Interfacial Structural Transition in Hydration Shells of a Polarizable Solute.

Electrostatics of polar solvation is typically described by harmonic free energy functionals. Polarizability contributes a negative polarization term that can make the harmonic free energy negative. The harmonic truncation fails in this regime. Simulations of polarizable ideal dipoles in water show that water's susceptibility passes through a maximum in the range of polarizabilities zeroing the harmonic term out. The microscopic origin of the nonmonotonic behavior is an interfacial structural transition involving the density collapse of the first hydration layer and enhanced number of dangling OH bonds.