Abstract:

The ion-pairs of the anionic surfactants (dodecyl sulfonate, dodecyl carboxylate) and the cations (Na+, Ca2+, Mg2+) were optimized using density functional theory (DFT) at the B3LY/6-31G level and the interaction between the surfactants and ions were studied at the molecular level. The results showed that: i) a 2:1 type of ion-pair was formed in which two oxygen atoms from the polar group in the surfactant bound with one ion; ii) the α-methylene nearest the headgroup should be classified as the part of the polar head because of its negative charge before ion bonding; iii) the charge of the α-methylene group can be converted from a weak negative into a weak positive charge by the cations, which decreased the effect of the polar headgroup. This calculation also showed that the tail chain had a weak positive charge in the micelle 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: Density functional theory, Anionic surfactant, Interaction, Micelles, Charge