Cross-diffusion in a colloid–polymer aqueous system

The thermodynamic behavior of colloid–polymer systems has been extensively investigated. However, relatively few studies have been reported on the corresponding transport properties. We have measured the four multicomponent diffusion coefficients for a ternary colloid–polymer-water system by Rayleigh interferometry at 25 °C. The colloidal particles and polymer coils in our system are the tyloxapol micelles and polyethylene glycol (molecular weight of 2.0 kg mol−1) respectively. Both solutes are neutral and preferentially interact with water. Our investigation focused on the behavior of the two cross-diffusion coefficients in dilute solutions. These coefficients characterize how the concentration gradient of colloidal particles (polymer coils) affects the diffusion rate of polymer coils (colloidal particles). Our data were used to extract two fundamental parameters describing the thermodynamic and kinetic factors responsible for cross-diffusion. The comparison of our experimental results with a proposed reference model based on the hydrodynamic volume of particles indicates that excluded-volume thermodynamic interactions and solute hydration represent the main contributions to cross-diffusion coefficients. The role of the aggregation number of micelles and the effect of polydispersity on cross-diffusion parameters were also examined.

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