Improvement of Ca2+-tolerance by the introduction of EO groups for the anionic surfactants: Molecular dynamics simulation

The influence of Ca2+ on the aggregation of sodium dodecyl sulfate (SDS), sodium dodecyl oxyethylene sulfate (SDES), sodium dodecyl benzene sulfate (SDBS) and sodium dodecyl benzene oxyethylene sulfate (SDBES) at air–water interface were compared to investigate the role of oxyethylene unit by means of the molecular dynamics simulation. In the SDES and SDBES systems, the interaction between OSO3− groups and Na+ions decreases, the diffusion coefficient of Na+ increases, while the interaction between surfactants and Ca2+ ions increases, and the diffusion coefficient of Ca2+ ions decreases. Unlike SDS and SDBS systems, the EO groups introduced may bind Ca2+ ions and the interaction between Ca2+ ions OSO3− groups is weakened, consequently, the precipitation of anionic surfactant by Ca2+ ion is restrained, i.e., the Ca2+-tolerance of anionic surfactants via the introduction of EO groups may be improved. The present results could help in choosing of anionic surfactants used in aqueous solution with inorganic salts.

In the SDS and SDBS systems, the Ca2+ ions are mostly around the head groups, and tend to precipitate surfactants, while more Ca2+ ions should close to the Op in the SDES and SDBES systems, and only a few Ca2+ ions appear to the head groups, then the surfactants would not precipitate easily, so the Ca2+-tolerances of SDES and SDBES molecules are higher than that of SDS and SDBS molecules.Figure optionsDownload as PowerPoint slideHighlights
► Some Ca2+ ions enter into the interface only in the SDES and SDBES systems.
► The appearance of Ca2+ ions decreases the interactions between OSO3− groups and Na+ ions.
► The introduced EO groups can bind Ca2+ ions and restrict the mobility of Ca2+ ions.
► Only a few free Ca2+ ions interact with the head groups for the existence of EO groups.
► The presence of EO groups also increases the hydration ability of OSO3− groups.