Phase diagram of hard-core repulsive Yukawa particles with a density-dependenttruncation: a simple model for charged colloids

Abstract

Using computer simulations we study the phase behaviour of hard spheres with repulsiveYukawa interactions and with the repulsion set to zero at distances larger than adensity-dependent cut-off distance. Earlier studies based on experiments and computersimulations in colloidal suspensions have shown that the effective colloid–colloid pairinteraction that takes into account many-body effects resembles closely this truncatedYukawa potential. We present a phase diagram for the truncated Yukawa system bycombining Helmholtz free energy calculations and the Kofke integration method. Comparedto the non-truncated Yukawa system we observe (i) a radical reduction of the stability ofthe body centred cubic (BCC) phase, (ii) a wider fluid region due to instability of the facecentred cubic (FCC) phase and due to a re-entrant fluid phase and (iii) hardly anyshift of the (FCC) melting line when compared with the (BCC) melting line forthe full Yukawa potential for sufficiently high salt concentrations, i.e. truncationof the potential does not affect the location of the solid–fluid line but replacesonly the BCC phase with the FCC at the melting line. We compare our resultswith earlier results on the truncated Yukawa potential and with results fromsimulations where the full many-body Poisson–Boltzmann problem is solved.