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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 pK a 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 pK a has shown an increase in apparent pK a with concentration. The increase in pK a 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 pK a as compared to that of soap monomers. Mixing of soap molecules of unequal chain length decreases the pK a 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 pK a of soap solutions.

The orientation of surface-active molecules at interfaces is extremely important in determining the dynamic and equilibrium properties of such systems. Films of fatty acid salts were studied at various pH values of the solutions. 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 pKa of the fatty acid salts. The experimental results are explained in terms of area per molecule, intermolecular spacing, and cooperativity among soap molecules at the interface. It was further shown that the chain length of the soap molecules can modulate the area per molecule, and hence the intermolecular distance in the film, and thereby influence the ionization behavior of the fatty acid carboxyl group. The pKa increases from 6.5 to about 9.0 as the chain length of the fatty acid salt increases from C8 to C16. Cooperativity among surfactant molecules at the interface is controlled by the area per molecule and intermolecular spacing in the adsorbed film. A small change in intermolecular distance of 0.03 Å can significantly influence various technological processes such as foaming, emulsification, wetting, and retardation of evaporation.

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