Margules expression, excess free energy

Thompson and Waldbaum (1967). Orville s hydrothermal ion-exchange data on the compositions of high-temperature alkali feldspars coexisting with aqueous NaCl-KCl solutions are used to derive two-coefficient Margules expressions for excess free energy and molar volume. These expressions are used to calculate the critical mixing curve and the imivariant curve for two feldspars coexisting with jadeite plus quartz (to 30 kbar). 

Comments The particular expressions fitted to the excess Gibbs free energy and the resulting activity coefficients are known as the Margules model. This and other models for the activity coefficient are discussed in Section 12.6. 

Margules activity coefficient model A simple thermodynamic model used to describe the excess Gibbs free energy of a liquid mixture. It uses activity coefficients that are a measure of the deviation from ideality of solubility of a compound in a liquid. See raoult s law. In the case of a binary mixture, the excess Gibbs free energy is expressed as a power series of the mole fraction in which the constants are regressed with experimental data. The activity coefficients are found by differentiation of the equation. Unlike other... 

Equations 12.10.5 through 12.10.12 are based on the use of the two-suffix (one-parameter) Margules expression for the excess Gibbs free energy (Eq. 12.10.1). It is a very simple model which, even with a temperature dependent A, is applicable to a very limited number of systems where is a symmetric function of a i, mainly hydrocarbons of similar size. 

The simplest single-parameter expression for the molar excess Gibbs free energy of a binary system is the two-suffix Margules equation used already in Section 12.10 ... 

---