Molecular Exchange in Colloidal Dispersions

University dissertation from Alex Evilevitch, Beväringsgatan 1, 212 33 Malmö, Sweden

Abstract: This thesis is a study of molecular exchange between the aggregates in a colloidal dispersion. The problem of oil molecular transport at resolubilization of big oil drops by smaller microemulsion droplets is considered as an experimental model system. The resolubilization kinetics was measured through a temperature jump into a droplet microemulsion phase from a two-phase region of microemulsion droplets and separating oil. The relaxation process was monitored by following the turbidity of the system. A quantitative model for the solubilization kinetics was formulated on the basis of experimental observations. The effects of concentrations and sizes of droplets are treated within a framework of a cell model. New computer simulation approach for molecular exchange studies is also introduced. The simulation model describes discreet spherical aggregates moving in a Brownian motion at the same time as small molecules, such as oil or surfactant, are allowed to exchange between the aggregates. The model allows a detailed study of the local exchange between neighbor micelles as well as the collective long-range exchange. The theoretical analysis from both models showed that as a result of high droplet concentration, the molecular exchange between the droplets occurs via diffusive monomer transport only in the vicinity of the droplets’ surface, and not across the entire system as predicted by the infinite dilution limit approximation. A quantitative agreement between the simulation, the cell model, and experiment was obtained in description of the resolubilization process. In addition, structure and transport properties of weakly charged oil-in-water microemulsion droplets, were also studied. Static and dynamic properties such as osmotic pressure, osmotic compressibility, self-diffusion, collective diffusion, and zero shear viscosity were analyzed by experimental and theoretical techniques.

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