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.