Mechanism responsible for flocculation in poly (n-isopropylacrylamide) temperature responsive dewatering-attractive interaction forces induced by surface hydrophobicity

Temperature responsive polymers such as Poly (N-isopropylacrylamide) (PNIPAM) have been shown to have potential to improve water recovery from mineral tailings by using the polymer as a flocculant at temperature above the lower critical solution temperature (LCST 32°C) and then as a consolidation aid at temperature below the LCST. The mechanism for the flocculation (at high temperature) is shown to be hydrophobic and the mechanism for the repulsion (at low temperature) is shown to be steric repulsion. The contact angle between water and air on PNIPAM covered silica glass surfaces increases with increasing temperature and increases with increasing polymer molecular weight. Atomic force microscope (AFM) colloid probe surface force measurements at temperature below the LCST show steric repulsion which increases with polymer molecular weight. At temperature above the LCST, attraction is observed between the PNIPAM covered surfaces which increases as molecular weight of the polymer increases. The polar acid-base interfacial interaction forces have been estimated using contact angle measurements of three different liquids (water, formamide and diiodomethane) on PNIPAM coated silica glass surfaces. The results of these calculations are consistent with hydrophobic attraction at temperature above the LCST and steric repulsion at temperature below the LCST. Since the mechanism for the attraction is due to an increase in particle hydrophobicity it is also possible to use PNIPAM as a flotation collector for fine particles because it can produce hydrophobic floccs.