High resolution neutron scattering on ionic surfactant micelles : sds in water

Abstract

An improvement over previous determinations of the structure of sodium dodecyl sulfate micelles has been obtained from small angle neutron scattering. The experiment measures the average distribution of distances between all nuclei within a micelle, and also those between deuterium labels attached at positions ω or γ on the SDS chains. At low resolution (15 Å), only the average structure of the micelle is observed : this is a dense sphere containing N = 74 SDS molecules; the hydrocarbon core has a radius of 18.4 Å and contains less than one water molecule per SDS molecule. At high resolution (5 Å), fluctuations away from the average structure are observed. If the internal structure of the core is not resolved, one sees mainly the dispersion in the aggregation numbers (σ N/N = 0.33) or radii (σR/R = 0.1) of the micelle. When the distributions of distances between deuterium labels are observed, shape fluctuations show up through distortions of the internal structure : indeed the chain ends (ω) are not concentrated near a « centre » of the micelle, and the γ methylene groups do not remain in a spherical shell