Surface forces arising from adsorbed ionic copolymers with hydrophobic and hydrophilic segments in colloidal dispersions

Abstract

The effects of ionic copolymers with hydrophobic segments on the yield stress–pH behavior of ZrO2 dispersions were evaluated. These polyelectrolytes were two alternating copolymers; diisobutylene-maleic acid and alpha-methyl styrene-maleic acid, and a random styrene-maleic acid copolymer. At low concentrations, all three copolymers caused a much greater reduction in the maximum yield stress, tau ymax (at the neutral charge condition), compared with hydrophilic polyacrylic acid and acrylic acid-ester polymers. The reduction in tau ymax was as much as 75%. The much larger reduction in tau ymax was attributed to a thicker steric layer formed by the ionic copolymers with hydrophobic segments. At higher concentrations, all three copolymers caused tau ymax to increase. This was attributed to significant hydrophobic effects and attraction. The degree of hydrophobic segment aggregation at the closest point of interaction between particles is now higher. This also caused an overall increase in the entropy relative to the state when the particles are far apart, as the degree of hydrogen bonding among water molecules is less when the hydrophobic segments are aggregated.