The Mixed Micellization of Cationic/Anionic Surfactants: Role of Matching Water Affinities Between Oppositely Charged Headgroups and That Between Oppositely Charged Constituent Counterions.

To explore the role of matching water affinities between the oppositely charged headgroups, the micellization of cetyltrimethylammonium bromide (CTA(+)Br(-))/sodium dodecanoate (Na+L-) mixed system and the CTA(+)Br(-)/sodium dodecylsulfonate (Na(+)AS(-)) mixed system has been investigated by the surface tension method and molecular dynamic (MD) simulation. In comparison with the CTA(+)Br(-)/Na+L- system, the CTA(+)Br(-)/Na(+)AS(-) system shows stronger micelle formation ability, smaller critical micelle concentration (cmc), and stronger synergistic effect arising from the higher degree of matching water affinities between CTA(+) and AS(-). To explore the role of matching water affinities between the oppositely charged constituent counterions, the micellization of CTA(+)X(-)/Y+L- mixed systems with various counterions has been investigated. The higher degree of matching water affinities between counterions X- and Y+ and the higher degree of mismatching water affinities between headgroups and counterions are unfavorable to the screening effect of counterions on the electrostatic attraction between headgroups CTA(+) and L-, leading to stronger micelle formation ability and smaller cmc and vice versa. MD simulation results also indicate that, for the mixed micellization of cationic/anionic surfactants, the role of matching water affinities between oppositely charged headgroups is more important than that between oppositely charged constituent counterions.