  In this paper we extend the familiar continuum electrostatic model with a
perturbation to the usual macroscopic boundary condition. The perturbation is
based on the mean spherical approximation (MSA), to derive a multiscale
hydration-shell boundary condition (HSBC). We show that the HSBC/MSA model
reproduces MSA predictions for Born ions in a variety of polar solvents,
including both protic and aprotic solvents. Importantly, the HSBC/MSA model
predicts not only solvation free energies accurately but also solvation
entropies, which standard continuum electrostatic models fail to predict. The
HSBC/MSA model depends only on the normal electric field at the dielectric
boundary, similar to our recent development of an HSBC model for charge-sign
hydration asymmetry, and the reformulation of the MSA as a boundary condition
enables its straightforward application to complex molecules such as proteins.
