Forces between aqueous nonuniformly charged colloids from molecular simulation

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Abstract
NVT Monte Carlo simulation results are presented for the forces between charged colloids within the primitive model for electrolytes. The calculations show that when charged colloids have a net dipole moment, a strong attraction can arise at short separations. The attractive force is not purely electrostatic; significant contributions follow from hard-sphere collisions between the electrolyte ions and the colloidal particles. In divalent electrolyte solutions, nonuniformly charged colloids show an oscillatory force profile as a function of separation, due to layering of electrolyte ions around the interacting colloids. Simulation results are compared to two analytical models derived from classical Debye–Hückel screened potentials. In the first model, contributions from charge–charge, dipole–dipole, and charge–dipole interactions are independently angle-averaged and then added to obtain the colloid–colloid potential. In the second model, the pair potential is obtained by simultaneously angle-averaging all interactions. Our results show that simultaneous angle-averaging of anisotropic interactions provides significant improvement over the commonly used additivity approximation.