On the diffusion and phase transitions of confined colloid-polymer mixtures
Robert, Marc A.
Doctor of Philosophy
Diffusion and phase transitions of confined neutral colloid-polymer mixtures are studied theoretically in one dimension, and theoretically and experimentally in two dimensions. For colloids in a channel, their short-time self- and collective diffusion coefficients and their long-time mobility are calculated, assuming the colloid-polymer interactions to be of depletion origin and described by the Asakura-Oosawa model. The colloid-polymer mixture is mapped onto an effective one-component system in which the size of the colloids, the hydrodynamic interactions, and the wall effects are taken into account. It is found that depletion interactions reduce the diffusion of colloids for short times and enhance their mobility for long times. For a single polymer in a colloidal suspension confined to a channel, the self-diffusion coefficient of the polymer center-of-mass is calculated in the ground-state dominance regime as function of suspension density, degree of confinement, and quality solvent quality. The scaling exponents describing the variations of the self-diffusion coefficient with the degree of polymerization and the radius of the channel are computed. These exponents are found to have higher values than those of a polymer in the absence of colloids. It is also shown that the influence of colloids on polymer diffusion under theta and good solvent conditions is much more pronounced for the latter case. Monolayers of mixtures of poly(lactic acid) (PLA) and two types of particles, magnetic colloids and Cd-Se nanoparticles, are prepared using the Langmuir-Blodgett technique. Pressure-area isotherms show that the transition from the isotropic phase to the liquid-crystalline smectic-A phase, observed for pure PLA, is suppressed at a critical concentration of the magnetic colloids, whereas it persists in the presence of nanoparticles, even at high concentrations. The theory developed by McMillan for the smectic-A phase in three dimensions is extended to the case of two dimensions, and its predictions are compared to those of the latter as well as to experiment.
Physical chemistry; Chemical engineering; Condensed matter physics