Dissociation constants and micelle-water partition coefficients of hydroxybenzoic acids and parabens in surfactant micellar solutions

The dissociation constants of 2- and 4-hydroxybenzoic acids and methyl, ethyl, propyl and butyl esters of 4-hydroxybenzoic acid (parabens) were determined in aqueous and micellar solutions of sodium dodecylsulfate (SDS) and Brij 35. It was shown that micelle concentration of surfactant influences on apparent dissociation constants to a greater extent than surfactant type. It was found that surfactant micellar pseudophase doesn't bind monoanionic and dianionic forms of 4-hydroxybenzoic acid as well as anionic forms of 2-hydroxybenzoic acid and parabens. The binding constants of molecular forms of 2-hydroxybenzoic acid and parabens were calculated from apparent dissociation constants on surfactant concentration dependences. Values of binding constants of parabens at Brij 35 micellar solutions are significantly changed with the increasing of Brij 35 concentration. The partition coefficients between water and micellar pseudophase values were calculated from obtained binding constants values. The correlations between log P(water-micelle) and log P(water-1-octanol) for parabens in SDS and Brij 35 micellar solutions are different.