ИСТИНА

Polyelectrolytes are widely used to modify surface properties and to stabilize or destabilize colloidal dispersions. In such applications, we have investigated the effect of low-molecular-mass salt (NaCl) on properties of non-stoichiometric polyelectrolyte complexes (nPEC) bearing an excess of cationic charges. Tested nPEC were formed by polydiallyldimethylammonium chloride (PDADMAC) and copolymers of maleic acid with propylene (P(MS-P)) or methylstyrene (P(MS-α-MeSty)). It is known that low-molecular-mass salts exhibit a strong influence on properties of nPEC. The addition of salt to nPEC solutions can be accompanied by a marked conformational changes of complex particles and can lead to phase separation in such systems. At sufficiently high salt concentrations, dissociation of nPEC to their polyelectrolyte components is observed. The aim of our study has been to find out which properties of the nPEC dispersions that can be adjusted by a variation in the salt content are most suitable for stabilization or for flocculation of silica particles. For this purpose, two different principles of surface modification by nPEC have been used in our experiments: (i) previously prepared nPEC dispersions were contacted with dispersions of silica, (ii) polycation was preadsorbed on silica surface and then the same amount of polyanion was added. For the characterization of nPEC dispersions as well as modified silica particles, we used polyelectrolyte titration by a particle charge detector, quasi-elastic light scattering, microelectrophoresis, and turbidimetry. A dependence of turbidity of a dispersion (molar ratio of charges n(-)/n(+) = 0.3) of nPEC formed by P(MS-P) and PDADMAC on the salt concentration was obtained. At low salt concentrations, the formation of soluble and relatively small nPEC particles is favored but further increase in the salt concentration results in a pronounced aggregation of them due to decreasing repulsion forces (I); at moderate salt concentrations, a formation of a precipitate, i.e., phase separation, is observed (II), and at high salt concentrations, a dissolution of the precipitate takes place because of a progressive dissociation of interpolymer salt bonds (III). The effect of the different molecular structure of the used polyanions on the stability of nPEC is demonstrated by a marked shift of a 'critical' NaCl concentration at which the dissolution of the precipitate occurs (II -> III). We found that this 'critical' salt concentration shifts from 0.5 mol/l for nPEC containing P(MS-P) to 0.8 mol/l for nPEC containing P(MS-α-MeSty). From our results on surface modification of silica with preformed nPEC dispersions, we also obtained a significant influence of nature of the used polyanion. It is interesting to note that both types of nPEC dispersions show a very pronounced effect of salt on the efficiency of surface modification of silica. At low salt concentrations, silica dispersions can be stabilized and near a salt concentration at which the phase separation is detected the maximum of flocculation is reached. From an analysis of modified silica particles carried out by charge compensating titration and zeta-potential measurements, it seems that the efficiency of cationic charge modification by nPEC dispersions is controlled predominantly by the properties of the formed nPEC particles that can be influenced by the low-molecular-mass salt.