Proton Transfer Voltammetry at Electrodes Modified with Acid Thiol Monolayers

Abstract

By combining a description of the potential profile at electrodes coated with acid thiol monolayers with a quadratic relationship between activation energy and electrode potential, a rather simple expression for proton transfer voltammograms is derived. Our electrostatic analysis shows that proton transfer can only produce narrow voltammetric peaks when the immobilized acid groups lie close to the metal substrate. Quantitative fits of experimental voltammograms obtained with an Au(111) electrode modified with a 11-mercaptoundecanoic monolayer at pH 8.5 reveal that less than 1% of the carboxylic groups in the monolayer participate in the potential induced proton transfer process and that these groups lay close to the metal surface. A preliminary analysis of the kinetic parameters suggests that the interfacial electric field facilitates an intrinsically slow proton exchange between a proton donor and acceptor pair that are not in close contact with each other at the interface.

Potentiostatic infrared titration of 11-mercaptoundecanoic acid monolayers

“…These results can be classified into two groups. The first group [8][9][10][11][12][13] displays an increase of the monolayer acidity as the potential is made more positive (in agreement with predictions from electrostatic models [17][18][19]), whereas the second group [14][15][16] shows the opposite trend (which has been rationalized in terms of a cation-exchange model [14][15]). The potential-induced ionization of 11-mercaptoundecanoic acid (MUA) monolayers has been reported to display both of these opposite trends, under the scrutiny of different experimental techniques and in the presence of different electrolyte solutions.…”

“…Usually, this control is exerted through the solution pH, but attachment of the thiol group to a metallic substrate opens the possibility of applying a potentiostatic control on the ionization degree of the terminal carboxylic groups. Evidence supporting the influence of the substrate potential on the acid dissociation of thiol monolayers has been gathered from a variety of techniques, including voltammetric [8][9][10][11][12], surface Raman [13], quartz microbalance [14] and infrared absortion [15,16] experiments. These results can be classified into two groups.…”

Nanoporous gold electrodes modified with self-assembled monolayers for electrochemical control of the surface charge

“…36,37 The more precise pzc for npAu/MHDA clearly reveals that long chains are favorable to control the surface charge on porous surfaces because they suppress side-effects caused by the rough surface. This stands in contrary to conclusions drawn for planar gold electrodes 12 where highly ordered long chains appear to be less suitable as the long chains also suppress acid-base reactions on the head group due to the large diffusive layer (the dielectric layer between the metallic surface and the ionisable head group). Based on these findings, the focus of the subsequent study lies on MHDA due to the better control of the surface charge.…”

“…3B, it is obvious that the deprotonation peak is significantly broader than the protonation peak, almost exhibiting a double peak shape. This has been observed in simulations 12 as well and is dependent on the diffuse layer potential and can be expected to be more pronounced for a large concentration of surface acid groups. Worthwhile to mention is the underlying slope in the cathodic current (Fig.…”

“…7 The electrochemical control of the proton transfer reaction at the head group of SAMs has already been the subject of numerous theoretical and experimental studies. [8][9][10][11][12][13] However, it has also been shown that SAMs on polycrystalline surfaces exhibit a significantly different behaviour compared to single crystals 14 as their final ordering differs greatly, which influences the overall (electro-)chemical behavior. 15,16 This makes it impossible to directly transfer the available knowledge from the planar Au to porous substrates which exhibit very rough and strongly curved surfaces with high defect concentrations.…”