Temperature-dependent colloidal stability of hydrophobic nanoparticles caused by surfactant adsorption/desorption and depletion flocculation.

Nanoparticles coated with self-assembled dodecyltrimethylammonium bromide shells are shown to undergo colloidal destabilization at higher temperatures. This is caused by two different mechanisms depending on the surfactant concentration. Up to a surfactant concentration of 55 mM, the surfactant micelles dissolve before the breakdown of the dispersion. In this case, the breakdown is triggered by desorption of surfactant molecules from the particle surface causing flocculation via hydrophobic interactions. Since the surfactant concentration influences the adsorption-desorption equilibrium, the breakdown temperature increases with increasing surfactant concentration from approximately 100 to 160 degrees C. Beyond 55 mM, surfactant micelles are still present when the dispersion breaks down and destabilization is caused by high temperature depletion flocculation. Since higher surfactant concentrations result in a larger number of micelles in solution, the breakdown temperature for concentrations above 55 mM decreases with increasing surfactant concentration.