Spin-glass charge ordering in ionic liquids

Abstract

Ionic liquids form intricate microstructures, both in the bulk and near charged surfaces. In this Letter, we show that, given the ionic positions from molecular simulations, the ionic charges minimize a "spin-glass" Hamiltonian for nearest-neighbor interactions with remarkable accuracy, for both room-temperature ionic liquids (RTIL) and water-in-salt electrolytes (WiSE). Long-range charge oscillations in ionic liquids thus result from positional ordering, which is maximized in ionic solids, but gradually disappears with added solvent. As the electrolyte becomes more disordered, geometrical frustration in the spin-glass ground state reduces correlation lengths. Eventually, thermal fluctuations excite the system from its ground state, and Poisson-Boltzmann behavior is recovered. More generally, spin-glass ordering arises in any liquid with anti-ferromagnetic correlations, such as 2D vortex patterns in super-fluids or bacterial turbulence.