Thermoresponsive colloidal stability of core-shell nanoparticles with self-assembled ionic surfactant shells

The topic of this thesis is the thermoresponsive dispersion stability of nanoparticles coated with self-assembled ionic surfactant shells. The objective is to provide experimental evidence for a loss of colloidal stability at high temperatures and to explore the characteristics of and the reason for the dispersion breakdown. The influence of the properties of the system, e.g. concentrations or the structure of the surfactant, should be investigated in order to control the temperature of dispersion breakdown. The dispersion stability of hydrophilic boehmite-nanoparticles in aqueous sodium bis(2-ethylhexyl) sulfosuccinate (AOT) solutions was studied at room temperature. On increasing surfactant concentration and decreasing particle concentration, the system changes from stable via moderately stable to unstable and back. Fully redispersed particles are present being stabilised, in first approximation, by a self-assembled surfactant bilayer on the surface. The position of two transitions, namely for complete precipitation and for beginning redispersion, can be fitted accurately by using a simple model based on an H-type adsorption and including the specific surface area of the particle and molar area of the surfactant. In order to assess influence of the surfactant chain length on the dispersion stability, dispersion stability diagrams of hydrophobic boehmite nanoparticles in aqueous n-alkyltrimethylammonium bromide solutions (CnTAB, alkyl chain lengths 10-16) were investigated in detail. On increasing surfactant concentration and decreasing particle concentration, the dispersions change from unstable to fully stable. At very high surfactant concentration, an additional instability region caused by depletion flocculation is found. At low particle concentrations, the transition from the intermediate to the stable region, i.e. the disappearance of the precipitate, occurs at a constant surfactant concentration. This concentration is introduced as the “critical dispersion concentration” (cdc). The logarithm of the cdc shows a linear dependence on the surfactant chain length, thus a cmc-analogous behaviour. Turbidity measurements in a pressurised vessel give clear evidence that dispersions of nanoparticles with self-assembled ionic surfactant shells (both hydrophilic and hydrophobic particles) breakdown at elevated temperatures of 100-200 °C. For hydrophobic boehmite particles in CnTAB solution at constant particle concentration, the temperature of dispersion breakdown increases strongly up to a maximum with increasing surfactant concentration. The breakdown is triggered by the desorption of the surfactant molecules from the particle surface and subsequent flocculation of the particles by hydrophobic interactions. The adsorption-desorption equilibrium, which is influenced by the surfactant concentration, is the origin of the increase of the dispersion breakdown temperature. For C10TAB and C12TAB, the dispersion breakdown temperature drops again with increasing surfactant concentration after the maximum, because the mechanism of the dispersion breakdown changes. Since from here on the surfactant micelles are still present when the dispersion breaks down, destabilisation is caused by depletion flocculation. For C14TAB and C16TAB, the dispersion breakdown temperature deceases only slightly with increasing surfactant concentration after the maximum. Since additionally micelles are dissolved before the dispersions break down, nanoparticles coated with C14TAB and C16TAB generally are only subject to surfactant desorption. Long surfactant chains are found to give higher dispersion breakdown temperatures than short ones. For a quantitative comparison with respect to the much different surfactant concentrations of C10TAB and C16TAB, the concentration of not-adsorbed, thus free surfactant molecules in solution, cfree, is normalised to the corresponding cmc. At a normalised concentration of 1, the temperatures of dispersion breakdown show a linear dependence on the surfactant chain-length. The dispersion breakdown temperature depends strongly on the particle concentration gammaboehmite. The decrease of the breakdown temperature with increasing gammaboehmite is in good qualitative agreement with the dependence on surfactant concentration, since cfree decreases, when the total surfactant concentration is kept constant. However, for a quantitative comparison at constant cfree, higher particle concentrations gives lower dispersion breakdown temperatures than low gammaboehmite, which is most likely affect by a different number of the counterions dissociated from the ionic-surfactant shell on the particles. A redispersion of high-temperature flocculated dispersions upon cooling is possible, even though heavily dependent on the surfactant concentration. There is a minimum concentration of around the cmc required for redispersion, and depletion flocculated samples at high concentration cannot be redispersed