PROPYLENE GLYCOL-WATER COSOLVENT MIXTURES.The deviations observed in the solubility of ibuprofen (IBP) and naproxen (NAP) in propylene glycol (PG) + water (W) cosolvent mixtures with respect to the logarithmic-linear model proposed by Yalkowsky have been analyzed at 25.00 ± 0.05 °C.Negative deviations were obtained in all cosolvent compositions for both drugs; they were greater for IBP.Another treatment, based on Gibbs free energy relationships, was also employed showing an apparent hydrophobicity chameleonic effect, because at low PG proportions NAP is more hydrophobic, whereas at high PG proportions IBP is more hydrophobic.The results are discussed in terms of solute-solvent and solvent-solvent interactions.