Less bound cations and stable inner salt structure enhanced the salt tolerance of the bio-based zwitterionic surfactants

Molecular interfacial behavior and the salt tolerance of two representative bio-based zwitterionic surfactants, the sulfonated and carboxylated bio-based zwitterionic surfactants derived from waste vegetable oils, have been studied in the presence of monovalent and divalent inorganic salts using molecular dynamics simulation. The sulfonated bio-based zwitterionic surfactant formed a more stable "inner salt" structure and the molecules were arranged more closely at the interface. Both of the inner salt structure and the solvent layer of the sulfonated biobased zwitterionic surfactant were kept with the increase of salt concentrations, which partially accounted for its better solubility in high salinity brines. PMF results showed that the energy barriers to form ion pair between the headgroup and cations in the SPODP systems (8.10-21.6 kJ/mol) were higher than those in the POAPMB systems (3.52-11.7 kJ/mol), and the fraction of bound cations in SPODP solutions (2.4%-13.1%) were lower than those in POAPMB solutions (12.1%-51.6%). In contrary, the solubility of the carboxylated bio-based surfactant was significantly affected by the increasing salt concentration. This work provides new views in understanding the salt tolerance of the bio-based zwitterionic surfactants at a molecular level and facilitates the design of bio-based zwitterionic surfactants with high salt tolerance.