Depletion interactions in model microemulsions.

The effects of temperature changes and polymer addition on the behavior of droplet microemulsions of nonionic surfactant, water, and decane are reported and analyzed within polymer depletion theory. Dilution viscometry and dynamic light scattering were used to confirm that these microemulsions behave essentially as hard-sphere dispersions, providing us with an ideal reference system. Addition of poly(ethylene glycol) (PEG) lowers the emulsification failure boundary, where excess oil is expelled, which can be qualitatively understood by an analysis of the available volume for the polymer. Sufficient addition of PEG causes a fluid-fluid phase separation in qualitative accord with experiments on mixtures of rigid colloidal hard spheres and nonadsorbing polymer. Addition of PEG or raising the temperature causes the collective diffusion coefficient D(C) to decrease. From theory, the initial linear slope of D(C) versus droplet concentration can be used to discriminate between attractions and repulsions. The measured D(C) data for the droplets in the presence of PEG are modeled using the Asakura-Oosawa theory of depletion. Fitting the theory to the measured D(C) data permits for extracting the only unknown parameter, the polymer radius of gyration. Quantitative agreement is found with literature data, demonstrating that polymer depletion occurs in the system and that the Asakura-Oosawa theory provides a faithful description of the phenomenon.