Phase diagram of highly asymmetric binary mixtures: A study of the role of attractive forces from the effective one-component approach

Abstract

The phase diagram of an asymmetric solute-solvent mixture is investigated at the level of the effective one-component fluid. The solvent is taken into account by computing the potential of mean force between solute particles at infinite dilution for different models of solvent-solvent and solute-solvent short range interactions. Fluid-fluid and fluid-solid coexistence lines are determined from the free energy in the reference hypernetted chain theory for the fluid branch and from a variational perturbation theory for the solid one. The phase boundaries so determined compare well with recently published Monte Carlo data for mixtures of pure hard spheres. The influence of solute-solvent and solvent-solvent short range attractive forces is then investigated. When compared with pure hard core interactions, these forces are found to produce dramatic changes in the phase diagram, especially on the solvent packing fractions at which a dense fluid of solutes can be stable and on the separation of the fluid-fluid and fluid-solid coexistence lines. Finally, the connection of these results with the behavior of some colloidal suspensions is emphasized.

When mixtures of hard-sphere-like colloids do not behave as mixtures of hard spheres

“…The accuracy of the hybrid method was shown in Ref. [15] for the fluid-solid (FS) and fluid-fluid (FF) transitions, by comparison with the simulation data of Dijkstra et al [6], and confirmed more recently by simulations from our group [17] for the FF transition. Concerning a possible solid-solid (SS) transition between two solid states with different densities, it was shown in [26] that the perturbation treatment is suitable to describe a dense solid near close packing but is more problematic for a "softer" one with lower density.…”

“…Since it is expected to operate at the scale D s of the small (solvent) particles, it falls in the category described by model (2). This is the situation more often considered in the papers discussing the non-HS interactions effect in colloidal suspensions [14][15][16][17].…”

“…[15,17]: for the fluid phase, the very accurate RHNC integral equations [24] were used with the bridge function of Malijevski and Labik [25] as unique input (see Ref. [26] for details).…”

“…First, it is generally difficult to anticipate in this case the complex interplay of non-HS contributions (see Refs. [14][15][16][17] for a discussion of true molecular solvent/colloid binary mixtures). Second, the fundamental features of the phase diagram of pure HS mixtures are critically dependent on the specific characteristics of the HS depletion well.…”

Influence of polydispersity on the critical parameters of an effective-potential model for asymmetric hard-sphere mixtures

“…Interestingly, our estimates for the criticalη s values lie much closer than those of ref. [3] to the results of a computation using integral equation theory of both the depletion potential and its phase behaviour [34]. In ref.…”