Adsorption of polycations on clays: A comparative in situ study using Cs-133 and Na-23 solution phase NMR

Na-23 solution phase NMR has been evaluated as an in situ probe to study the adsorption of tetramethylammonium (TMA(+)) and two polycations, FL17 ([(Me2NCH2CHOHCH2)(n)]Cl-n+(n)) and Magnafloc 1697, ([(CH2CHCH2N(Me)(2)CH2CHCH2)(n)]Cl-n+(n)), onto clays in aqueous suspensions containing 2.5 mass% low iron Texas bentonite. The NMR data shows the effectiveness of the organocations at displacing Na+ from the bentonite surface. This information has been correlated with that obtained from particle-size and electrophoretic measurements in aqueous solution, together with information from adsorption isotherms. These results have been compared to those obtained in parallel studies using Cs-133 solution phase NMR. FL17 and 1697 both exhibited high affinity adsorption isotherms on Na+- and Cs+-clay, whereas the adsorption of TMA(+), which represents the cationic portion of the polymers was of lower affinity. Na+-bentonite adsorbed almost twice the amount of polycation required to fulfill the cation-exchange capacity (CEC) of the bentonite. The electrophoretic and particle size data indicated significant differences in the size of the polycation/clay flocs and the amount of polymer adsorbed on the external faces of the flocs in the presence of Na+- and Cs+ exchange ions. Correlation of this data with the NMR results suggests that the Na+ -bentonite/polycation flocs are large, of low density, and that the polycation is concentrated in the interior while the Na+-ions occupy exchange sites on the external faces. (C) 1997 Academic Press