(591g) The Effect of Solute-Solute Interactions On Hindered Diffusion of SDS Micelles in Agarose Gels

The hindered diffusion of ionic micelles in gels is not well understood, and yet plays an important role in numerous applications including foods, drug delivery, and environmental technologies. We have used holographic interferometry to measure the diffusivity of sodium dodecyl sulfate (SDS) in solution and in agarose gels at various SDS, agarose, and sodium chloride concentrations. We have also measured aggregation numbers of the micelles, both in solution and gels, by using time-dependent fluorescent quenching. For SDS micelles, we find the rate at which the micelle diffusivity increases with micelle volume fraction to be dependent both on gel concentration and ionic strength. The fluorescence measurements show a slight increase in the aggregation number of the SDS micelles when in agarose gels, but that increase does not explain the observed changes in rates of diffusion. Those changes can be very significant: at micellar volume fractions of ~3%, solute-solute interactions can more than double the diffusion coefficient. At moderate to high ionic strength, the diffusion data are in quantitative agreement with predictions of a theory that accounts for volume exclusion by the gel fibers, and electrostatic and hydrodynamic interactions between the micelles. At the lowest ionic strengths studied, for which salt concentrations were less than 0.03M, the measured rates of diffusion were significantly higher than the theory predicts. This difference could be a result of the assumption of pairwise-additive solute-solute interactions in the theory.