Molecular simulation and theory of soft matter and biophysical systems often use implicit solvent models that coarse-grain solvent degrees of freedom into an effective interaction potential. Such an approach embeds entropic contributions of the solvent into the effective interaction. In systems containing charge, the solvent degrees of freedom are lumped into the dielectric constant. Properly accounting for the solvent entropy is essential to understand the thermodynamic driving force for interactions between charged species and in charged assembly. Our study highlights the significance and universality of solvent entropy in mediating interactions between ions in polar solvents. By utilizing dipolar self-consistent field theory and coarse-grained molecular dynamics, we demonstrate that the dominant factor governing the interaction between two oppositely charged ions in polar solvents is the solvent entropy, rather than their energetic attraction.
