Adsorption of Polyvinylpyrrolidone onto Polystyrene Latices and the Effect on Colloid Stability

Abstract

Isotherms have been determined for the adsorption of polyvinylpyrrolidone (PVP) samples of various molecular mass (10 4 to 2.5 × 10 6 ) onto anionic polystyrene (PS) latices from both water and 0.5 M NaCl. The adsorption capacity, Γ ads, was found to be about 0.7 mg m -2 from 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 molecular mass material. In the presence of 0.5 M NaCl, however, the amount 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, δ, were also very different for adsorption from the two solvents. In water, thicknesses of 1−3 nm were obtained, indicating that the molecules were lying flat on the surface in the form of trains. In 0.5 M NaCl, the values of δ increased with increasing molecular mass and were between 4 and 29 nm, indicating a more extended configuration with loops and tails protruding away from the surface into solution. In order to explain the different behavior in the two solvents, it was concluded that, in water, interaction with the PS occurred through the PVP hydrophobic methylene/methine groups and the positive dipole of the amide nitrogen of the pyrrolidone ring. The negative dipole associated with the amide oxygen is directed away from the surface into the solution. In 0.5 M NaCl, interaction occurred predominantly through the hydrophobic groups, since the polar interactions would be screened by the electrolyte present. The effect of PVP on the stability of PS was monitored by turbidity measurements. In the absence of electrolyte, stability was achieved due to electrostatic repulsions between the PS particles whereas in 0.5 M NaCl stability was achieved through steric repulsions for the higher molecular mass PVP samples (i.e. >4 × 10 4 ).