Growth of mixed nonionic micelles

Static and quasielastic light-scattering measurements were utilized to investigate the shape, size, and polydispersity of mixed micelles composed of the nonionic surfactants dodecyl hexaoxyethylene glycol monoether (C(12)E(6)) and dodecyl octaoxyethylene glycol monoether (C(12)E(8)) in aqueous solutions. We determined the molecular weight and diffusion coefficient of these micelles and showed that they are rodlike. The average diffusion coefficient of the mixed micelles was measured at various total surfactant concentrations in the range between approximately 30 and 1000 times the critical micellar concentration of the surfactant mixture. Pure C(12)E(6), pure C(12)E(8), and three different mixtures of C(12)E(6) and C(12)E(8) were studied in the temperature range 10 degrees C less than or equal to T less than or equal to 55 degrees C. We found that the mixed micelles could be satisfactorally described as prolate ellipsoids with an average long axis which grows in size as the total surfactant concentration increases. The extent of this growth is more pronounced when the relative proportion of C(12)E(6) in solution is increased or as the temperature approaches the boundary of phase transition into coexisting micelle-rich and micelle-poor phases. We propose a phenomenological model to quantitatively describe the linear growth of rodlike mixed micelles. Two parameters of this model which control the observed linear growth of the mixed C(12)E(6) and C(12)E(8) micelles were deduced from the experimental data and compared with the values predicted by a recently developed molecular-thermodynamic theory of mixed micellization.