The influence of salicylate counterions on the aggregation behaviour of a polymerizable cationic surfactant

The aggregation behaviour of the cationic polymerizable surfactant dimethylhexadecyl-(2-acrylamidoethyl)ammonium bromide (AM16) has been studied with time-resolved fluorescence quenching (TRFQ) and viscometry. The quencher-averaged aggregation number N-q was determined in the absence and presence of strongly binding salicylate ions using TRFQ. In the absence of sodium salicylate (NaSal) AM16 behaves very much like other cationic surfactants with an aggregation number increasing with surfactant concentration and decreasing with increasing temperature. In the presence of NaSal at constant temperature, N-q first increases slowly with NaSal concentration C-Sal, until the critical concentration C-Sal* above which N-q increases sharply. At constant C-Sal, with increasing temperature N-q decreases more strongly at C-Sal > C-Sal* than at C-Sal < C-Sal*. Viscosity measurements have confirmed the trends observed with TRFQ. The values of C-Sal*, as determined with the two different techniques, agree well. The copolymerization of the surfactant with acrylamide leads to water-insoluble copolymers, because of the large compositional drift during copolymerization. (C) 1998 Elsevier Science B.V.