems were presented as point charges interacting only via Coulomb forces. The effects of fluid (electrolyte) and matrix concentration on the screening of the ion–ion interactions in the fluid phase were investigated. The effects of the prequenching conditions were also examined. It was shown that augmenting the matrix concentration promotes attraction between equally charged ions and repulsion between ions of opposite sign. This peculiar behavior, observed first in the simulation study of Bratko and Chakraborty [J. Chem. Phys. 104, 7700 (1996)], follows straightforwardly from the ROZ equations. Moreover, we generalized the expression for the disorder averaged ion–ion potential for an arbitrary fluid concentration and prequenching conditions. In addition to these results, which are consistent with computer studies, we present some new results that have not been observed in simulations. For example, alternating ionic ordering, generated by the influence of the charged matrix was observed. This contribution can be considered as a first step toward a study of primitive model electrolytes adsorbed in disordered matrices of hard-sphere ions. The solution of this problem will be presented elsewhere.