Phase Separation in Charge-Stabilized Colloids: Influence of Nonlinear Screening

A range of experiments provide evidence that suspensions of like-charged colloids can separate into macroion-rich and -poor phases at low salt concentrations. An apparently related phenomenon is predicted by theories that map the mixture of macroions and microions (counterions and salt ions) onto an effective one-component system and include a one-body volume energy in the effective Hamiltonian\footnote{R. van Roij, M. Dijkstra, J.-P. Hansen, Phys. Rev. E {\bf 59}, 2010 (1999).}$^,$\footnote{P. B. Warren, J. Chem. Phys. {\bf 112}, 4683 (2000).}. Theoretical studies of charged colloids commonly assume that electrostatic interactions between macroions are linearly screened by microions. However, as recently shown within the framework of response theory\footnote{A. R. Denton, Phys. Rev. E {\bf 70}, 031404 (2004).}, nonlinear microion screening induces triplet attractions, weakens pair repulsions, and modifies the volume energy. Taking these nonlinear interactions as input to a thermodynamic perturbation theory, equilibrium phase diagrams for aqueous suspensions of highly charged colloids are computed. The main conclusion: nonlinear effects do not suppress, and can even promote, the predicted phase separation of deionized suspensions.