G. M. Torrie

Electrical double layers. I. Monte Carlo study of a uniformly charged surface

This paper reports grand canonical Monte Carlo computations on the diffuse double layer in an ionic system next to a uniformly charged plane surface. The boundary conditions and the grand canonical techniques are discussed. Calculations were carried out for the 1:1 restricted primitive model at several concentrations and over a range of surface charge densities. The results are compared with the modified Gouy–Chapman theory, and some remarks are also possible with respect to the modified Poisson–Boltzmann and the hypernetted chain approaches. At high concentrations and surface charge densities the counterions are packed closely at the surface and begin to show a layered structure. This results in a large electrostatic potential drop, but only very slight charge oscillations are observed in the solution. None of the theories seems able to describe this behavior.

Colloid stability: The forces between charged surfaces in an electrolyte

Monte Carlo methods are presented for the evaluation of the various components of the force between parallel charged surfaces due to the presence between them of an electrolyte, represented by the restricted primitive model. This is a significant problem because some recent theoretical results, and computations on even simpler models, have put in question the adequacy of traditional Derjaguin–Landau–Verwey–Overbeek (DLVO) theory in offering a basic understanding of colloid stability. The methods are applied principally to 2:2 electrolytes at a variety of surface charge densities. We find, in contrast to DLVO theory, that the force between the surfaces resulting from the electrolyte is almost everywhere attractive; in detail these forces turn out to have a remarkably rich behavior resulting from the interaction of their various contributions. The results bolster the perception that we need to take a new look at some basics of colloid science.

Electrical double layers. II. Monte Carlo and HNC studies of image effects

The effect of surface polarization (i.e., image forces) on the properties of electrical double layers is studied by means of Monte Carlo calculations on a primitive model electrolyte next to a planar charged surface bounding a semi‐infinite dielectric. Two cases are considered, that of a conducting material for which an ion is attracted by its own image and that of an insulator for which the self‐image force is repulsive. At low surface charge densities the image forces cause quite dramatic changes in the ionic densities near the wall; the effect on the electrostatic potential is small but increases with surface charge density. The modified Poisson–Boltzmann theory of Outhwaite is quite successful in describing the Monte Carlo results for the range of parameters studied. A screened self‐image model of image effects is also considered for which both Monte Carlo calculations and numerical solution of the HNC equation have been obtained.

The electrical double layer. III. Modified Gouy−Chapman theory with unequal ion sizes

This article examines the consequence of assigning to the cations and anions different distances of closest approach to the charged surface, within the simple Poisson–Boltzmann theory of the diffuse part of the electrical double layer. This introduces asymmetry into electrocapillary and differential capacitance curves. It also leads to a diffuse double layer with zero charge on the surface, and thus to a concentration‐dependent potential of zero charge (pzc). These phenomena are usually discussed in terms of solvent‐structure next to the surface and of adsorption isotherms for anions on the surface, but it seems that a contribution to such effects must come from the diffuse layer itself.

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 17.5 μC/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...

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.

Electrical double layers. VI. Image effects for divalent ions

The effect of image forces on the behavior of primitive model double layers is investigated by means of Monte Carlo computer simulation and modified Poisson–Boltzmann theory for two types of systems containing divalent ions. For the symmetric 2:2 electrolyte the fractional change in the diffuse layer potential caused by image forces differs from the previously studied 1:1 case both by being much larger in the 2:2 system and by decreasing once the surface charge is increased beyond a certain point. For 2:1 electrolytes a small charge separation is found at an uncharged surface and the structural effects associated with it can be significant if image forces are present.

Modeling and structure of mercury-water interfaces

The modeling and nature of the physisorption of water at the metal (Hg)-water interface is explored in this paper. We have evaluated potential models that fit into three general classes that are employed in the literature. These classes are distinguished by the manner in which the isotropic interactions between the metal and the water are modeled: namely, as non-attractive, weakly attractive, and strongly attractive. In the present studies the metal is described by a jellium model. In our model, in addition to the isotropic water potential there is an interaction between the jellium and the water molecules which depends on the orientation of the water molecule with respect to the metal surface. We find that hard potentials without isotropic attractive terms dewet. The density of water near hydrocarbon-like potentials remains close to the bulk value but the interaction isn’t strong enough to structure the water near the interface, nor are the adsorption energies sufficiently high. The strongly attractive p...

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 C m−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.

Heat capacity of the restricted primitive model near criticality

This concerns the supercritical heat capacity of the “restricted primitive model” of Coulombic systems, one aspect of a more general attempt to study the phase transition and critical behavior of this model using thermodynamic-scaling Monte Carlo techniques. We see no indication of Ising-type divergence of the heat capacity; the strength of the evidence in this regard is discussed.