Effect of a low-molecular-weight salt on colloidal dispersions of interpolyelectrolyte complexes

Colloidal dispersions of an interpolyelectrolyte complex were prepared by mixing dilute aqueous solutions of poly(dimethyldiallylammonium chloride) and the sodium salt of the alternating copolymer of maleic acid propene in amounts providing about a threefold excess of the charged groups of the cationic polyelectrolyte over those of the anionic polyelectrolyte. These dispersions were examined by means of analytical sedimentation, quasielastic light scattering, and laser Doppler microelectrophoresis. The experimental results obtained suggest that the particles of the interpolyelectrolyte complex are multicomplex aggregates bearing cationic charge. Such aggregates were assumed to consist of a hydrophobic core formed by coupled oppositely charged macromolecules and a hydrophilic shell formed by cationic macromolecules. Hydrodynamic and electrophoretic properties of these aggregates were found to be rather sensitive to variations in the ionic strength of the surrounding medium: with rising salt concentration, their sedimentation coefficient and hydrodynamic size increase, these increases becoming more strongly pronounced at higher salt concentrations, whereas their electrophoretic mobility gradually decreases. The salt effects revealed suggest that the aggregation level of the particles of the interpolyelectrolyte complex rises in response to an increase in the ionic strength of the surrounding medium. This phenomenon was associated with the salt-induced decrease of the stabilizing effect of the hydrophilic shells that protect such particles from progressive aggregation.