Phase diagram of symmetric binary mixtures at equimolar and nonequimolar concentrations: A systematic investigation

Abstract

We consider symmetric binary mixtures consisting of spherical particles with equal diameters interacting via a hard-core plus attractive tail potential with strengths ǫ ij , i, j = 1, 2, such that ǫ 11 = ǫ 22 > ǫ 12 . The phase diagram of the system at all densities and concentrations is investigated as a function of the unlike-to-like interaction ratio δ = ǫ 12 /ǫ 11 by means of the hierarchical reference theory (HRT). The results are related to those of previous investigations performed at equimolar concentration, as well as to the topology of the mean-field critical lines. As δ is increased in the interval 0 < δ < 1, we find first a regime where the phase diagram at equal species concentration displays a tricritical point, then one where both a tricritical and a liquid-vapor critical point are present. We did not find any clear evidence of the critical endpoint topology predicted by mean-field theory as δ approaches 1, at least up to δ = 0.8, which is the largest value of δ investigated here. Particular attention was paid to the description of the critical-plus-tricritical point regime in the whole density-concentration plane. In this situation, the phase diagram shows, in a certain temperature interval, a coexistence region that encloses an island of homogeneous, one-phase fluid.

Monte Carlo Simulation of Phase Equilibria in Ising Fluids and Their Mixtures

“…This leads to the appearance of a van Laar point in the global phase diagram and at this point to a phase diagram containing an unsymmetrical tricritical point at finite magnetic field [32]. This is in agreement with the scenario found in mean field theory for symmetrical mixtures [33] and the HRT [30].…”

“…Our results show the same scenario as MC simulations in an Ising liquid with both interactions, the non-magnetic and the magnetic one, of square well [16] or Lennard-Jones type [29]. It turned out that the hierarchical reference theory (HRT) [30] did not agree with the scenario found in MC simulations. In the language of the magnetic fluid, for small values of R no critical end point [29], but a phase diagram of type II was found [30].…”

“…It turned out that the hierarchical reference theory (HRT) [30] did not agree with the scenario found in MC simulations. In the language of the magnetic fluid, for small values of R no critical end point [29], but a phase diagram of type II was found [30].…”

Ising fluids in an external magnetic field: An integral equation approach

“…Our calculations have shown that this special point is identified at R vl = 0.196 with in [30] (their δ vl = 0.67 corresponds to R vl = 0.197). In order to collect our results for the thermodynamic phase diagrams, the global phase diagram in R-space is presented in Fig.…”

“…On the other hand, the more cumbersome SCOZA technique can provide us with highly precise results for the phase boundaries and it remains accurate even near criticality [33]. The other concept is the hierarchical reference theory (HRT) [30,[34][35][36], that combines features of the renormalization group theory (RGT) and theoretical liquid-state approaches and allows to reproduce some critical exponents more precisely with respect to the MSA. For instance, the critical exponent β (which gives the curvature of the coexistence curve near the critical point) takes the values 7/20 and 0.345 within the SCOZA and HRT, respectively.…”