Analytic calculation of phase diagrams for charged dipolar colloids with orientation-averaged pair potentials

Molecular thermodynamics is used to calculate phase diagrams for aqueous charged dipolar colloids or globular proteins. Because normal pressures are not important for condensed systems, here a phase diagram is a plot of temperature versus colloid concentration. Properties of the fluid phase are obtained from the random-phase approximation, whereas those for the solid phase correspond to a perfect crystal. Crystal structures considered are face-centered and body-centered cubic. For each phase, the Helmholtz energy is determined by the sum of a hard-sphere reference term and a perturbation term that uses a potential of mean force for pairs of charged, dipolar colloids that also interact through dispersion forces. In view of different screening effects on charge–charge repulsion and dipolar attraction, the net electrostatic term features an extremum at intermediate ionic strengths leading to a non-monotonic dependence of the phase behavior on salt concentration. Illustrative phase diagrams are shown as a function of colloid charge, dipole moment, and ionic strength of the aqueous medium. Calculated results show that the phase diagram is sensitive to the structure assumed for the solid phase.