Effects of hydrodynamic mixing intensity coupled with ionic strength on the initial stage dynamics of bridging flocculation of polystyrene latex particles with polyelectrolyte

Effects of hydrodynamic mixing intensity on the initial stage dynamics of bridging flocculation induced by adsorbing polyelectrolyte were analyzed as an extension of previous report on the effect of ionic strength (J. Coll. Int. Sci. 204 (1998) 328). Mixing condition were changed by adopting forked flask of varying depth, mounted on an end-over-end rotation apparatus. At low ionic strength, no significant influence due to the difference of mixing intensity was observed. Flocculation proceeded in a way consistent with the picture of kinetically-controlled polymer adsorption and ultimate degree of flocculation increased with increasing mixing intensity. At high ionic strength, a qualitatively different behavior of the progress of flocculation was found. At higher mixing intensity, flocculation started slowly, implying a flat conformation of the adsorbing polyelectrolyte in the beginning of adsorption. This result was interpreted as an immediate spreading of attached polyelectrolyte chains on the bare surface. However, a remarkable enhancement of the rate of flocculation sets in after a while. The point of this onset roughly corresponds to the half coverage of the surface. This is an indication of the formation of a thicker adsorption layer presumably caused by the fact that pre-adsorbed chains hamper the spreading of new comers in their neighborhood. These results mean that the rate of reconfirmation is a function of surface coverage. It was also confirmed that the ultimate degree of flocculation converged to a constant level independent of mixing intensity, implying a complete relaxation of the meta-stable state. These observations are explained in terms of the effect of salt on (i) polyelectrolyte conformation and (ii) strength of the ion pair bond between charged segments and charged groups on the particle surface. (C) 2002 Elsevier Science B.V. All rights reserved.