Density Functional Theory of Ionic Screening: When Do Like Charges Attract?

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Abstract
We describe a simple local density functional theory for screened interactions between macroions and apply it to electric double layers. The approach incorporates the strong correlations between screening ions at high densities, and reduces to the Poisson-Boltzmann equations at low densities. Results for monovalent ions are in excellent agreement with Monte Carlo (MC) and inhomogeneous hypernetted chain (IHNC) calculations. Nonlocal effects are important for divalent ions at small macroion separations and high surface charge densities. A simple weighted density approximation qualitatively reproduces MC and IHNC results in this regime, and provides a simple explanation for the onset of attractive interactions between like-charged plates.