Electrolytes at charged interfaces: Ion–ion–interface three-body correlation functions

Abstract

Inhomogeneous correlation functions for model ‘ soft’ 2–2 and 1–1 electrolytes at a charged interface have been determined by the so–called ‘pair’ approximation of integral equation theory. The solvent is modeled as a structureless dielectric continuum at 25°C. The wall–ion–ion structure is calculated using the inhomogeneous Ornstein–Zernike relation, together with the hypernetted chain closure, and one of two choices for the functional relationship between the singlet and pair correlation functions. Both the interfacial density profiles and the inhomogeneous pair correlation functions are calculated. For most cases, the inhomogeneous pair correlation functions near the interface vary significantly from the homogeneous pair correlation functions. This deviation generally becomes stronger as the charge on the surface increases, and the deviation generally extends out further from the interface as the surface charge increases. The density profiles predicted by the pair approximations generally show less structure than the singlet approximation density profiles, whereas the inhomogeneous pair correlation functions generally predict more structure than would be expected by simply assuming bulk pair correlation functions. The density profiles and inhomogeneous correlation functions are also found to agree qualitatively with previous simulations which used ‘charged hard–sphere/charged hard—wall’ potentials.