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The interfacial charge formation of self-assembled monolayers (SAMs) of thiol derivatives on gold was investigated by streaming potential and streaming current measurements in aqueous electrolyte solutions. All experiments were performed at a slit channel formed by two parallel sample plates. For the first time, ζ potentials of planar SAM−solution interfaces were determined experimentally. Surface pK values were calculated from the pH dependence of the ζ potential. Monolayers carrying ω-carboxylic moieties show a shift of the pK toward the alkaline direction as compared to the carboxy-terminated alkanethiol molecules in solution. Contact angle titrations using a captive bubble method confirmed the dissociation behavior of monolayers with acidic groups. Monolayers of methyl-terminated thiols exhibit ζ potential−pH plots that account for unsymmetrical ion adsorption. Models of the electric double layer are discussed to describe the interface between self-assembled monolayers and electrolyte solutions. Two contrary situations were found depending on the process which generates surface charges. For methyl-functionalized SAMs, the main part of the surface charge is compensated within the diffuse double layer. In contrast, at monolayer surfaces bearing carboxylic acid groups, the main part of the countercharge was found to be located in the stagnant part of the double layer.

Force−distance studies suggest that electrostatic forces play a dominant role in the resistance of oligo(ethylene glycol) (OEG) terminated self-assembled monolayers (SAMs) to adsorption from aqueous solution, whereas for end grafted poly(ethylene glycol) (PEG) coated surfaces inertness is explained by a steric repulsion effect. To study the role of electrostatics in the repulsive forces measured on methoxytri(ethylene glycol) (EG3OMe) terminated alkanethiolate SAMs on gold surfaces, their electrokinetic potentials and charging properties were determined with the microslit electrokinetic setup (MES) using rectangular microchannels formed by SAM-covered sample carriers. The zeta potentials measured on the (EG3OMe) SAMs, both on gold and on glass substrate, indicate preferential hydroxide ion adsorption from the aqueous phase resulting in a net negative surface charge at pH >4. Calculated charge densities agree well with the results from force−distance studies. We also studied the electrokinetic properties of poly(ethylene glycol) (PEG) SAMs (HS(CH2)11(OCH2CH2)44−OCH3). All these films show a negative zeta potential in neutral and alkaline solutions which decreases with increasing packing density and optical film thickness, reflecting the changes in frictional properties of the PEG chains with increasing coverage. An explanation why the negatively charged (EG3OMe) surfaces exhibit repulsive electrostatic forces in aqueous solution, but other negatively charged surfaces do not, is proposed.