Salt-induced diffusiophoresis of a nonionic micelle: Roles of salting out and proximity to surfactant cloud point

Abstract

• Salt-induced diffusiophoresis is the migration of a colloidal particle in water caused by a salt concentration gradient. • Tyloxapol micelle was identified as model particle with surface properties governed by polyethylene glycol (PEG) chains. • Na 2 SO 4 was chosen because it is a common salt and a strong salting-out agent according to Hofmeister series. • Gradient of Na 2 SO 4 drives micelle diffusiophoresis from high to low salt concentration due to PEG preferential hydration. • Near surfactant cloud point, diffusiophoresis becomes dominant mechanism responsible for micelle transport. Salt-induced diffusiophoresis is the migration of colloidal particles in water caused by salt concentration gradients. A widespread example of colloidal particles is represented by those with interfacial properties governed by polyethylene glycol (PEG) functionalities. Since salting-out salts strongly affect PEG-water interactions, their gradients are expected to generate diffusiophoresis of PEG-based particles. In this work, diffusiophoresis of nonionic micelles of tyloxapol, a polyoxyethylene surfactant, was characterized in the presence of Na 2 SO 4 . Specifically, multicomponent-diffusion coefficients were measured at 25 °C for the ternary tyloxapol-Na 2 SO 4 -water system using Rayleigh interferometry. These transport data, together with measurements of cloud points and dynamic-light-scattering diffusion coefficients, were used to determine the effect of salt concentration on micelle mobility, salt-induced micelle diffusiophoresis and salt osmotic diffusion induced by micelle gradients. It was found that gradients of Na 2 SO 4 induce micelle diffusiophoresis from high to low salt concentration. Moreover, diffusiophoresis becomes the dominant mechanism responsible for micelle transport near surfactant cloud point due to increase in both micelle size and osmotic compressibility with salt concentration. A hydration model was employed to theoretically describe salt salting-out effect on micelle diffusiophoresis and salt osmotic diffusion. This work is the first report showing salt-induced diffusiophoresis of a neutral PEG-based colloidal particle and characterizing its magnitude near miscibility gaps.