Effect of ionic strength and salt identity on poly(N-isopropylacrylamide) brush modified colloidal silica particles

The thermoresponse of poly(N-isopropylacrylamide) stabilised particles is influenced by the presence of salt and is dependent on the concentration, and ions present. The conformation and electrophoretic mobility of core/shell PNIPAM brush modified silica particles is expected to vary as a function of these specific ion effects.

The thermoresponse of PNIPAM brush modified silica particles was investigated via dynamic light scattering and electrophoretic mobility measurements between 5 and 45 °C in the presence of 11 different salt solutions.

Specific ion effects were observed in the presence of salt solutions for concentrations between 50 and 1000 mM. The temperature response could be mapped to a master curve unlike PNIPAM brush behaviour on planar substrates. The magnitude of brush layer lower critical solution temperature reduction was found to follow the order F^- > CH₃CO₂^- > Cl^- > NO₃^- ∼ Br^- > I^- > SCN^- for the potassium series and Na⁺ > K⁺ > Cs⁺ > Li⁺ ∼ NH₄⁺ for the chloride salts. The electrophoretic mobility of the modified particles in the presence of 100 mM potassium salts increased in magnitude as the brush layer collapsed and also with the chaotropic nature of the anion.