Synthesis, surface activities, and aggregation behavior of phenyl-containing carboxybetaine surfactants

A series of carboxybetaine surfactants, 2-((4-(alkoxy)-3,5-dimethylbenzyl)dimethyl-ammonio)acetate (C OBCb, where n represents the hydrocarbon chain length of 12, 14, 16 and 18), were synthesized by an efficient and high-yield route for the first time. The surface activities and aggregation behavior of C OBCb in aqueous solution were investigated by equilibrium surface tension, interfacial tension, steady-state fluorescence, dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryo-TEM) and negative-staining transmission electron microscopy (TEM) measurements. In comparison with conventional N-alkylbetaine surfactants (C Cb), the C OBCb species, with a phenyl group introduced in the hydrophobic tail, exhibited excellent surface activities, including lower critical micelle concentration (cmc), lower surface tension and stronger adsorption tendency at an air/water interface. C OBCb also displayed high efficiency in reducing the toluene/water interfacial tension, with C₁₂OBCb achieving an ultralow interfacial tension (10^-3 mN m^-1) at concentrations from 0.2 to 1 mmol dm^-3. The fluorescence intensity ratio and the scattering intensity in DLS measurements changed remarkably at concentrations around the cmc. Furthermore, the C OBCb species spontaneously formed vesicles above the cmc in aqueous solution, and the size of the aggregates increased with increasing surfactant concentrations. Flooding experiments showed that C OBCb could effectively improve oil recovery by 7.85-10.55%.