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

Ultrasonic relaxation (0.6 < f < 210 Hz) and conductance studies of mixed micelles containing decyltrimethylammonium bromide (DTAB), water, and 1-butanol or 1-pentanol are reported at 25-degrees-C. Measurements have been carried out as a function of surfactant and alcohol concentrations. In all systems two relaxation frequencies are found. The lower and higher relaxation frequencies have been assigned respectively to the exchange of monomer surfactant (f(r1)) and of alcohol (f(r2)) between mixed micelles and the bulk phase. Conductance measurements were made to obtain the critical micellar concentration (cmc), the degree of micelle ionization (beta), and the distribution coefficients (K) of the alcohol between the micellar and bulk phase. Time-resolved fluorescence measurements were used to obtain estimates of the mean aggregation number of the surfactant (n) in the micelles. By using these results together with the binding constant of the alcohol to the micelles and the amount of free alcohol in the bulk phase, estimates of the mean aggregation number of the alcohol (a) have also been obtained. The ultrasonic results of the present work together with the results previously published by us for 1-propanol-DTAB-water were analyzed according to two models: the theory of Aniansson and the phenomenological model recently proposed by Hall and Wyn-Jones. Both models have been used to obtain information about the forward and backward rate constants (k1+, k1-, k2-, and k2+), polydispersity of the mixed micelles (sigma(s)2 and sigma(A)2), and the change of volumes of the two exchange processes (DELTA-V1 and DELTA-V2). The results obtained with both models have been interpreted and compared to each other. The influence of the alcohol chain length on the magnitude of the kinetic and thermodynamic parameters is discussed as well.