| Abstract
| - The present article describes the preparation and preliminary characterization of a novel phosphate-functionalized self-assembled monolayer (SAM) and the determination of the surface ionization states of the phosphateheadgroup in aqueous solutions by chemical force microscopy (CFM). The phosphate headgroup used was PO(OH)2, a diprotic acid. The adhesion force between anAFM probe and a flat substrate, both of which werechemically modified with the same phosphate SAM, wasalso measured as a function of pH and ionic strength. Atlow ionic strength (10-4 M), two peaks were observed inthe force titration curve (adhesion force versus pH) at pH4.5 and 8.4. The two peaks are positioned 2.4 and 1.2pH units higher, respectively, than the acid dissociationconstants obtained for the phosphate group free in aqueous solution. At high ionic strength (10-1 M), the adhesion forces were reduced by 1 order of magnitude andthe peaks were replaced by shoulders similar to thosepreviously reported for acid force titrations. On the basisof JKR theory, the surface pKa values of the phosphategroup in high ionic strength solutions were found to be4.5 and 7.7, respectively. However, in light of the effectsof ionic strength on the force titration curves, we discussthe applicability of JKR theory to nanoscopic measurements of adhesion force and surface pKa.
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