Heats of mixing and vapor-liquid equilibrium calculations



In the literature, when the available integrated forms of the Gibbs-Duhem equation are employed for extrapolating vapor-liquid equilibrium data at isobaric conditions, the relative partial molal enthalpies at constant liquid compositions, L, are assumed either to be zero or to be independent of temperature. An investigation of the temperature effect on heats of mixing for non-ideal solutions reveals that these assumptions are invalid and that the quantity L varies linearly with temperature. Therefore the liquid activity coefficients at constant compositions should involve three temperature terms of the form a + (b/T) + c log T for extrapolating purposes. As a result, more general equations are proposed.