Synergistic interactions in the mixed micelles of anionic amino acid surfactant with cationic gemini surfactant

The synergistic interactions in the mixed micelles of anionic amino acid surfactant SMS with cationic gemini surfactant 12-3-12 and single-chain surfactant DTAB have been investigated. The fluorescence probe measurements reveal that the critical micelle concentration (CMC) and micellar micropolarity (I1/I-3Micelles) of SMS/12-3-12 and SMS/DTAB mixed micelles are always lied below the individual surfactants and reach the minimum values at equimolar compositions, which indicates the synergistic electrostatic attraction and hydrophobic interaction in the mixed micelles. Comparing with SMS/DTAB mixed micelles, the double charged headgroups and two alkyl chains lead 12-3-12 to have stronger synergistic interactions with SMS to form SMS/12-3-12 mixed micelles, which can be confirmed by the experimental values of CMC, I1/I-3Micelles, zeta potential, and the various micellar parameters calculated from Clint's and Rubingh's models. When sodium bromide (NaBr) is added, SMS/DTAB mixed micelles have more markedly decreased values of CMC and I1/I-3Micelles, but there are slight changes in SMS/12-3-12 mixed micelles. This result is ascribed to more tightly packed structure of SMS/12-3-12 mixed micelles than SMS/DTAB mixed micelles. The salt effect is also explained by the fact that the chaotropic ion Br- and the kosmotropic ion Na+ have different counterion specificities on two mixed micelles. (C) 2021 Elsevier Ltd.