GROWTH OF MIXED NONIONIC MICELLES

Static and quasielastic light-scattering measurements were utilized to investigate the shape, size, and polydispersity of mixed micelles com posed of the nonionic surfactants dodecyl hexaoxyethylene glycol monoe ther (C(12)E(6)) and dodecyl octaoxyethylene glycol monoether (C(12)E( 8)) in aqueous solutions. We determined the molecular weight and diffu sion coefficient of these micelles and showed that they are rodlike. T he average diffusion coefficient of the mixed micelles was measured at various total surfactant concentrations in the range between approxim ately 30 and 1000 times the critical micellar concentration of the sur factant mixture. Pure C(12)E(6), pure C(12)E(8), and three different m ixtures of C(12)E(6) and C(12)E(8) were studied in the temperature ran ge 10 degrees C less than or equal to T less than or equal to 55 degre es C. We found that the mixed micelles could be satisfactorally descri bed as prolate ellipsoids with an average long axis which grows in siz e 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 o f phase transition into coexisting micelle-rich and micelle-poor phase s. 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-thermodyna mic theory of mixed micellization.