Studies on Physicochemical Properties and Aggregation Behavior of Two Pairs of Betaine Surfactants

Two pairs of betaine surfactants with the same hydrophobic moieties but different hydrophilic headgroups, including alkylbetaine surfactants (ACB12 and ASB12) and phenyl-containing betaine surfactants (BCB10 and BSB10), were synthesized, and their chemical structures were confirmed by ESI-HRMS, 1H NMR, and 13C NMR. Their surface activities and aggregate morphologies were also systematically investigated. Phenyl-containing betaine surfactants exhibit better surface activities than alkylbetaine surfactants, including higher short-term thermal stabilities, lower critical micelle concentrations (CMC) and surface tensions, and greater tendencies of adsorption and aggregation. The minimum interfacial tension (IFTmin) of BSB10 and BCB10 solution against Xinjiang crude oil achieved an ultralow value (10–3 mN/m), while the IFTmin only reached a 100–10–1 mN/m order of magnitude for ASB12 and ACB12. In addition, phenyl-containing betaine surfactants displayed considerably higher wetting ability than alkylbetaine surfactants, while the contact angles of BSB10 and BCB10 solution on a paraffin film decreased to 31.89 and 25.19°, respectively. Moreover, DLS and cryo-TEM measurements verified that phenyl-containing betaine surfactant molecules could spontaneously form vesicles in an aqueous solution above the CMC, while alkylbetaine surfactant molecules only aggregate into small micelles in the solution. The BSB10 and BCB10 molecules could further assemble into larger vesicles with increasing surfactant concentration.