Vapor-liquid critical-point properties of a symmetric binary fluid mixture

We propose a microscopic approach for investigating the behavior of a symmetric binary fluid mixture in the vicinity of the vapor-liquid critical point. The problem can be reduced to calculating the partition function of a 3D Ising model in an external field. For a square-well symmetric binary mixture, we express the parameters of the critical point as functions of the microscopic parameter r measuring the relative strength of interactions between the particles of dissimilar and similar species. The calculations are performed at intermediate (λ=1.5) and moderately long (λ=2) intermolecular potential ranges. The obtained results agree well with the results of computer simulations.