Structure of ionic liquids and concentrated electrolytes from a mesoscopic theory

Abstract

Recently, underscreening in concentrated electrolytes was discovered in experiments and confirmed in simulations and theory. It was found that the correlation length of the charge-charge correlations, λs, satisfies the scaling relation λs/λD∼(a/λD)n, where λD is the Debye screening length and a is the ionic diameter. However, different values of n were found in different studies. In this work we solve this puzzle within the mesocopic theory that yielded n = 3 in agreement with experiments, but only very high densities of ions were considered [A. Ciach and O. Patsahan, J.Phys.: Condens. Matter33, 37LT01 (2021)]. Here we apply the theory to a broader range of density of ions and find that different values of n in the above scaling can yield a fair approximation for λs/λD for different ranges of a/λD. The experimentally found scaling holds for 2<a/λD<4, and we find n = 3 for the same range of the reduced Debye length. For smaller a/λD, we find n = 2 obtained earlier in several simulation and theoretical studies, and still closer to the Kirkwood line we obtain n = 1.5 that was also predicted in different works. It follows from our theory that n = 3 (i.e. λs is proportional to the density of ions) when the variance of the local charge density is large, and λs is proportional to this variance times the Bjerrum length. Detailed derivation of the theory is presented.