A comparison of two models of ternary excess free energy

Wohl's model and Kohler's model are two empirical excess free energy of mixing models which have been formulated for ternary solutions. The two models are identical when the binary systems are regular solution model systems. When it is assumed that the binary systems are subregular model systems the two ternary models differ. This difference is examined using the subregular model parameter values suggested by Saxena to approximate the experimental work of Seck on coexisting feldspars in the system Anorthite-Albite-Orthoclase at 900° C and 0.5 Kb. For these conditions Wohl's model is closer to the experimental data than Kohler's model in generating the position of the solvus isotherm and is better in matching the shape of the albite partitioning curve. With regard to the slope of the tie-lines, Kohler's model provides a better fit to the experimental results than Wohl's model. Experimentally determined activities for this system are not yet available so that there is no absolute way of selecting the model which would provide the most realistic activities.A ternary solvus can be completely displayed on one diagram when two sets of contours are overlain on a ternary plot. One set of contours consists of isotherms while the other set consists of icophases which are usually at a reasonable angle to the isotherms. Not only are icophases a clear way of coding tie-line information, they also assist in the positioning of the consolute or critical line. The simple new activity matching algorithm that is used in the comparative calculations has the ability to produce icophases directly.Symbols N mole fraction - a activity - T temperature °K - R gas constant, 1.987 cals/mole - G ^x excess free energy of mixing - u _i ^x excess chemical potential of mixing of component i - f activity coefficient - E _AC , W _AC characteristic parameter (cals/mole) for binary AC in a solution model