Pair connectedness functions and percolation in highly charged electrolyte solutions

Abstract

The structure of highly charged electrolyte solutions is investigated through a percolation approach. The systems, simulated by standard Monte Carlo methods, include several 2:2 aqueous solutions with concentrations ranging from 0.25 to 2 M and one 1:1 electrolyte at 1 M concentration. The ion–ion pair connectedness functions and related quantities have been computed. Two ions are considered connected if they are closer than a predefined distance. Besides this geometric criterion, an energetic condition (i.e., only particles with unlike charge signs are allowed to form directly connected links) has also been employed. The clusters obtained via the geometric and the energetic conditions are substantially different for the 1:1 electrolyte while the 2:2 solutions show a cluster structure almost independent on the criterion used. The percolation thresholds exhibit a nonmonotonic variation with concentration. A minimum in the percolation threshold has been observed at the higher concentrations investigated for the 2:2 systems; the shape of the curve and the limiting values for infinitely dilute solutions suggest the appearance of a maximum at concentrations lower than those studied in this paper. In addition, the critical exponents α, γ, and τ have been computed. The result for α significantly differs from the values reported for three dimensional lattices but it agrees with those obtained for the Lennard-Jones fluid. In contrast, the γ and τ exponents for 3D lattice systems and for our electrolyte solutions seem to be coincident.