ADSORPTION OF POLYVINYLPYRROLIDONE ONTO POLYSTYRENE LATICES AND THE EFFECT ON COLLOID STABILITY

Isotherms have been determined for the adsorption of polyvinylpyrrolid one (PVP) samples of various molecular mass (10(4) to 2.5 x 10(6)) ont o anionic polystyrene (PS) latices from both water and 0.5 M NaCl. The adsorption capacity, Gamma(ads), was found to be about 0.7 mg m(-2) f rom water and was independent of the PVP molecular mass. These results were consistent with gel permeation chromatography experiments which indicated that there was no preferential adsorption of high or low mol ecular mass material. In the presence of 0.5 M NaCl, however, the amou nt adsorbed was between two and four times greater than that in water, depending on the molecular mass, and high molecular mass material was found to adsorb preferentially. The adsorbed layer thicknesses, delta , were also very different for adsorption from the two solvents. In wa ter, thicknesses of 1-3 nm were obtained, indicating that the molecule s were lying flat on the surface in the form of trains. In 0.5 M NaCl, the values of delta increased with increasing molecular mass and were between 4 and 29 nm, indicating a more extended configuration with lo ops and tails protruding away from the surface into solution. In order to explain the different behavior in the two solvents, it was conclud ed that, in water, interaction with the PS occurred through the PVP hy drophobic methylene/methine groups and the positive dipole of the amid e nitrogen of the pyrrolidone ring. The negative dipole associated wit h the amide oxygen is directed away from the surface into the solution . In 0.5 M NaCl, interaction occurred predominantly through the hydrop hobic groups, since the polar interactions would be screened by the el ectrolyte present. The effect of PVP on the stability of PS was monito red by turbidity measurements. In the absence of electrolyte, stabilit y was achieved due to electrostatic repulsions between the PS particle s whereas in 0.5 M NaCl stability was achieved through steric repulsio ns for the higher molecular mass PVP samples (i.e. >4x10(4)).