The structure of zwitterionic surfactant sulfobetaine, i.e. N-dodecyl-N,N-dimethyl-3-ammonio- 1-propanesulfonate, was optimized using density functional theory (DFT) at the B3LYP/6-31＋g(d) level and the interactions between the surfactant and Ca^2＋ or Cl^－ ions were studied at the molecular level. The results showed that: i) a 2∶1 type pair between zwitterionic negative center ( ) and Ca^2＋ was formed, in which two oxygen atoms of the polar group ( ) in the surfactant bound with one Ca^2＋ ion|ii) the positive center (—N^＋(CH₃)₂—) bound with one Cl^－ through two methyl groups and one methylene which connect to N atom. Since there are some weak charges on the methylene nearest to the polar groups, the negative and positive centers in the polar group of surfactant should be re-divided. The calculation also showed that the tail chain has a weak charge resulting in the core of the micelle having polarity. This core polarity of the micelle is somewhere between the oil phase polarity and the water phase polarity, which favors surfactant aggregation in solution.

Key words: sulfobetaine surfactant, interaction, density functional theory, micelles