Forces between sulfonate self-assembled monolayers (SAMs) and zirconia particles in aqueous solutions

An atomic force microscope was used to measure the interaction forces between sulfonate-terminated, siloxane-anchored, organic self-assembled monolayers (SAMs) on silicon and a 20-micron-sized zirconia sphere in aqueous electrolyte solutions. The jump-in distance and adhesive force were measured in KNO3 solutions with pH ranging from 2 to 9 and ionic strength from 0.45 M to 4.5 6 10(-5) M. The observed forces in all cases were larger for sulfonate SAM-covered silicon than for bare silicon. With the sulfonate surface, the force was repulsive at high pH and attractive at low pH, and it increased with decreasing pH and ionic strength. Experimental results were in most cases consistent with predictions from Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, indicating that DLVO forces play a key role for the interactions between these surfaces under such conditions. The results further indicate that DLVO interactions may contribute to the formation of zirconia and other oxide films via attachment of particles present in aqueous media.