Anomalous surfactant diffusion in a gel of chemically cross-linked ethyl(hydroxyethyl) cellulose

The interactions of sodium dodecyl sulfate (SDS) with chemically cross-linked gels of ethyl(hydroxyethyl) cellulose (EHEC) were studied. Above the so-called critical association concentration (cac), binding of SDS gives rise to an increased swelling of the EHEC gels. The binding of SDS to the gels was measured with flame emission analysis of the sodium ion. The self-diffusion of the surfactant ion (DS) in the gels was studied by the NMR pulsed field gradient spin-echo technique. Both experiments were performed on gels swollen to equilibrium in SDS solutions of varying concentrations. Comparisons with the DS diffusion in a solution of non-crosslinked EHEC were also made. In the EHEC solutions the observed spin-echo decays for DS were always describable in terms of a single surfactant diffusion coefficient (Gaussian diffusion). In contrast, the DS diffusion in the gels above the cac (SDS) was clearly non-Gaussian, or anomalous. The echo decays in the gels could be fitted to a log-normal distribution of diffusion coefficients. When the time during which the diffusion was measured was increased, the width of the distribution increased, while the average diffusion coefficient remained constant. An increase in the width of distribution was also seen when the SDS concentration was increased. The anomalous diffusion is ascribed to inhomogeneities in the gel.