In this thesis we report on a system of colloidal plates and spheres developed to model the key particles found in paint and other industrial formulations. From theories of mixtures of particles of different shapes and sizes one would expect entropic phase sep- aration to occur at sufficiently high volume fractions, due to the gains in free volume. An aqueous system consisting of synthetic nano-gibbsite Al(OHh plates and alumina coated silica nano-spheres were sterically stabilized with a commercial stabiliser in wa- ter to create near-hard particle interactions. The stabilizer was chosen, by using in the first instance an NMR spin-spin relaxation technique. This gave a pseudo-adsorption isotherm. This technique was complemented with electrophoresis and elemental anal- ysis. The addition of the stabilizer to the suspension of plates modified the kinetics of the isotropic-nematic phase formation, with larger domains forming than in a surfac- tant free system. Colloidal plate-sphere mixtures were investigated using small-angle neutron scattering (SANS), optical microscopy (polarising and differential interference contrast). In SANS,' for dilute suspensions using contrast matching, the addition of colloidal plates to colloidal spheres showed an increase in density fluctuations of the spheres, which agreed well with theoretical predictions. The birefringence of the sys- tems was also observed in order to study the isotropic-nematic (I-N) transition. On adding spheres, the I-N transition occurs at a lower platelet concentration. The impact of the plate __ ~(Ls<plre"'te size ratio was also studied and found to have a noticeable effect , on the phase behaviour of the mixtures. SANS results showed a larger increase in the density fluctuations of the spheres at higher size ratios, and these fluctuations are a precursor to phase separation. Increases in the size ratio led to phase separations of the mixtures occurring at lower colloid volume fractions. This is due to an increase in the effective attraction between both particle types when mixed together. The main conclusion of the thesis is that mixtures of colloidal spheres and plates tend to phase separate, even at low concentrations of the particles. The phase separation is driven by . entropy. This is in contrast with observations in the coating industry where such mix- tures supposedly remain intimately mixed. Presumably these systems are at sufficiently high concentrations to be dynamically arrested.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:559381 |
| Date | January 2011 |
| Creators | Doshi, Nisha |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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