Double-Layer-Capacitance Titration of Self-Assembled Monolayers of ω-Functionalized Alkanethiols on Au(111) Surface

Abstract

The acid-base equilibrium of ω-functionalized alkanethiol monolayers on Au(111) has been studied using the change in double-layer capacitance accompanied by the protonation-deprotonation of ω-terminals. The pK of ω-carboxyl alkanethiols on Au(111) increases by four pH units. The shift becomes greater as the alkyl chain length increases. The same magnitude of the pK shift to the acidic side occurs in the monolayers of aminoethane thiol. The surface pK varies little with the electrode potential, whereas the increase in the supporting electrolyte concentration slightly diminishes the pK shift. The drawn-out shape of the titration curves is consistent with the mean-field model taking account of the repulsive interaction between adsorbed molecules, indicating the significance of the strong electrostatic repulsion between adsorbed thiol molecules in its charged states. The magnitude of the pK shift is, more than that predicted by the mean-field model, however, and suggests the considerable contribution from other factors that stabilize the uncharged state, for example, hydrogen bonding and low dielectric constant in the vicinity of the self-assembled monolayer. The possibility of counterion binding when the degree of deprotonation is large has been suggested.

Controlled Orientation of DNA in a Binary SAM as a Key for the Successful Determination of DNA Hybridization by Means of Electrochemical Impedance Spectroscopy

“…In addition, the pK a value of w-carboxyl-terminated thiol monolayers can be lowered by dilution with alkyl thiols. [41] The used electrodes had a surface roughness of 1.5 to 2. Hence, we believe that at a pH value of 7.4, the MPA in the mixed monolayer isat least partially-deprotonated, which leads to localized negative charges within the binary SAM.…”

Molecular Structure Dependence of the Phase Transition Spike Response of Viologens at an HOPG Electrode Using Bisviologen and Carboxylated Viologen

“…Another approach that includes explicitly the intermolecular interaction has been reported by Aoki, Kakiuchi, and corworkers 46,47 for the case of a self-assembled monolayer (SAM) of a ω-carboxylated alkanethiol. In the case of the SAM, predominant hydrogen bonding attraction as a short-range interaction over electrostatic repulsion as a long-range interaction has successfully explained the experimentally observed pK a shift to a larger value by more than 3 pH-units.…”

Study of electrode potential effect on acid-base behavior of ?-functionalized self-assembled monolayers using Fourier transform surface-enhanced Raman scattering spectroscopy

“…6 For example, it was found by Kane and Mulvaney 21 that the ionization behavior of ω-mercaptoundecanoic acid SAMs could not be interpreted quantitatively using the Gouy-Chapman (GC) theory when they used atomic force microscopy to study double-layer interactions between the SAMs on gold surfaces. In order to explain the experimental data more adequately, they proposed two additions to the basic GC model.…”