The middle phenyl-group at the hydrophobic tails of bio-based zwitterionic surfactants induced waved monolayers and more hydrated status on the surface of water

Bio-based zwitterionic surfactants have received increasing attention owing to their structural diversity and good interfacial activity. Therefore, understanding the relationship between the structure of bio-based zwitterionic surfactants and interfacial behaviors has significance for its industrial applications. In this study, molecular dynamics simulation was employed to investigate the interfacial behaviors of bio-based zwitterionic surfactants (BZS) with seven structures at the air/water interface. The four of them were isomers of BZS, in which the phenyl groups were located at the 7-, 10-, 13-, and 16- carbon atom of the octadecanoyl group. The other three were the homologues of 10B, and were different in the structures of hydrophilic groups. The structure-property relationship was demonstrated by the structure of surfactant monolayer, the overall arrangement of surfactant molecules at the interface, and the dynamic properties of binding pairs between surfactant and water/cations. The results showed that waved monolayers were formed when the phenyl group located at the 7- carbon atom of the hydrophobic tail of BZS, and there were more hydration water and cations surrounding the hydrophilic group of surfactants. As for the surfactants with the phenyl group located at 16- of the hydrophobic tail of BZS, molecules were arranged more perpendicular at the interface and formed a flat monolayer and construct stable binding pairs with water and cations. Morphology of monolayer and molecular orientation were independent of the change in structure of headgroup, but surfactant with less methylene on the hydrophilic group had the less steric hindrance, which induced more hydration water, more neighboring surfactants, and more stable of binding pairs between COO- and water.