Solubilization of amphiphilic carboxylic acids in nonionic micelles:: Determination of partition coefficients from pKa measurements and NMR experiments

The solubilization of octylamidotartaric acid (C8T) and octanoic acid (C8C) in Triton X-100 and Brij 58 nonionic micelles has been studied by pHmetric and H-1 NMR self-diffusion experiments. As both C8C and C8T exhibit acidbase properties, a distinction between the partition of the neutral acidic form, in terms of the partition coefficient K-PH, and the partition of the charged basic form, in terms of the partition coefficient Kp-, has been made. The acidity constants, K-a, of C8T and C8C in the presence of micelles have been evaluated from pHmetric experiments. For both solutes, an increase in the pK(a) is observed in micellar media due to the difference in the partition of acidic and basic forms of the solutes. A model has been developed to determine K-PH and Kp- from the pK(a) shifts observed. The values obtained by this pK(a) shift modeling method and those from self-diffusion coefficient measurements are in good agreement. The acidic form of C8C is incorporated to a larger extent into the Brij 58 micelles than the acidic form of C8T, whereas the opposite trend is observed for the basic forms. both the acidic and basic forms of C8T are more easily incorporated into Brij 58 micelles than into Triton X-100 micelles. The influence of the structure of the polar head on the solubilization properties is demonstrated. Moreover, evidence for the localization of the solutes in the micelles is obtained from the comparison of the partition coefficients and from H-1 NMR data.