Control of adhesion and surface forces via potential-dependent adsorption of pyridine.

The orientation and extent of adsorption of pyridine on a gold electrode is known to depend on applied potential and is well characterized. By use of the electrochemical surface forces apparatus, we measured the potential dependence of the double-layer interactions and adhesive forces between a gold electrode and a mica surface for different pyridine concentrations. We observed that, unlike mica-mica interactions, the gold-mica interactions were strongly affected by the presence of small concentrations of pyridine. We are able to reach high negative surface potentials (as determined by applying Derjaguin-Landau-Verway-Overbeek theory to our force measurements), which is similar to what is observed in the absence of pyridine. This demonstrates the electronic nature of the forces measured and shows that pyridine does not displace potential-determining ions on the surface. At positive potentials, where the interaction between gold and mica is attractive, pull-off measurements are a strong function of applied potential. The major effect of the presence of pyridine is on the observed shift in the potential of zero force (PZF), moving it to more negative potentials. This effect is caused by the strong dipole of the pyridine molecule. When the applied potential is cast as a deviation from the PZF, the effect of pyridine is to reduce adhesion between gold and mica. We modeled the potential-dependent adhesion of this system using an electrocapillary framework developed previously, and in doing so, we establish the relationship between the gold-liquid and gold-mica surface energies. In addition, we show that pyridine adsorption affects the capacitance of the gold-mica interface.