Separation of TiO2 particles from water and water/methanol mixtures by cationic dextran derivatives

Cationic polysaccharides with N-alkyl-N,N-dimethyl-N-(2-hydroxypropyl)ammonium chloride pendent groups attached to a dextran backbone were used to flocculate titanium dioxide (TiO2) particles suspended in water as well in water/methanol mixtures (90:10 and 50:50 v/v %). The investigations were performed with respect to the polycation dose, the suspension medium composition and the length of alkyl substituent at the quaternary nitrogen (alkyl=ethyl, octyl, dodecyl). The supernatant residual turbidity values were much lower for TiO2 particles dispersed in solvent mixtures than in water. This finding was explained by the solvent mixtures effect on the charges of both the particle surface and polyelectrolyte chains. The alkyl substituent length did not affect in a dramatic way the separation of TiO2 particles. The supernatant zeta potential dependence on the polycation dose when water/methanol mixtures were used as dispersion medium indicated a charge patch mechanism for the flocculation of TiO2 particles, which was supported by particle aggregates size measurements.