Dissociation constants of long chain fatty acids in methanol-water and ethanol-water mixtures

The pK_a values of lauric acid, myristic acid, palmitic acid and stearic acid in methanol-water and ethanol-water mixtures of different compositions at 303 K and ionic strength, I=0.1M(KNO₃) have been determined. The pK_a values of the fatty acids have been found to increase with the alcohol content of the solvent. It has also been found that, in general, the pK_a values in the ethanolic solvent are greater than those in the methanolic solvent. In a given solvent mixture the observed pK_a value decreases in the order of stearic acid > palmitic acid > myristic acid > lauric acid. Departure from the predicted linearity of the plots of pK_a against the inverse of the dielectric constant, 1/D indicates that the electrostatic effects taken into account by the Born theory are not adequate enough to represent the total medium effects on pK_a.