Interaction between oppositely charged micelles or globular proteins

Monte Carlo simulations and the hypernetted chain theory are used to study the interaction between spherical macroions of opposite charge immersed in a solution of monovalent or divalent simple electrolyte. These calculations represent the first step toward studying phase behavior and precipitation kinetics in solutions containing a mixture of macroions with positive and negative net charges. The potential of mean force between colloidal particles is determined as a function of colloid-colloid separation. In addition to having an opposite sign, the calculated potential of mean force is found to be stronger and longer-ranged than observed in the case of equally charged macroparticles. The difference is more pronounced in the presence of divalent counterions and is especially noticeable when we compare distinct Coulombic and hard-core collision contributions to the interaction between equally and oppositely charged colloids. The present observations suggest the dominance of attractive forces between globally neutral but electrostatically heterogeneous macroparticles. While our numerical results cannot be successfully analyzed by existing theories, they provide useful guidance and benchmark data for the development of advanced analytic descriptions.