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Abstract
The formation of micelles and the composition and concentration of sub-micellar components of surfactant solutions was studied for sodium lauryl sulfate (SLS), alkyl polyethoxy (average of 7 EO) sulfate (AEOS 7EO) and mixtures of these two anionic surfactants both above and below the critical micelle concentrations (c.m.c). Radiolabeled surfactant was added to the solutions to follow the concentration and identity of the components. To separate micelles from sub-micellar components, an Amicon microfiltration apparatus containing either a 2, 5, or 30 thousand molecular weight cut off membrane was used. Results using a hydrophobic fluorescent probe demonstrated minimal disruption of the micelles during separation. Separations for single surfactants showed breaks in the concentration curves at the c.m.c. The observed sub-micellar composition and concentration were evidence of non-ideal behavior. Concentration dependence and surfactant selectivity in micellization (the “micro c.m.c.”phenomena) is described. This previously undocumented behavior reveals that the compositions of micelles and the sub-micellar phase for mixed surfactant systems do not reflect the composition of the original solution mixture.