Micellar Exchange Kinetics of Quaternary Ammonium Type Gemini Surfactants Monitored by Nuclear Magnetic Resonance

Abstract

The dynamic NMR (DNMR) method was used to detect kinetic parameters of the molecular exchange process between monomers in bulk solution and those in the micelle for Gemini surfactants, 12-s-12 and 14-s-14 (s=2, 3 and 4). The escape rate constant, k−, was derived based on the simplified equations of DNMR theory, and the apparent activation energy of escape, Ea−, was obtained based on the Arrhenius equation through temperature variation experiments. Results show that the orders of magnitude of k− for 14-s-14 and 12-s-12 are respectively 10 and 103 s−1, Ea− of 14-s-14 and 12-s-12 are respectively 54.04–73.64 and 33.42–47.09 kJ/mol. Furthermore, k− increases and Ea− decreases with the spacer length growing. In combination with the micro-polarity measurements, it was revealed that molecules of 14-s-14 and 12-s-12 have to experience conformation changes when escaping from the micelles. The two-step molecular exchange mechanism for Gemini surfactants was therefore supported.