The adsorption kinetics of thioctic acid (ToA) self-assembled monolayer (SAM) formation has been observed by monitoring small changes in the extinction of a gold nanoparticle surface, interrogated by evanescent wave cavity ring-down spectroscopy. A direct measurement of the adsorption kinetics for charged and neutral SAM formation has been made and modeled and shows significant differences in the lateral interactions between surface moieties: the interaction parameter for the charged species is 20 times larger than for the neutral species. Titration of ToA SAMs deposited from basic conditions demonstrate a pH-switchable surface with charge densities varying from 0.1 to 1.0 e nm(-2). Similar measurements were performed from SAMs deposited under acidic conditions producing charge densities of 1.8 e nm(-2). The surface potential varies between -44 and -198 mV for all SAMs, which produces an interfacial pH 0.8-3.4 units more acidic than the bulk and interfacial capacitances varying from 5.5 to 88 mu F cm(-2).