Aggregation behavior of a pH-responsive cationic–nonionic silicone surfactant in aqueous solution

A novel pH-sensitive cationic–nonionic silicone surfactant with a tertiary nitrogen head group and poly(ethylene oxide) hydrophilic chain was synthesized and characterized by ¹H NMR and FT-IR spectroscopies. The surface activity and micellization behavior under acidic, neutral, and basic conditions were characterized by static surface tension (SST), dynamic surface tension (DST), fluorescence, particle size, and zeta potential techniques. The results showed that the cationic–nonionic silicone surfactant exhibited excellent surface activity, which depended on the aqueous solution pH. Both the critical micelle concentration (CMC) and micelle particle size were significantly different at different pH values. The SST results revealed a change in the CMC of the surfactant from 1.88 g·L⁻¹ at pH 4 to 0.32 g·L⁻¹ at pH 10. Moreover, the mesoequilibrium time, determined by DST, significantly decreased with increasing pH, demonstrating that the interface effectiveness was higher at high pH. The micelle morphology changed from 10 to >100 nm when the pH was varied from 3 to 10. Owing to these properties, the proposed surfactant shows great potential for application in many pH-response-related fields such as coatings, drug carriers, and pesticides.