Double-layer interactions between self-assembled monolayers of ω-mercaptoundecanoic acid on gold surfaces

The atomic force microscope was used to investigate the interaction between 11-mercaptoundecanoic acid surfaces and 2-mercaptoethanesulfonic acid surfaces as a function of electrolyte and pH. The surface potential was found to peak at -80 mV at pH 6-8 in 0.1 mM 1:1 electrolyte. The Hamaker constant for the gold-gold interaction was found to be decreased from the value of 1 x 10(-19) J in water to 4 x 10(-20) J in ethanol. The dispersion interaction between gold surfaces following derivatization with 11-mercaptoundecanoic acid was drastically reduced. The terminal carboxyl groups did not ionize in ethanol. Even in water, extremely low degrees of ionization were observed, with just 1-2% of surface sites being ionized at pH 10. The force curves showed no jump-in nor adhesion above pH 7. We could not reconcile the psi(afm)-pH results with a simple diffuse layer model for the interface; however the ionization behavior could be explained with sodium ion binding to surface carboxyl groups with a binding constant pK(Na) = 6.3. The addition of micromolar amounts of Pb2+ caused almost complete neutralization of charged surface groups. These data could be modeled with K-Pb = (1-4) x 10(-10). An alternative modeling procedure based on incorporation of a zero-order Stern layer could only explain the experimental values if an extremely low value of the inner layer capacitance was assumed of 1-4 mu F cm(-2). However, this model may be able to explain the lack of jump-in in alkaline solution and the absence of adhesion at high pH.