Phase Separation in Charge-Stabilized Colloidal Suspensions: A Simulation Study

A variety of experiments\footnote{J. C. Crocker and D. G. Grier, Phys. Rev. Lett. {\bf 77}, 1897 (1996).}$^,$\footnote{C. P. Royall, M. E. Leunissen, and A. van Blaaderen, J. Phys.: Condens. Matter {\bf 15}, S3581 (2003).} suggest that deionized suspensions of highly charged colloids may exhibit an unusual fluid phase separation. Here we report results of Gibbs ensemble Monte Carlo simulations of a model system consisting of charged colloidal macroions and microions (dissociated counterions and salt ions) dispersed in water. As input to the simulations, we use effective electrostatic interactions -- a screened-Coulomb macroion-macroion pair potential and a one-body volume energy -- predicted by mean-field linear response theory\footnote{A. R. Denton, Phys. Rev. E {\bf 62}, 3855 (2000).}. The volume energy -- a natural byproduct of integrating out from the partition function the microion degrees of freedom -- plays a crucial role in the acceptance probabilities for trial volume changes and particle transfers in the Gibbs ensemble. We present the bulk fluid phase diagram, which exhibits vapor-liquid coexistence at low salt concentrations ($c_s<1$ mM), and compare with theoretical predictions.