Solvent-specific gel-like transition via complexation of polyelectrolyte and hydroxyapatite nanoparticles suspended in water-glycerin mixtures: A rheological study

Polyelectrolytes are routinely used in many applications including hydrogels and aqueous particle suspensions. Although the rheology of charge-stabilized colloidal particle systems involving polyelectrolyte in a single solvent has been studied both theoretically and experimentally, the influence of a second solvent has received considerably less attention. We report here on the system comprising ellipsoidal nano-hydroxyapatite particles suspended in water-glycerol mixtures containing dissolved polyelectrolyte "dispersant." The nano-hydroxyapatite content ranged from 0 to 20 vol%, while the concentration of the neutralized poly(acrylic acid) dispersant was varied between 0 to 0.08 wt% in the suspension (0 to 0.108% with respect to the HA component). In contrast to earlier reports on similar suspensions, an increase in apparent viscosity by three orders of magnitude with increasing polyelectrolyte content was observed. At the highest concentrations of polyelectrolyte, the suspensions exhibited solid-like behavior as indicated by the presence of a yield behavior and a shear storage modulus one order of magnitude larger than the loss modulus. These observations led to the hypothesis that the polyelectrolyte contributed to the formation of polymer-rich water bridges between the particles, although no macrophase separation was found in the absence of the particles.