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 n egative net charges. The potential of mean force between colloidal particle s is determined as a function of colloid-colloid separation. In addition to having an opposite sign, the calculated potential of mean force is found t o be stronger and longer-ranged than observed in the case of equally charge d macroparticles. The difference is more pronounced in the presence of diva lent counterions and is especially noticeable when we compare distinct Coul ombic and hard-core collision contributions to the interaction between equa lly and oppositely charged colloids. The present observations suggest the d ominance of attractive forces between globally neutral but electrostaticall y heterogeneous macroparticles. While our numerical results cannot be succe ssfully analyzed by existing theories, they provide useful guidance and ben chmark data for the development of advanced analytic descriptions.