Potentiostatic infrared titration of 11-mercaptoundecanoic acid monolayers

Abstract

In situ IR difference spectra of 11-mercaptoundecanoic acid (MUA) monolayers deposited on gold, and in the presence of a NaF solution in deuterated water, have been recorded as a function of both solution pH * and substrate's potential. IR spectra recorded under voltammetric conditions indicate that the potential-induced ionization of the monolayer is a slow process, involving a simultaneous rearrangement of the hydrocarbon chains, and that a significant population of carboxylic groups are not dissociated at positive potentials, even in the presence of a pH * 9 solution. Steep potentiostatic IR titration curves, consistent with a monolayer pKa = 4.3, are obtained at negative potentials. As the potential is made more positive, titration curves become lower, broader and are shifted towards more basic pHs, as would be expected when only a fraction of the carboxylic groups become ionized in the presence of less favorable interactions. Rather than acting directly on the electrostatic energy of the carboxylate groups, the electrode potential seems to control the ionization of the monolayer by attracting (or repelling) cations from the electrolyte and by reorienting the thiol head groups inside and outside the low permittivity layer of methylene chains.

Squaric acid adsorption and oxidation at gold and platinum electrodes

“…the uncoordinated carboxylic group seems also to be lowered in comparison to the second pKa for the corresponding dissolved species in an extent that decreases as the size of the molecule increases from (hydrogen)oxalate to (hydrogen)succinate [71]. In other cases, such as bisulfate [72] and mercaptoundecanoic acid [73] dissolved carbon dioxide (this latter species being hardly detected in the ATR-SEIRA spectra). In the case of platinum electrodes, it was clear from previous infrared external reflection data for SQA oxidation that carbon dioxide is produced [24,28].…”

Spectroscopic Evidence of Size-Dependent Buffering of Interfacial pH by Cation Hydrolysis during CO2 Electroreduction

“…This is similar to a recent work in which the pH-and potential-dependent ratio between the integrated intensities of the carbonyl and carboxylate bands in ATR-SEIRA spectra of self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid (MUA) was used to determine the pK a of MUA in the SAM. 22 The proximity between ߥ COO, asym and the H-O-H bending mode of water (ߜ HOH ) also needs to be taken into account. As shown recently by Yamakata et al using CO-covered Pt electrodes, [23][24][25] ATR-SEIRAS can be used to obtain the vibrational spectrum of the hydration layer of the cations of the supporting electrolyte at the electrode-electrolyte interface.…”

Acid deprotonation driven by cation migration at biased graphene nanoflake electrodes

“…Degree of acid dissociation (pK a ) is a function of solution composition [1][2][3] and identity of neighbouring charged species. 4,5 For example, it is common for acid groups within enzymes to have pK a values that vary as a function of local charge in the active site.…”

“…1 Protonation states of electrode-immobilized acids are also found to vary with applied potential. 2,3,6,7 Some studies find that acid association takes place on application of a negative potential and have proposed that protonation is driven by the applied electric field. 6,7 This electrochemically driven reversible acid protonation reaction has been exploited to fabricate a fast charge-discharge supercapacitor from 3,4,9,10-perylene tetracarboxylic acid adsorbed on graphene layers.…”

“…8 Conversely, other studies have found that deprotonation takes place when a negative potential is applied and is driven by a local increase in cation activity at the electrode interface that lowers the effective pK a of the acid groups. 2,3 pK a is defined in terms of acid dissociation but neglects the effect of concomitant interaction of the conjugate base with solution cations. The resulting apparent pK a (pK a (app)) can be shown 2 (S1, ESI †) to depend on monovalent cation activity a M + and K as , the equilibrium constant for the association between conjugate base and solution cations, as shown in eqn (1): pK a (app) = pK a + pK as À log(a M +)…”