Recently, we proposed a new technique for atomistic MC simulations of strongly inhomogeneous and dense systems [1]. The gauge cell MC method was applied to nanoconfined fluids, nanodroplets, and liquid junctions. Here, we extend this technique to coarse-grained mesoscale systems composed of quasi-particles or beads of molecules, rather than of individual particles. Similarly to molecular systems considered previously, the sample system cell is placed in equilibrium with the gauge cell that represents a finite reservoir of ideal quasi-particles. The chemical potentials in the sample system are calculated from the hystogram distribution of the particles in the gauge cell. The gauge cell method allows one to determine the relationship between the system composition and chemical potentials. We apply the gauge cell method to mesoscale simulation of polyelectrolyte membranes. Coarse-grained simulations of polyelectrolytes are challenging due to a divergence of the electrostatic energy between two point charges at small distances that poses a significant problem in simulations with soft potentials. We introduce a "hydrated ion" type of beads, which represents the ion in an aqueous shell; the charge is "smeared out" over the bead according to the prescription of Groot [2]. Using the gauge cell method, we determine the activity coefficients of the model electrolyte solutions, and fit the DPD parameters (repulsion and smearing radius) to experimental data on selected electrolyte solutions. These parameters are further employed in DPD simulation of hydrated Nafion membranes.

(1) Neimark A.V., Vishnyakov A. A simulation method for the calculation of chemical potentials in small, inhomogeneous, and dense systems J. Chem. Phys. 2005, 122, 234108

(2) Groot R. D. Electrostatic interactions in dissipative particle dynamics-simulation of polyelectrolytes and anionic surfactants J. Chem.Phys. 2003, 118, 11265.