Phase behavior and structure of Janus fluids.

The equilibrium phase behavior of Janus fluids is examined based on a model potential for the interaction between their constituents. Janus fluids consist of axisymmetric particles possessing two different "faces," e.g., one hydrophobic and one hydrophilic surface, and the interaction depends on the relative orientation. Starting from a short range, isotropic potential we make an ansatz for an anisotropic model interaction potential. Two types of symmetries of the particles are considered. One leads to a polar phase. The Helmholtz free energy and the pressure are calculated by the help of an augmented van der Waals approximation. A qualitative phase diagram is obtained. The appearance of a polar phase and the corresponding transition temperature are examined adapting a Landau-de Gennes expansion of the orientational part of the free energy. Monte Carlo simulations are performed and the results are compared with the ones obtained by the analytical description.