β-Hydroxyl Group in the Sulfonic Biobased Zwitterionic Surfactant Improves Its Thermal Tolerance of Interfacial Activity

The hydrophilic head structure can significantly influence the interfacial performance of biobased zwitterionic surfactants. Among various hydrophilic groups, the hydroxyl group plays a considerable role in the influence on interfacial activity. However, the relevant mechanism remains to be elucidated. To study this point, in this work two new biobased zwitterionic surfactants, anisole-based oleamide ethyl hydroxypropyl sulfonyl quaternary ammonium salt (AEHSQA) and anisole-based oleamide ethyl sulfonylpropyl quaternary ammonium salt (AESPQA), were synthesized from biomass material methyl oleate, and the interfacial performances of the two surfactants were evaluated. It showed that AEHSQA can lower the interfacial tensions between crude oil and groundwater to the ultralow level (<10^–2 mN/m) at a temperature of up to 120 °C and a concentration of Ca²⁺ of up to 2500 mg/L with one more β-hydroxyl group, while the corresponding tolerances of AESPQA are only 90 °C and 250 mg/L. Molecular dynamics simulation (MDS) was employed to study the interfacial behaviors of surfactant molecules at the oil–water interface under conditions of different temperatures and salinities. The results of MDS implied that introducing a hydroxyl group could improve the thermal resistance and salt tolerance of zwitterionic surfactants via resisting the hydrophilicity decline and interfacial looseness of surfactant molecules resulting from the increases in temperature and salinity.