EVIDENCE OF CHARGE REVERSAL FROM DIRECT FORCE MEASUREMENTS INVOLVING DISSIMILAR METAL SULFIDES IN AQUEOUS-ELECTROLYTE

PbS cubes with close to atomically smooth surfaces have been synthesis ed using a relatively simple precipitation method. Forces between the 10 mu m size faces of these PbS cubes and ZnS spheres have been measur ed in dilute electrolyte at pH 4, 5.8 and 10 using colloid probe atomi c force microscopy (AFM). At pH 5.8 and 10 the measured interactions w ere 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 potent ial, has been observed at separations >15 nm. At pH 4, dissolution and softening of the PbS surface resulted in instability of the interacti on data after ca. 15 min. Prior to this, modifications to the ZnS surf ace were evident in the approach data. In addition a non-DLVO force si milar in magnitude to that measured between ZnS spheres in acidic cond itions was evident. This was attributed to hydrophobicity, with incong ruent dissolution resulting in a sulfur-rich surface. A highly variabl e adhesion (<0.3 mJ m(-2)), which is reduced by the addition of electr olyte, has been measured between ZnS and PbS under all conditions.