Phase transitions and critical behaviour of binary liquid mixtures

Compared to the simple one-component case, the phase behaviour of binary li quid mixtures shows an incredibly rich variety of phenomena. fn this contri bution we restrict ourselves to so-called binary symmetric mixtures, i.e. w here like-particle interactions are equal (Phi (11)(r) = Phi (22)(r)), wher eas the interactions between unlike fluid particles differ from those of li kes ones (Phi (11 Phi) (r) not equal Phi (12)(r)). Using both the simple me an spherical approximation and the more sophisticated self-consistent Ornst ein-Zernike approximation, we have calculated the structural and thermodyna mic properties of such a system and determine phase diagrams, paying partic ular attention to the critical behaviour (critical and tricritical points, critical end points). We then study the thermodynamic properties of the sam e binary mixture when it is in thermal equilibrium with a disordered porous matrix which we have realized by a frozen configuration of equally sized p articles. We observe-in qualitative agreement with experiment-that already a minute matrix density is able to lead to drastic changes in the phase beh aviour of the fluid. We systematically investigate the influence of the ext ernal system parameters (due to the matrix properties and the fluid-matrix interactions) and of the internal system parameters (due to the fluid prope rties) on the phase diagram.