same theory against more complex mixtures such as CS 2 / We report a study of the surface tension of three binary liquid CH 2 Cl 2 and CS 2 / CCl 4 ; the authors found out that by fitting mixtures of molecular fluids. A microscopic mean field theory the energy parameters (e) of the pure components and one (MFT) has been used to calculate the theoretical results enabling binary parameter to the surface tension, the theory could the comparison with the experimental data. The mean field theory explain the qualitative differences between the two systems. has been successfully used in the prediction of the surface proper-In this paper we use the MFT to study the interfacial structure ties of simple systems composed by quasi-spherical molecules. In of binary liquid mixtures involving one component with a the present study the MFT was able to reproduce the essential features of the interfacial properties of the systems CH 4 / Kr, Kr high percentage of dimerized molecules. The importance of / NO and CH 4 / NO. The pure components were modeled by mixtures such as Kr / NO and CH 4 /
NO is due to the Lennard-Jones potentials with a set of intermolecular parameters fact that the NO molecules in the liquid phase associate taken from the literature for Kr and calculated from the fitting of between themselves to form dimers (but not higher aggrethe energy parameters (e) to the surface tension, for CH 4 and NO. gates), while the NO vapor is mainly constituted by mono-In the case of the mixtures, it was found that reasonable agreement mers. Moreover, nitric oxide is one of the few strongly assowith experiment can only be obtained by allowing deviations from ciating fluids which does not also have a strong electrostatic the Lorentz-Berthelot combining rules. For the CH 4 / Kr system interaction (m Å 0.16 D). This circumstance makes the apwe used the binary energy parameter j obtained through a fitting to the bulk properties; for the Kr / NO and CH 4 / NO systems plication of MFT (which does not account for electrostatic the binary parameter was adjusted to the interfacial properties. multipole interactions) particularly attractive. The bulk prop-᭧ 1997 Academic Press erties of Kr / NO and CH 4 / NO have already been mea-Key Words: surface tension; liquid mixtures; mean field theory. sured (3-5); both systems exhibit large deviations from ideality, leading to positive azeotropy. The interpretation of these experimental results suggests that the strong departure