Nuclear magnetic resonance relaxation in micelles. Deuterium relaxation at three field strengths of three positions on the alkyl chain of sodium dodecyl sulphate

Abstract

A ²H n.m.r. relaxation study of sodium dodecyl sulphate (SDS) in the micellar phase is presented. The surfactant is specifically deuterated in the α-, γ- and ω-positions. Relaxation data are obtained at two temperatures and at three field strengths, and show a dependence on the field strength. The data are analysed with the ‘two-step’ model of relaxation, and the fast correlation times as well as the order parameters for the three deuterated positions on the hydrocarbon chain are presented. The order parameters obtained are very similar in magnitude to those found in the SDS/water hexagonal phase. In particular, the order parameter is very low (ca. 0.03) for the ω-methyl deuterons. In addition, a common slow correlation time is obtained from the data. The slow correlation time is discussed in terms of the micellar rotational tumbling process and surfactant lateral diffusion along the micellar surface, and a value for the diffusion coefficient of the latter process is obtained.