Self-consistent structural and dynamic study of concentrated micelle solutions

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Abstract
We have performed static and dynamic neutron small-angle scattering experiments on sodium dodecyl sulphate solutions, in the concentration range 0.04–0.8 mol dm–3, with up to 0.2 mol dm–3 of added NaCl. We have analysed the data taking intermicellar correlations specifically into account by calculating an analytic structure factor for the fluid of interacting charged micelles. We have thus been able, for the first time, to derive micellar charge and aggregation numbers from the small-angle scattering patterns of concentrated micelle solutions. Our results largely reconcile the two opposed current views of ‘wet’ or ‘dry’ micelle structure. We effectively retain the basic micelle geometry proposed by Hartley, but take dynamic thermal fluctuations into account by allowing a rough outer layer from which monomers may penetrate into the solvent. We find the micelles are generally less than 25% ionised. Dynamic small-angle neutron spin-echo measurements of the effective micellar diffusion coefficients have also been performed, to provide independent self-consistent verfication of the calculated structure factors. The techniques used are directly applicable to the interpretation of light-scattering and small-angle X-ray studies, as well as neutron data. They form a basis for extending the study of solutions and suspensions of charged spherical particles to the concentrated regime.

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