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

Using computer simulations we study the phase behaviour of hard spheres with repulsive Yukawa interactions and with the repulsion set to zero at distances larger than a density-dependent cut-off distance. Earlier studies based on experiments and computer simulations in colloidal suspensions have shown that the effective colloid–colloid pair interaction that takes into account many-body effects resembles closely this truncated Yukawa potential. We present a phase diagram for the truncated Yukawa system by combining Helmholtz free energy calculations and the Kofke integration method. Compared to the non-truncated Yukawa system we observe (i) a radical reduction of the stability of the body centred cubic (BCC) phase, (ii) a wider fluid region due to instability of the face centred cubic (FCC) phase and due to a re-entrant fluid phase and (iii) hardly any shift of the (FCC) melting line when compared with the (BCC) melting line for the full Yukawa potential for sufficiently high salt concentrations, i.e. truncation of the potential does not affect the location of the solid–fluid line but replaces only the BCC phase with the FCC at the melting line. We compare our results with earlier results on the truncated Yukawa potential and with results from simulations where the full many-body Poisson–Boltzmann problem is solved. (Some figures in this article are in colour only in the electronic version)