Probing Dynamics and Mechanism of Exchange Process of Quaternary Ammonium Dimeric Surfactants, 14-s-14, in the Presence of Conventional Surfactants

Abstract

In this Article, we investigated effects of different types of conventional surfactants on exchange dynamics of quaternary ammonium dimeric surfactants, with chemical formula C(14)H(29)N(+)(CH(3))(2)- (CH(2))(s)-N(+)(CH(3))(2)C(14)H(29)·2Br(-), or 14-s-14 for short. Two nonionic surfactants, TritonX-100 (TX-100) and polyethylene glycol (23) laurylether (Brij-35), and one cationic surfactant, n-tetradecyltrimethyl ammonium bromide (TTAB), and one ionic surfactant, sodium dodecyl sulfate (SDS) were chosen as typical conventional surfactants. Exchange rates of 14-s-14 (s = 2, 3, and 4) between the micelle form and monomer in solution were detected by two NMR methods: one-dimensional (1D) line shape analysis and two-dimensional (2D) exchange spectroscopy (EXSY). Results show that the nonionic surfactants (TX-100 and Brij-35), the cationic surfactant (TTAB), and the ionic surfactant (SDS) respectively accelerated, barely influenced, and slowed the exchange rate of 14-s-14. The effect mechanism was investigated by the self-diffusion experiment, relaxation time measurements (T(2)/T(1)), the fluorescence experiment (I(1)/I(3)) and observed chemical shift variations. Results reveal that, nonionic conventional surfactants (TX-100 and Brij-35) loosened the molecule arrangement and decreased hydrophobic interactions in the micelle, and thus accelerated the exchange rate of 14-s-14. The cationic conventional surfactant (TTAB) barely changed the molecule arrangement and thus barely influenced the exchange rate of 14-s-14. The ionic conventional surfactant (SDS) introduced the electrostatic attraction effect, tightened the molecule arrangement, and increased hydrophobic interactions in the micelle, and thus slowed down the exchange rate of 14-s-14. Additionally, the two-step exchange mechanism of 14-s-14 in the mixed solution was revealed through interesting variation tendencies of exchange rates of 14-s-14.