Double Flocculation in Aqueous Dispersion of Layered Double Hydroxide Particles Induced by Sodium Polyacylate

The effect of sodium polyacrylate(PAAS) on the stability of aqueous dispersion of Mg/Al layered double hydroxide(MgAl-LDH) nanoparticles was studied. The adsorption of PAAS on LDH particles was determined by total organic carbon(TOC) analysis and the electrical properties of LDH particles were characterized by zeta potential measurement. The experimental results show that the LDH-PAAS mixtures appeared to be flocculation-dispersion-reflocculation as the PAAS concentration increased from 0.006 to 2.400 mmol/L in a 1.0% (mass fraction) LDH aqueous dispersion. At first, the zeta potential of LDH particles changed from positive to zero and then to negative with the increase of PAAS concentration, which was due to the adsorption of PAAS on LDH surfaces. As a result, the electrostatic repulsion between LDH particles firstly decreased and then increased. Therefore, the PAAS LDH mixture first flocculated and then dispersed. When the PAAS concentration further increased, the LDH particles remained negatively charged, and the depletion attraction between LDH particles should be the major reason causing the reflocculation of the particles. The examination of FTIR spectra of LDH, PAAS, and LDH after PAAS adsorption suggests that the adsorption of PAAS on LDH particles was mainly through the interaction between the -COO- in PAAS molecules and the LDH particles.