Effect of Alcohols and Oils on the Kinetics of Micelle Formation-Breakdown in Aqueous Solutions of Ionic Surfactants

Article abstract of DOI:10.1021/j100412a072

The temperature-jump method has been used for an extensive study of the dependence of the relaxation time τ₂ characterizing the process of micelle formation-breakdown on the surfactant concentration C and upon additions of alcohols and oils to moderately concentrated surfactant solutions.The 1/τ₂ vs.C curves have been found to be V-shaped, as in previous studies.For the four surfactants investigated, the increase of 1/τ₂ at high C has been interpreted in terms of rapid coagulation-fragmentation processes postulated by Kahlweit (Kahlweit, M.Pure Appl.Chem. 1981, 53, 2060.J.Colloid Interface Sci. 1982, 90, 92).The changes of τ₂ have been found to be very sensitive to the nature, chain length, and concentration of both the added alcohol and oil.Additions of medium chain length alcohols (up to 1-hexanol) increase dramatically 1/τ₂ by up to a factor of 10⁴.Additions of oils to mixed surfactant + alcohol micellar solutions result in the opposite effect.Longer chain alcohols behave like medium chain length alcohols at low alcohol concentration and like oils at high alcohol concentration, when they become solubilized in the micelle core.Additional measurements by means of time-resolved fluorescence probing and quasi-elastic light scattering have been performed in order to obtain information on the micelle aggregation number and the micelle size and polydispersity of some of the investigated systems.The observed changes of 1/τ₂ upon addition of alcohols and oils have been interpreted in terms of variations of the concentration of the species around the minimum of the size distribution curve and, in turn, in terms of the polydispersity of the systems and its effect on the micelle breakdown-formation, in agreement with the quasi-elastic light-scattering results.The relationship between the present chemical relaxation results on micellar kinetics and those obtained recently by time-resolved fluorescence probing is discussed.