Primitive model electrolytes in the near and far field: Decay lengths from DFT and simulations

Inspired by recent experimental observations of anomalously large decay lengths in concentrated electrolytes, we revisit the Restricted Primitive Model (RPM) for an aqueous electrolyte. We investigate asymptotic one-body ionic density profiles RPM contact with a planar electrode using classical Density Functional Theory (DFT) and compare these corresponding two-body correlation functions bulk systems, obtained previous Integral Equation (IET) studies. Extensive Molecular Dynamics (MD) simulations are employed to complement DFT IET predictions. Our calculations incorporate electrostatic interactions between ions three different (existing) approaches: one is based on simplest mean-field treatment Coulomb (MFC), while other two employ Mean Spherical Approximation (MSA). The MSAc invokes only MSA direct function, whereas MSAu also incorporates internal energy. Although yields that excellent agreement MD near field, far observe consistent IET, MSAc, simulations, those from MFC deviate significantly. Using DFT, calculated solvation force, which relates directly surface force experiments. find its length neither qualitatively nor quantitatively close measured experiments conclude latter cannot be accounted primitive model. found measurements require explanation lies beyond models.