| Abstract | Salt-induced diffusiophoresis is the migration of a colloidal particle caused by salt concentration gradients in water. A common example of colloidal particles is represented by those with interfacial properties governed by polyethylene glycol (PEG) functionalities. This dissertation provides the first report showing salt-induced diffusiophoresis of a neutral PEG-based colloidal particle. The nonionic micelle of tyloxapol, a commercially available polyoxyethylene surfactant oligomer of Triton X-100, was chosen in this investigation. This dissertation also includes the development of a new single-sample method for the determination of surfactant critical micelle concentration (cmc) based on the diffusion-driven dilution of a micellar solution. This method was sufficiently precise to show that the cmc of non-ionic surfactant Triton X-100 decreases as the concentration of salting-out salt Na2SO4 increases. Salt-induced diffusiophoresis of Tyloxapol micelles in water was experimentally characterized for two salt cases, Na2SO4 and MgSO4. Specifically, multicomponent-diffusion coefficients were measured at 25 °C for the ternary tyloxapol-salt-water system using Rayleigh interferometry. Measurements of cloud points allowed us to establish the experimental conditions for our diffusiophoresis studies. Dynamic-light-scattering diffusion coefficients were used to determine the effect of salt concentration on micelle mobility and size, salt-induced micelle diffusiophoresis, and salt osmotic diffusion induced by micelle concentration gradients. In both salt cases, micelle diffusiophoresis was found to occur from high to low salt concentrations (positive diffusiophoresis). Interestingly, diffusiophoresis becomes the dominant mechanism responsible for micelle transport near surfactant cloud point. This is related to an increase in both micelle size and osmotic compressibility with salt concentration. A preferential-hydration model was employed to theoretically describe the effect of salt salting-out on micelle diffusiophoresis and salt osmotic diffusion. Although micelle diffusiophoresis can be attributed to preferential hydration of PEG surface groups, salting-out salts also promote an increase in the size of micellar aggregates. This complicates diffusiophoresis description because it is not clear how surfactant aggregation contributes to micelle diffusiophoresis. We, therefore, developed a two-state aggregation model describing the observed salt concentration effect on the size of tyloxapol micelles and theoretically evaluated the contribution to diffusiophoresis. Our analysis shows that aggregation promotes micelle diffusiophoresis from low to high salt concentration (negative diffusiophoresis). However, we also find that this mechanism marginally contributes to overall diffusiophoresis, indicating that preferential hydration is the main mechanism causing micelle diffusiophoresis. We believe that concentration gradients of salting-out agents such as MgSO4 and Na2SO4 may be employed for achieving migration of PEG-based colloidal particles such as those utilized as drug carriers and extracting agents with applications in the fields of microfluidics, enhanced-oil recovery, soil remediation, and controlled release technologies. |
