Monomer-Aggregate Exchange Rates in Dialkyl Chain Cationic-Nonionic Surfactant Mixtures.

The monomer-aggregate exchange rate in self-assembled dialkyl chain cationic-nonionic mixed surfactant aggregates has been studied using small-angle neutron scattering, SANS, and a stopped-flow apparatus. SANS was used to follow the evolution of the structure with time of an equimolar mixture of the dialkyl chain cationic surfactant dihexadecyl dimethyl ammonium bromide, DHDAB, in D(2)O with the nonionic surfactant dodecaethylene monododecyl ether, C(12)E(12), in D(2)O at a solution concentration of 1.5 mM. With increasing time, the bilamellar vesicle structure, blv, of DHDAB and the globular micellar structure, L(1), of C(12)E(12) evolved to a lamellar (L(beta) or L(alpha))/micellar (L(1)) coexistence. Measurements were made for the isotopically labeled combinations of hydrogeneous DHDAB (h-DHDAB) and alkyl chain deuterium-labeled C(12)E(12) (d-C(12)E(12)) in D(2)O such that the lamellar contribution is the predominantly visible contribution to the scattering. From the variation (decrease) in the scattering intensity with time (measured at a scattering vector of approximately 0.014 A(-1)), a characteristic time was measured at 32 degrees C (T < L(beta)/L(alpha) transition temperature) and at 46 degrees C (T > L(beta)/L(alpha)). The characteristic time was approximately 130 min and a few seconds respectively, indicating a dramatic change in the monomer/aggregate exchange rate between the solid-like L(beta) and fluid-like L(alpha) phases. The characteristic time of approximately 130 min in the L(beta) phase is indicative of a slow monomer-aggregate exchange rate and is consistent with the slow kinetics of adsorption of DHDAB and DHDAB/nonionic surfactant mixtures observed at the air-water interface. This slow adsorption kinetics was assumed to arise from near-surface depletion effects associated with slow monomer/aggregate exchange rates, and these results support and reinforce that hypothesis