A study of the effect of electrolyte on the swelling and stability of poly(N-isopropylacrylamide) microgel dispersions

Poly(N-isopropylacrylamide) (poly(NIPAM)) microgel dispersion stability has been investigated in the presence of a range of electrolytes. The stability of the dispersions to flocculation is strongly related to the free energy change of formation in aqueous solution for the anions (Delta G(f)(0)) and their position in the lyotropic series. The ability of the anions to induce particle deswelling and flocculation has been investigated using photon correlation spectroscopy and turbidity-wavelength measurements, respectively. Anions with a high charge density (and high negative values for Delta G(f)(0)) have the ability to dehydrate the poly(NIPAM) chain and induce flocculation. The ability of cations to induce particle deswelling and flocculation was also investigated. The ability of the cations to induce particle deswelling did not coincide with their values for Delta G(f)(0) and their position in the lyotropic series. Time-dependent turbidity measurements of collapsed particles in the presence of NaCl were used to calculate the Hamaker constant (A,) for poly(NIPAM). The value found was A(p) = 6.2 x 10(-20) J, which compares favorably to that reported for polystyrene and poly(NIPAM)polystyrene composites. Turbidity-wavelength measurements for poly(NIPAM) particles dispersed in aqueous NaCl solutions revealed that the critical flocculation temperature of the dispersion coincided with the lower critical solution temperature.