Implicit solvent models that reduce solvent degrees of
freedom
into effective interaction potentials are widely used in the study
of soft materials and biophysical systems. For electrolyte and polyelectrolyte
solutions, coarse-graining the solvent degrees of freedom into an
effective dielectric constant embeds entropic contributions into the
temperature dependence of the dielectric constant. Properly accounting
for this electrostatic entropy is essential to discern whether a free
energy change is enthalpically or entropically driven. We address
the entropic origin of electrostatic interactions in a dipolar solvent
and provide a clarified physical picture of the solvent dielectric
response. We calculate the potential of mean force (PMF) between two
oppositely charged ions in a dipolar solvent using molecular dynamics
and dipolar self-consistent field theory. We find with both techniques
that the PMF is dominated by the entropy gain from the dipole release,
owing to the diminished orientational polarization of the solvent.
We also find that the relative contribution of the entropy to the
free energy change is nonmonotonic with temperature. We expect that
our conclusions are applicable to a broad range of problems involving
ionic interactions in polar solvents.