The Sphere-Rod Transition in SDS Micelles
We have employed laser light scattering methods to investigate the size, shape, and polydispersity of alkyl sulfate micelles in aqueous NaCl solutions. Our studies have demonstrated the existence of a continuous transition from roughly spherical aggregates (R ~ 25 Å) to polydisperse rod-like structures whose mean size increases with increasing NaCl, decreasing temperature and increasing detergent concentration. We present here a brief description of a thermodynamic model for this sphere-to-rod transition. In this model, detergent molecules are assumed to form a distribution of micelles with aggregation numbers n = no, no +1, ..., no +k, where no corresponds to the minimum spherical micelle and no +k corresponds to a prolate sphero-cylinder with K molecules in the cylindrical portion between hemispherical endcaps each having no /2 molecules. The theory describes the micellar size distribution using two parameters: (X-XB), the amount of detergent in micellar form, and K, which reflects the difference in chemical potential between a molecule in the spherical and cylindrical portions of the micelle. We first present experimental data which tests the ability of our theory to predict successfully the dependence of the mean hydrodynamic radius Rn, on detergent concentration and the shape of the micellar size distribution. New data on the influence of chain length, counterion size and the addition of urea are discussed.
KeywordsThermodynamic Model Hydrodynamic Radius Aggregation Number Spherical Micelle Spherical Aggregate
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