Evidence of charge reversal from direct force measurements involving dissimilar metal sulfides in aqueous electrolyte

Abstract

PbS cubes with close to atomically smooth surfaces have been synthesised using a relatively simple precipitation method. Forces between the 10 µm size faces of these PbS cubes and ZnS spheres have been measured in dilute electrolyte at pH 4, 5.8 and 10 using colloid probe atomic force microscopy (AFM). At pH 5.8 and 10 the measured interactions were entirely due to overlap of electrical double layers (edl) with no evidence of attractive van der Waals forces on approach. An electrical attraction, predicted by the Poisson–Boltzmann equation to occur as a result of charge reversal on the surface of lower and constant potential, has been observed at separations >15 nm. At pH 4, dissolution and softening of the PbS surface resulted in instability of the interaction data after ca. 15 min. Prior to this, modifications to the ZnS surface were evident in the approach data. In addition a non-DLVO force similar in magnitude to that measured between ZnS spheres in acidic conditions was evident. This was attributed to hydrophobicity, with incongruent dissolution resulting in a sulfur-rich surface. A highly variable adhesion (<0.3 mJ m⁻²), which is reduced by the addition of electrolyte, has been measured between ZnS and PbS under all conditions.