ULTRASONIC-ABSORPTION STUDIES OF SURFACTANT EXCHANGE BETWEEN MICELLES AND BULK PHASE IN AQUEOUS MICELLAR SOLUTIONS OF NONIONIC SURFACTANTS WITH A SHORT ALKYL CHAIN .2. C6E3, C6E5, C8E4, AND C8E8

Spectrofluorimetry (with pyrene as a probe), density and ultrasonic absorption measurements, and static and time-resolved fluorescence quenching have been used to obtain the critical micelle concentration (cmc), the isothermal volume change upon micellization and the micelle aggregation number N of the ethoxylated nonionic surfactants C6E3, C6E5, C8E4, and C8E8 (tri- and penta(ethylene glycol) monohexyl ethers and tetra- and octa(ethylene glycol) monooctyl ethers, respectively). The kinetics of the exchange of these surfactants between micelles and bulk phase has been investigated by the ultrasonic relaxation method. The results obey the predictions of Aniansson and Wall theory for micellization kinetics (J. Phys. Chem. 1975, 79, 857). The full analysis of the relaxation data on the basis of this theory showed that the association of the surfactant to micelles is nearly diffusion-controlled, as for ionic surfactants. The rate constant k- for the dissociation of the C(i)E(j) surfactants from micelles decreases by a factor of about 10 for i increasing from 6 to 8 and can be taken as 5 X 10(9) cmc, to within a factor of 2. The results also show that the effect of temperature on the rate constants is not large and that the micelle polydispersity is low. The volume changes determined from density measurements and from the ultrasonic relaxation amplitudes are in good agreement Finally in the case of C6E3 solutions where the measurements extend up to only 5-degrees-C below the critical temperature, the results showed no contribution of the critical fluctuations of micelle concentration to the ultrasonic absorption.