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Phase Transitions and Interactions in Ionic Systems

We are investigating phase transitions in simple models for ionic melts and solutions. These systems are extremely difficult to handle by either theoretical or numerical methods because of the presence of infinite-range ionic forces and strong counterion association.  Work by our group and others has generated estimates for the phase behavior of  "primitive models" for ionic systems consisting of charged hard spheres in a dielectric continuum. Our recent work has extended the charge asymmetry to ratios of 2000:1, appropriate for charged colloids. A key factor controlling the interactions between surfaces in aqueous solutions is the surface charge density. Surfaces typically become charged though a titration process where surface groups can become ionized based on their dissociation constant and the pH of the solution. We have developed a a Monte Carlo method to treat this process in a system with two planar surfaces with explicitly described ionizable sites in a salt solution, as shown in the snapshot below (from S. A. Barr and AZP, Langmuir, 2011).