Screening, Structure, and Simulation of Ionic Fluids: The Long and Short of It

John D. Weeks, Institute for Physical Science and Technology and Department of Chemistry and Biochemistry, University of Maryland

Charged ions in dielectric solvents play an important role in many chemical and biological processes. The simplest primitive model for ionic fluids consists of charged hard spheres where the Coulomb interaction is modified by a static dielectric constant. More realistic models consider point charges embedded in soft molecular cores described by some short ranged interaction potential, which can also take account of local variations in the dielectric constant. We describe a new theoretical approach to such systems by exactly separating the point charge Coulomb interaction into a long-ranged and slowly varying part u1(r) that arises from a rigid Gaussian charge distribution with width σ, and the remainder u0(r), which is short-ranged and can be added to the other short-ranged core interactions. By proper choice of σ of order the molecular core size, the Gaussian charge potential u1 decays rapidly in k-space on the scale of short ranged intermolecular correlations. As a result a local molecular field treatment of the effects of the slowly-varying u1 for both uniform and nonuniform fluids is exceptionally accurate. At very low densities the exact Debye-Hückel theory is recovered, and at higher densities in uniform fluids the short-ranged structure is very accurately described by that of a Coulomb "mimic system" where u1 is set equal to zero. Characteristic phenomena such as ion-pairing at low density and charge-ordering at higher density can be seen in the mimic system with only short-ranged interactions. Relations to other treatments of long-ranged interactions such as Ewald sum and fast mutipole methods are briefly discussed.

This work is supported by the National Science Foundation under grant NSF CHE0111104.

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