Roman Melnyk, Oksana Patsahan

Institute for Condensed Matter Physics, National Academy of Sciences of Ukraine
Binary mixtures, in contrast to their constituent components, manifest three different types of two-phase equilibrium states: vapour-liquid, liquid-liquid, and gas-gas. Which of these states is realized and to what extent does it depend on both the external conditions and the microscopic parameters of a mixture. The study of the influence of interparticle interactions on the critical properties of a binary mixture is an interesting and relevant problem. During the last decade, this problem has been intensively studied by integral-equation methods. However, this approach, although it reproduces different types of phase diagrams under variation of the microscopic parameters, it only gives a qualitative picture of the phenomenon under consideration.
In the present lecture, we talk about a microscopic approach to studying the vapour-liquid critical point of a symmetric binary mixture. This approach is based on the method of collective variables, which was effective in describing the second-order phase transition of the 3D Ising model and the vapour-liquid critical point of a one-component fluid. Both universal and nonuniversal quantities are obtained on the basis of this approach. The phase diagram of the symmetric mixture is examined within the framework of the Gaussian approximation.
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