Salt effect on formation and properties of interpolyelectrolyte complexes and their interactions with silica particles

The influence of low-molecular-mass salt, namely NaCl, on the properties of intel-poly electrolyte complexes (PECs) formed as a result of interactions between poly(diallyldimethylammonium chloride)(PDADMAC) and copolymers of maleic acid with propene [P(MS-P)] or methylstyrene [P(MS-alpha-MeSty)] in their salt-containing nonstoichiometric mixtures has been studied by means of turbidimetry, quasi-elastic light scattering, and laser-Doppler microelectrophoresis. It has been found that stable dispersions of positively charged PEC particles could be prepared at low salt concentration. The hydrodynamic radius of these PEC particles formed at a salt concentration <0.05 mol/L was shown to be nearly constant and to be in the range of a few hundred nanometers. A pronounced salt-induced increase in their size was observed at higher concentrations of NaCl. The complex dispersions formed with P(MS-alpha-MeSty) are more stable to salt than dispersions formed with P(MS-P). The flocculation behavior of silica was investigated by use of such cationic PEC microparticles as well as by PDADMAC, a water-soluble linear polycation. It can be seen that in contrast to the results obtained with PDAMAC alone, the PEC microparticles are able to flocculate silica particles with a high velocity of sedimentation and in a very broad range of polymer concentration. A restabilization could not be observed at high polymer concentration. On the basis of the experimental results, different flocculation mechanisms were proposed.