Effect of many-body interactions on the solid-liquid phase-behavior of charge-stabilized colloidal suspensions

The solid-liquid phase-diagram of charge-stabilized colloidal suspensions is calculated using a technique that combines a continuous Poisson-Boltzmann description for the microscopic electrolyte ions with a molecular-dynamics simulation for the macroionic colloidal spheres. While correlations between the microions are neglected in this approach, many-body interactions between the colloids are fully included. The solid-liquid transition is determined at a high colloid volume fraction where many-body interactions are expected to be strong. With a view to the Derjaguin-Landau-Verwey-Overbeek theory predicting that colloids interact via Yukawa pair-potentials, we compare our results with the phase diagram of a simple Yukawa liquid. Good agreement is found at high salt conditions, while at low ionic strength considerable deviations are observed. By calculating effective colloid-colloid pair-interactions it is demonstrated that these differences are due to many-body interactions. We suggest a density-dependent pair-potential in the form of a truncated Yukawa potential, and show that it offers a considerably improved description of the solid-liquid phase-behavior of concentrated colloidal suspensions. The classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory [1] predicts that an isolated pair of charged colloidal spheres in an aqueous salt solution interacts via a repulsive Yukawa potential at large separations. Direct experimental measurements have confirmed the general validity of this prediction [2, 3, 4]. Though originally being designed only for pairs of colloids in isolation, it is common practice nowadays to apply the DLVO theory also to the much more complex case of colloids in concentrated suspensions, the situation which occurs in most technical applications. As we show in this letter, treating a colloidal suspension as a system of pairwise interacting Yukawa particles is valid only in a limited parameter regime. To this end a full Poisson-Boltzmann (PB) mean-field description of the colloidal interactions has been combined with a molecular-dynamics (MD) simulation, similar to Ref. [8], to calculate the solid-liquid phase-behavior of charge-stabilized colloidal suspensions. While correlations between the microions are neglected in this approach, many-body interactions between the colloids, mediated by the screening ionic fluid between and around the colloids, are fully included. By calculating the effective DLVO parameters for the same system, it is seen that at large volume fraction of colloids and low salt concentration, the description in terms of pair-wise interacting Yukawa particles fails dramatically. 1 [email protected]