PREFERENTIAL BINDING WITH REGARD TO CHAIN-LENGTH AND CHEMICAL-STRUCTURE IN THE REACTIONS OF FORMATION OF QUASI-SOLUBLE POLYELECTROLYTE COMPLEXES

Viscometry, UV spectroscopy, and analytical ultracentrifugation (AUC) were applied to study preferential binding of poly(diallyldimethylammo nium chloride) added to mixtures of sodium poly(styrenesulfonates) (PS S) of quite different molecular weights and PSS/sodium poly(methacryla te) (PMA). In salt-free solutions the decrease of the relative viscosi ty during complex formation was compared with model systems containing amounts of the free PSS components and NaCl, corresponding to special cases of preferential binding. The analysis suggests some preference for binding of the shorter polyanions. These findings were supported b y AUC measurements of the composition of supernatant solutions after s pinning down the polyelectrolyte complexes (PECs). In the presence of low molecular salt, the systems exhibit even binding at first (at ioni c strengths of about 5 x 10(-3) N NaCl) and at higher salt contents a pronounced binding of the high molecular component. Even an exchange l ow molecular weight (LMW) PSS for high molecular weight (HMW) PSS was found in long-term experiments at higher ionic strength. In the case o f polyanions of different natures, only a weak preference of PSS was o bserved by AUC and UV spectroscopy in salt-free media, whereas in the presence of NaCl, PSS molecules were strongly favored. PSS was also de monstrated to substitute PMA in the complexes at I > 7.5 x 10(-3) N Na Cl. The peculiarities of preferential binding in salt-free solutions c an be explained by thermodynamic nonequilibrium conditions.