P. G. Kusalik

On the molecular theory of aqueous electrolyte solutions. I. The solution of the RHNC approximation for models at finite concentration

This paper describes a theoretical study of the thermodynamic, dielectric, and structural properties of model aqueous electrolyte solutions. The model considered consists of hard sphere ions immersed in a hard polarizable dipole tetrahedral–quadrupole solvent with water‐like parameters. The calculations involve the solution of the reference hypernetted‐chain (RHNC) approximation for ion–solvent mixtures at finite concentration and some details of the general method are discussed. The influence of the molecular polarizability of the solvent particles is treated at the self‐consistent mean field (SCMF) level and, surprisingly, the mean dipole moment of the solvent is found to be nearly independent of the salt concentration. Numerical results are reported for model alkali halide solutions and other 1:1 electrolytes, and comparisons are made with experimental results at 25u2009°C. The agreement obtained between theory and experiment is variable depending upon the particular property and solution considered. In ad...

The solution of the reference hypernetted-chain approximation for water-like models

In this paper we examine the dielectric and structural properties of hard polarizable multipolar models for liquid water. The theoretical results were obtained by solving the self-consistent mean field (SCMF) approximation together with the reference hypernetted-chain (RHNC) theory. The dielectric constants are in good agreement with experiment over a large range of temperatures and pressures. The rather poor agreement between the radial distribution functions determined for our water-like fluids at 25°C and those measured for liquid water is discussed.

Molecular solvent model for an electrical double layer: Reference hypernetted‐chain (RHNC) results for solvent structure at a charged surface

We report results of solving the full reference hypernetted‐chain (RHNC) theory for a large, multiply charged macroion at infinite dilution in a solvent of hard spheres with point dipole and quadrupole moments chosen to represent liquid water. We obtain results for the restructuring of this model solvent next to the macroion surface for a range of surface charges and macroion sizes up to 30 solvent diameters. Although we are unable to solve the theory for larger particles, when this largest macroion is neutral we find a solvent orientational structure in the surface layer that is in good qualitative agreement with computer simulation results for water‐like models at a planar surface. Our RHNC calculations show that this surface structure proves surprisingly resistant to the effect of surface charges as high as 17.5u2009μC/cm2, apparently because even such strong fields cannot compete with the still stronger intermolecular forces of water‐like models. There is, nevertheless, strong oscillatory behavior in both...

The thermodynamic properties of electrolyte solutions: Some formal results

The Kirkwood–Buff approach is used to obtain exact determinate expressions for the thermodynamic properties of electrolyte solutions. The solvent is treated at a molecular level and the thermodynamic functions are expressed in terms of ion–ion, ion–solvent, and solvent–solvent correlation functions. The equations obtained are particularly useful when used in conjunction with integral equation theories. The low concentration limiting behavior of the microscopic expressions is examined and it is shown that the Debye–Huckel limiting law for the activity coefficient can be readily extracted from the molecular theory. Also the partial molecular volume of the salt is considered in some detail and microscopic relationships are given for the infinite dilution value.

The solution of the hypernetted-chain and Percus-Yevick approximations for fluids of hard nonspherical particles. Results for hard ellipsoids of revolution

In this paper we describe a general approach which allows the hypernetted chain (HNC) and Percus–Yevick (PY) integral equation theories to be solved numerically for fluids of hard nonspherical particles. Explicit results are given for fluids of hard ellipsoids of revolution and comparisons are made with recent Monte Carlo calculations. It is found that for dense systems of highly anisotropic ellipsoids the HNC and PY closures give significantly different results. The HNC theory is superior predicting the existance of a nematic phase in qualitative agreement with computer simulations. The PY approximation strongly and erroneously suggests that the isotropic phase is stable throughout the liquid regime.

On the molecular theory of aqueous electrolyte solutions. II. Structural and thermodynamic properties of different models at infinite dilution

This paper describes a comparative theoretical study of the infinite dilution properties of solutions of univalent ions in different water‐like solvents. The theoretical results are obtained by solving the reference hypernetted‐chain (RHNC) approximation as described in the first article of this series [J. Chem. Phys. 88, 7715 (1988)]. It is shown that the inclusion of the octupole moment of the water molecule in the model solvent leads to strong preferential solvation of negative ions, and that this has important effects upon certain thermodynamic properties and upon the ion–solvent and ion–ion structure. Questions concerning the definition and physical interpretation of individual ion partial molar volumes are raised and considered in detail. It is argued that in general the individual ion results obtained experimentally may not accurately reflect the true microscopic ion–solvent structure.

Solvation energy of ions in dipolar solvents

The solvation energy of hard spherical ions immersed in dipolar hard sphere solvents is investigated as a function of ion diameter. We apply both the mean spherical and linearized hypernetted‐chain approximations and show that for ions of physically realistic size both theories give qualitatively similar results. The ion solvation energy is obtained as the sum of two competing terms: UID, the direct ion–solvent interaction energy, which is negative, and UDD, the change in the solvent–solvent interaction energy per ion at infinite dilution. UDD is found to be positive and to make an important contribution to the solvation energy for all ion diameters.

Theory of the electrical double layer: ion size effects in a molecular solvent

Recent application of the reference hypernetted‐chain theory to the calculation of the structure of electrical double layers in wholly molecular models of aqueous electrolytes [J. Chem. Phys. 90, 4513 (1989)] is extended to NaCl solutions. The strong interaction of the small Na+ ion with the solvent molecules leads to a number of novel features, notably a very high adsorption of Na+ into a region about 4 A from the surface next to the first solvent layer. This is sufficient to produce a positive potential of zero charge of about 29 mV and to cause the effective surface charge experienced by the diffuse layer to be positive even when the surface carries a substantial negative charge. In most cases, however, the surface potential is found to be remarkably insensitive to the details of the structure of the double layer for a given surface charge.

Computer simulation and theoretical results for a polar-polarizable fluid

In this paper we present molecular dynamics (MD) results for a fluid of polarizable Lennard-Jones particles with permanent dipole and quadrupole moments. Detailed comparisons are made with theoretical results obtained using a previously developed self-consistent mean field (SCMF) approximation together with the LHNC and QHNC integral equation theories. The SCMF approximation is found to give a reasonably accurate description of the polarizable system. The SCMF/QHNC and SCMF/LHNC calculations both give values for the dielectric constant and average dipole moment which are in good agreement with the MD simulations. The SCMF/QHNC theory gives better results for the pair correlation function and for some thermodynamic properties.

Molecular solvent model for an electrical double layer: Reference hypernetted chain results for potassium chloride solutions

The reference hypernetted‐chain theory is solved for the structure of the double layer at the surface of large spherical macroions in a wholly molecular model of aqueous KCl. Detailed results are reported for the solvent and ionic structure throughout the double layer for surface charges up to 0.175 Cu2009m−2 and salt concentrations of 0.1 to 1.0 M. Concentration effects on the short‐range structural features of the interface are discussed and related to the behavior of the same model at infinite dilution. There is a very rapid neutralization of the surface charge by a Coulombic adsorption of counterions into a narrow region near contact beyond which the system responds as though to a much lower effective surface charge. The relationship of these results to the properties of continuum solvent models and the implications for the electrostatic potential and differential capacitance of the double layer are discussed.