A Singlet-RISM Theory for Solid/Liquid Interfaces Part I:  Uncharged Walls

Abstract

We present a novel integral equation method for the calculation of fluid structure in the vicinity of a plane impenetrable wall. The theory is based on the well-known RISM equation and is capable of dealing with arbitrary interaction site model (ISM) fluids at a solid/liquid interface. In conjunction with several closure approximations, the equations are solved numerically and wall-fluid site density distributions as well as charge density, field, and potential profiles are calculated for pure water and aqueous electrolyte solutions with varying concentrations adjacent to an uncharged soft wall. The results show reasonable agreement with corresponding computer simulation data.

Application of the singlet reference interaction site model to the primitive model double layer

“…Detailed, three-dimensionally resolved solvation structures of irregularly shaped solutes have been calculated from its three-dimensional version (3D-RISM) [34,35]. Furthermore, the RISM equation has been extended to treat polymeric fluids (PRISM) [36,37], liquids sorbed into a matrix (Replica RISM) [38,39] and interfacial solid/liquid systems (SRISM) [25,26]. The latter allows for the calculation of density profiles of the individual interaction sites of an interaction site model (ISM) fluid adjacent to a plane, charged or uncharged wall and, thus, can be used for the theoretical treatment of EDLs involving explicit solvent molecules and simple or more complex, molecular ions.…”

“…For a detailed discussion of the differences, the reader is referred to [25]. The functions h i (x) are related to the corresponding density profiles i ðxÞ by…”

“…The numerical solution of the SRISM equation with a specified closure has been described in detail in [25]. Obviously, the density profiles are influenced by the type of closure relations used in the calculation of both the bulk radial distributions and of the singlet distributions.…”

“…The SRISM approach has been employed to study the electric double layer using an explicit model for water [25,26]. Comparison with simulation results and, very recently, with results of a computationally much more demanding 3DRISM integral equation approach [27] to EDLs showed this approach to be promising.…”

“…. From the earlier works [25,26] and as we will see later, various closure conditions for the SRISM equations are possible, some of which have not been adequately tested. Our goal is to do a comparative study of the closures and determine the optimum one for accuracy.…”