Excess free energy approach to the estimation of solubility in mixed solvent systems II: Ethanol-water mixtures.

The use of the reduced three-suffix solubility equation in characterizing solubility in ethanol-water mixtures is discussed. The equation states that ln xs2,m = z1 ln xs2,1 + z3 ln xs2,3 - A1-3z1z3(2z1 - 1)- (q2/q1) + A3-12z21z3(q2/q3) + C2z1z3, where xs2,1, xs2,3, and xs2,m are the mole fraction solubilities of the solute in ethanol (subscript 1), water (subscript 3), and in the mixture (m); A1-3 and A3-1 are solvent-solvent interaction terms; C2 is a solute-solvent interaction term; and the q- and z-values are molar volumes and solute-free volume fractions, respectively. The contributions of the various terms in the equation to solubility are examined, and the possible use of its derivative in indicating whether a maximum may exist in the solubility profile is discussed. Methods of obtaining the solvent-solvent interaction constant and the ternary constant C2 are described, and the general effectiveness of the equation in describing solubility is examined. The equation is shown to be applicable to 10 compounds with widely different physical properties and, thus, appears to combine both ease of use and general utility.