Multicomponent interdiffusion in micellar solutions and microemulsions.

摘要

This thesis focuses on multicomponent interdiffusion in micellar solutions and microemulsions. The Taylor dispersion technique is used to measure ternary diffusion coefficients for aqueous micellar solutions of: sodium dodecylsulfate (SDS)/NaCl; SDS/alcohol solubilizates; and sodium cholate (SC)/alcohol solubilizates. Ternary diffusion coefficients are also reported for water/sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/n-heptane and water/2-propanol/n-hexane water-in-oil microemulsions. The results include cross-diffusion coefficients, which provide new insights into the interactions between diffusing surfactants and solubilizates.;The diffusion coefficient of SDS in aqueous salt solutions determined by Taylor dispersion is found to be smaller than the diffusion coefficient of the micelles indicated by the popular quasi-elastic light scattering (QELS) method. To clarify the disagreement between the two methods, an expression is developed for the spectrum of light scattered by concentration fluctuations in a multicomponent solution. The spectrum of light scattered by concentration fluctuations in a multicomponent solution. The spectrum shows that QELS gives the lower eigenvalue of the diffusion coefficient matrix for SDS/NaCl solutions rather than the diffusion coefficient of the SDS micelles.;In aqueous SDS/alcohol solubilizate solutions, the diffusion of the solubilized portion of each alcohol was expected to cotransport large amounts of SDS. In fact, diffusing octanol produces large countercurrent coupled flows of SDS. Nernst-Planck equations are used to develop a model for the diffusion of solubilizates and ionic micelles.;The binary diffusion coefficient of aqueous SC, an important biosurfactant, solutions is determined by the Taylor dispersion technique. The results are used to estimate the critical micelle concentration and the average aggregation number of SC micelles. Ternary diffusion coefficients reported for aqueous SC/n-decanol and SC/n-octanol solutions suggest that diffusion in SC/solubilizate systems is similar to that of SDS/octanol solutions.;In water/AOT/n-heptane water-in-oil microemulsions, the water and AOT components were expected to diffuse together through the heptane-continuous medium as surfactant-coated water droplets. Surprisingly, the Taylor results show that the diffusion coefficient of AOT is up to 3 times larger than the diffusion coefficient of water. Moreover, concentration gradients in AOT produce cocurrent coupled flows of water, but gradients in water produce counterflows of AOT. This unexpected behaviour is attributed to changes in the average droplet size along the diffusion path. A size distribution model is proposed to describe the interdiffusion in microemulsions. The diffusion coefficients predicted for water/AOT/n-heptane are in good agreement with the measured diffusion coefficients.;Ternary diffusion coefficients are also measured for water/2-propanol/n-hexane water-in-oil microemulsions. The results show that diffusing water produces significant counterflows of 2-propanol. A chemical equilibrium model is developed to describe coupled diffusion in water/2-propanol/n-hexane microemulsions.