Extended 1-fluid theory for mixtures containing non-spherical molecules

Abstract

An approximative method is proposed that relates the thermodynamic properties of mixtures of non-spherical molecules to those of mixtures of spherical molecules. It is formulated as a 1-fluid theory with density-dependent mixing rules containing two non-integer exponents. The spherical exponent retains the functional dependence on concentration and density that it has in the case of mixtures of spheres. The non-spherical exponent has been obtained by Monte Carlo simulation of mixtures of hard spheres and fused spheres (di- and tri- atomics); it depends little on density, molecular shape, or composition. For a long-ranged pair potential its value is close to 1; for a square-well potential of range 1.5 the value is close to 0.8. Therefore the effects of molecular size and of shape can be separated. The new mixing rule has been used in connection with a non-cubic equation of state for the calculation of phase equilibria in binary fluid mixtures under high pressure. The representation of isotherms at different temperatures and of critical curves has been improved significantly.