Molecular theory of phase equilibria in model and real associated mixtures. II. Binary aqueous mixtures of inert gases and n-alkanes.

The phase equilibrium properties of a molecular-based model of binary aqueous mixtures are investigated using an extended primitive model (EPW) for water, which incorporates a mean-field attractive term in addition to the interactions given by the primitive model studied in part I of this series of papers. The second component, representing a nonpolar fluid, is modeled by a general hard body with a mean-field attractive term. Analytical expressions for the Helmholtz free energy of this precisely defined molecular model are obtained from statistical mechanical theory, as a function of the molecular size, the mean-field interaction term, and the shape of the second component. The predictions of our model are compared with the behavior of two classes of real aqueous mixtures: Group A={water+inert gases, hydrogen} and Group B={water+n-alkanes}. The phase equilibrium properties are studied as a function of the ratio of the critical temperature τ and critical volume λ with respect to the corresponding quantities for water, and the global phase diagram (i.e., the type of phase behavior and its dependence on the model parameters) is determined. Since τ and λ are obtainable both from our theory and from experiment, our approach thus contains no adjustable parameters. The theory gives qualitatively correct predictions of the phase behavior of these two classes of mixtures, i.e., of the transition between Type IIIc and Type IIId critical line behavior in the Konynenberg and Scott classification scheme, of the presence or absence of pressure minima for Group B mixtures exhibiting Type IIIc behavior, and the dependence of the temperature and pressure of the Type IIIc temperature minimum on the size of the second-component molecule. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]