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Solubility of drugs in non-aqueous systems is very important in understanding the partitioning and transport behavior. The present study was undertaken to evaluate the entropic and enthalpic contribution to activity coefficient of organic compounds (polycyclic aromatic hydrocarbons, aliphatic acids, aliphatic alcohols etc.) in non-aqueous solvents. The activity coefficient can be written as: ln γ₁ = ln γ₁ᶜ + ln γ₁ʳ where superscript "c" and "r" denote entropic (combinatorial) and enthalpic contribution respectively. We selected three solvent systems: benzene, triolein and octanol. The different models considered in this study were Flory-Huggins, Scatchard-Hildebrand, UNIQUAC combinatorial and UNIFAC residual. A combination of Flory-Huggins and Scatchard-Hildebrand which accounts for both the entropic and enthalpic effects gives the best predictions in all the solvents considered.

The effect of triacetin in combination with common cosolvents on the solubility of phenytoin and diazepam was studied. The cosolvents were PEG 400 and propylene glycol. In addition, the data were used to test the following model: UNFORMATTED EQUATION FOLLOWS: log Sᵈ(c,p,w) = log Sᵈ(w) + f(c)σᵈ(c) + [Sᵖ(w)10(f(c)σᵖ(c))/D(p)] σᵈ(p). UNFORMATTED EQUATION ENDS. The term on the left side of the equation is the solubility of a drug in the ternary system. This is related to the aqueous solubility of the drug, the solubility of the drug in a completely miscible organic solvent (CMOS), and the solubility of the partially miscible organic solvent (PMOS). This model was proposed by Gupta et al. (1989) and predicts the solubility of a ternary system composed of a CMOS and PMOS. The results indicate the triacetin does increase the solubility of the two poorly water-soluble drugs. There is good correlation between the observed and predicted increase in the solubility of the drugs.