Effect of Premicellar Aggregation on the pKa of Fatty Acid Soap Solutions

Associative interactions of the various species found in the premicellar concentration region of aqueous fatty acid solutions have been investigated using acid−base titration. In previous studies, aqueous films of fatty acid salts were investigated at various bulk solution pH values. It was found that there exists a pH where minimum evaporation of water, maximum foamability, maximum foam stability, minimum contact angle on PMMA surface, maximum single-bubble stability, and maximum surface viscosity are observed. It was also found that this optimum pH value is near the measured pKa of the fatty acid, which in turn depends on the length of the fatty acid chains. Titration of the homologous series of C8−C12 fatty acids to determine the solution pKa has shown an increase in apparent pKa with concentration. The increase in pKa is maintained at concentrations well below the critical micelle concentration (cmc). Thus, similar to micelle formation, the submicellar aggregates must be responsible for the increase in pKa as compared to that of soap monomers. Mixing of soap molecules of unequal chain length decreases the pKa of the solution as compared to that of the two individual components because of disorder produced by the unequal chain length. Results indicate that premicellar surfactant aggregation and molecular association well below the cmc of the soap considerably affects ionization of the polar group. This molecular association results in an increase in the measured pKa of soap solutions.