Diffusion of Probe Particles in Surfactant Gels: Dynamic Light Scattering Study

Abstract

The growth and entanglement of certain aqueous surfactant solutions were studied by the concept of probe particle diffusion in polymeric networks. The cetyl trimethylammonium tosylate(CTAT)−water system forms elongated micelles, in the absence of any added electrolytes, at low and intermediate concentrations. The diffusion coefficients of spherical polystyrene particles embedded in CTAT micelles were measured using dynamic light scattering (DLS). The diffusion coefficient changes drastically near the overlap concentration of the micelles, and anomalous diffusion of the probe particle takes place in the entangled network of the micelles. The measured diffusion coefficient is shown to reflect the growth of CTAT micelles as investigated by other techniques such as fluorescence recovery after fringe pattern photobleaching (FRAP). The diffusion coefficients of the probe particles follow the same qualitative behavior as those of the self-diffusion coefficients of the micelles measured by FRAP. The effect of breakage of micelles with temperature and consequent decrease of the characteristic pore size of the network has been monitored. From the measured diffusion coefficient data, the microviscosity of the probe particles was calculated and is found to follow a rapid increase in the microviscosity above the overlap concentration as is expected for the bulk zero shear viscosity. The effects of hydrophobicity of the alkyl chain on the growth of micelles in cationic−anionic surfactant mixtures were studied in a mixture of monoalkylated disulfonated diphenyl oxide surfactants (CnMADS) and dodecyl trimethylammonium bromide (DDTAB). The increased hydrophobicity of the long alkyl chain anionics seems to have an increased growth rate and hence low overlap concentration in the presence of DDTAB. Possible relevance of this technique in application is pointed out.