Interactions between dissimilar surfaces in high ionic strength solutions as determined by atomic force microscopy

Abstract

The recent non-equilibrium electrophoretic mobility measurements by laser-Doppler electrophoresis coupled with the stability and prevalence of collector colloids in soluble salt flotation systems suggest that the selective flotation of alkali halides is due to the adsorption of oppositely charged collector colloids by heterocoagulation. Previously reported flotation results confirm this surface charge/collector colloid adsorption model. However, the nature of the interparticle forces responsible for heterocoagulation of such oppositely charged particles at high ionic strengths remains to be determined. In this regard, atomic force microscopy was used for interparticle force measurements at high ionic strengths. Model systems such as polystyrene/quartz and silica/sapphire were studied to measure the particle interaction forces prevalent at high ionic strengths. Results from this study indicate that while repulsive hydration forces exist between similarly charged hydrophilic particles, attractive forces exist between oppositely charged particles in high ionic strength solutions. The repulsive forces between similarly charged hydrophilic surfaces at high ionic strengths (2–4 m) are described by a double exponential function with decay lengths of 0.17 and 1.4 nm, depending upon the surfaces involved. On the other hand, attractive forces were observed between oppositely charged hydrophilic surfaces at high ionic strengths (2–4 m) and were described by a single exponential function with decay lengths of up to 9 nm, depending upon the surfaces involved.