Cooperativity among Molecules at Interfaces in Relation to Various Technological Processes: Effect of Chain Length on the pKa of Fatty Acid Salt Solutions

Abstract

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 incre...