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