ULTRASONIC RELAXATION STUDIES OF MIXED MICELLES FORMED FROM PROPANOL DECYLTRIMETHYLAMMONIUM BROMIDE WATER

Ultrasonic absorption (0.6 < f < 210 MHz) and conductance studies of decyltrimethylammonium bromide (DTAB), water, and propanol (Pr) mixtures have been carried out as a function of surfactant and alcohol concentrations at 25-degrees-C. Two relaxation frequencies (rates) are found for all systems and are assigned to exchange processes involving monomer surfactant (f(rl)) and alcohol (f(r2)) species between mixed micellar aggregates and the bulk phase. The lower frequency relaxation is believed to be due to the exchange of monomer surfactant, and the higher frequency relaxation, due to the exchange of the alcohol. The ultrasonic results were analyzed according to the theory of Aniansson to obtain information about the polydispersity of the mixed micelles (sigma(1)2 and sigma(2)2), the rate constants (k1+, k2+, k1-, and k2-), and the change of volumes of the exchange processes (DELTA-V1 and DELTA-V2). Time resolved fluorescence measurements were made to obtain estimates of the mean aggregation number of the surfactant in the micelles, directly, and of the alcohol, indirectly, from the binding constant of the alcohol to the micelles and the free alcohol in the bulk phase. The exit rates of the DTAB monomer (k1-) and alcohol molecule (k2-) from a mixed micelle decrease with increasing stoichiometric alcohol concentration. Both the variance in the size distribution (sigma(1)2) and the mean aggregation number of the surfactant (nBAR) decrease as the Pr concentration is increased. These results can be explained by the change in the packing of the micelle with increased partitioning of Pr into the mixed micelle which results in a decrease in the charge density at the micelle surface. Analysis of the relaxation amplitude data gives rather large volumes for the surfactant exchange which appear to be consistent with a two-step process: its transfer from the mixed micelle to the bulk phase followed by its transfer to a propanol-water microphase.