Effect of three-body forces on the phase behavior of charged colloids

Statistical-thermodynamic theory for predicting the phase behavior of a col loidal solution requires the pair interaction potential between colloidal p articles in solution. In practice, it is necessary to assume pairwise addit ivity for the potential of mean force between colloidal particles, but litt le is known concerning the validity of this assumption. This paper concerns interaction between small charged colloids, such as surfactant micelles or globular proteins, in electrolyte solutions and the multibody effect on ph ase behavior. Monte Carlo simulations for isolated colloidal triplets in eq uilateral configurations show that, while the three-body force is repulsive when the three particles are near contact, it becomes short-ranged attract ive at further separations, contrary to a previous study where the triplet force is attractive at all separations. The three-body force arises mainly from hard-sphere collisions between colloids and small ions; it is most sig nificant in solutions of monovalent salt at low concentration where charged colloids experience strong electrostatic interactions. To illustrate the e ffect of three-body forces on the phase behavior of charged colloids, we ca lculated the densities of coexisting phases using van der Waals-type theori es for colloidal solutions and for crystals. For the conditions investigate d in this work, even though the magnitude of the three-body force may be as large as 10% of the total force at small separations, three-body forces do not have a major effect on the densities of binary coexisting phases. Howe ver, coexisting densities calculated using Derjaguin-Landau-Verwey-Overbeek theory are much different from those calculated using our simulated potent ial of mean force. (C) 2000 American Institute of Physics. [S0021-9606(00)5 1232-X].