Micelle Formation of Ionic Surfactants. Tracer Self-Diffusion Studies and Theoretical Calcualtions for Sodium p-Octylbenzenesulfonate

Article abstract of DOI:10.1021/j100206a045

An investigation of association phenomena in aqueous solutions of a surfactant, sodium p-octylbenzenesulfonate (SOBS), was performed by self-diffusion measurements.Self-diffusion coefficients were obtained for amphiphile ions, sodium counterions, solubilized decanol molecules, water molecules, and chloride co-ions as a function of surfactant concentration by using the open-ended capillary tube method employing radioactive labeling.Decanol self-diffusion provides information on micellar translation and size.Surfactant ion self-diffusion gives the concentration of free surfactant ions.It is found that, at higher concentrations, free-surfactant concentration falls well below the critical micelle concentration.Counterion self-diffusion provides information on the concentration of free counterions and on β, the ratio of counterions and surfactant ions in the micelles. β is invariant over wide concentration ranges corresponding to an ion condensation type behavior; however, at low concentrations β decreases with increasing micelle concentration.The co-ion self-diffusion coefficients are used to deduce an effective excluded volume; teh excluded volume per micelle decreases with increaseng surfactant concentration.The water self-diffusion coefficients give information on micelle hydration; although hydration numbers are difficult to obtain (and to define), it may be shown that they are small and that there is no marked water penetration deep into the micelles.The various types of information obtained give an overall view of micellar solutions which is compared with current studies by other approaches, experimental and theoretical.Furthermore, various methodological problems and advantages of self-diffusion investigations of micelle formation are considered.Using a recently developed theory, which treats the electrostatic effects according to the Poisson-Boltsmann equation, we calculated the concentrations of free and micellized amphiphile ions and counterions.All of the features of the experimental observations were displayed also by the theoretical results, and for the amphiphile concentrations good quantitative agreement was found.For β, quantitative differences between experiment and theory were found which can be referred to the somewhat ambiguous division into free and micellar counterions.