Interaction forces between a silica sphere and an a-alumina ments has been the surface force apparatus (2) in which substrate at various pH values were measured by atomic force forces between two macroscopic surfaces (usually mica) are microscopy (AFM). As expected, at pH values of 10.8 and 10.2 measured. In recent years, atomic force microscopy (AFM) when the surfaces are similarly charged, a repulsive force was has gained popularity as a technique for force measurements observed. On the other hand, at pH values of 5.5, 6.4, and 8.6 (7). Forces between a sharp tip and a flat surface have been when the surfaces are oppositely charged, attractive forces were measured both in air (8, 9) and in aqueous media (10, 11, observed. Experimental force vs separation distance curves were 12). However, these measured forces were not compared to found to be in good agreement with theoretical predictions based theoretical predictions because of the ill-defined geometry on electrostatic and van der Waals interactions. Interestingly, when the force/radius values at a particular separation distance of the tip. Recent advances in force measurements by atomic were plotted against pH, the transition from an attractive to a force microscopy now allow direct force measurements berepulsive force occurred at pH 9.3, which is very close to the point tween a sphere and a macroscopic surface (13-15) or a of zero charge (pzc) of a-alumina as determined from electrophosphere and a sphere ( ).
resis experiments. These results suggest that AFM force measure-
In the present work, forces between silica and a-alumina ments can be used to estimate the pzc of materials. This method surfaces have been measured by AFM to determine the intermay be of particular significance for soluble salt minerals where actions between these materials at different pH values. The conventional electrophoretic measurements are not possible at experimental force curves were then compared to the theorethigh ionic strengths. Finally, results from transmittance studies ical forces calculated based on electrostatic and van der further confirmed the interaction between silica and alpha alumina Waals interactions between charged surfaces. In addition, particles in suspensions at various pH values as would be expected the results from AFM force measurements were used to based on the results from atomic force microscopy measurements.