THE POLYNOMIAL REPRESENTATION OF THERMODYNAMIC PROPERTIES IN DILUTE-SOLUTIONS

Attempts to extend the interaction parameter formalism to higher-order polynomials and to render it thermodynamically consistent at finite s olute concentrations have resulted in much confusion. The literature i s reviewed with a view to clarifying the issues. The problem is best a nd most simply resolved through extension of the quadratic formalism, which has a sound theoretical foundation. A new and general set of equ ations for estimating higher-order parameters from binary parameters i s derived. The applicability of using molar ratios rather than mole fr actions in the polynomial expansions is discussed. The formation of as sociate species (such as the formation of ''AlO'' associates in molten Fe) is treated. In such cases of strong solute-solute interactions, t he usual practice of expressing the interaction parameters as linear f unctions of (1/T) is invalid. Finally, for more concentrated solutions , the advantage of using the Kohler or Toop interpolation models rathe r than the commonly used Muggianu model is shown.