The Wilhelmy plate method was used to study the interfacial tension changes and interaction between mixtures of hexane-, octane-, and decanesulfonic acid sodium salts and a nonionic polymer, hydroxypropylmethylcellulose (HPMC). The concentration of HPMC was fixed at 0.8, 2, and 10 ppm. The kinetics of the interfacial tension changes was explained in terms of adsorption of the polymer molecules and conformational changes of molecules that had already been adsorbed at the interface. The molecules above a critical aggregation concentration (cac), which was below the critical micellar concentration, formed clusters, which were evidenced by these surface tension measurements. The formation of the clusters was evidenced in two ways. No clusters were formed between HPMC and hexanesulfonic acid sodium salt. The cac decreased with increased chain length. A synergism in surface activity was observed below that cac for all the binary mixtures studied that was more pronounced with increased chain length. (C) 2003 Elsevier Science (USA). All rights reserved.