Mixed self-assembly of polyacrylic acid and oppositely charged gemini surfactants differing in the structure of head group

The complexation of a weak polyelectrolyte, polyacrylic acid (PAA) and oppositely charged gemini surfactants 16-6-16 and its hydroxyethylated counterpart 16-6-16(OH) was examined at spontaneous pH. These geminis demonstrate high aggregation capacity and can form micelles in a micromolar concentration range. Only one breakpoint was revealed on the concentration plots of the binary PAA–gemini systems by all the techniques used. The tensiometry and conductometry critical concentrations appeared to be much higher as compared to those revealed by fluorescence ratiometry, with critical aggregation concentration (cac) increasing with an increase in polyelectrolyte concentration in all cases. The cac values of 16-6-16 are higher as compared to 16-6-16(OH). The electrostatic contribution to the gemini–polyelectrolyte complexation is supported by the charge compensation in the systems, in particular by change of zeta potential from ca. −20 to ca. +60 mV with an increase in surfactant concentration, the points of the zero potential being in agreement with the maximum in the turbidity plots. The sizes of aggregates markedly decrease with an increase in surfactant concentration.