Potentiostatic infrared titration of 11-Mercaptoundecanoic acid monolayers

Antonio M. Luque, Angel Cuesta, Juan Jose Calvente, Rafael Andreu

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Abstract

In situ IR difference spectra of 11-mercaptoundecanoic acid monolayers deposited on gold 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 in 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. 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.
Original languageEnglish
Pages (from-to)13-16
Number of pages4
JournalElectrochemistry Communications
Volume45
Early online date17 May 2014
DOIs
Publication statusPublished - Aug 2014

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Titration
Monolayers
Infrared radiation
Acids
Ionization
Hydrocarbons
Sulfhydryl Compounds
Gold
Electrolytes
Cations
Electrostatics
Permittivity
Positive ions
Electrodes
11-mercaptoundecanoic acid
Substrates

Keywords

  • acid monolayers
  • ATR-SEIRAS
  • surface IR titration
  • potential-induced reorientation
  • potential-induced proton transfer

Cite this

Potentiostatic infrared titration of 11-Mercaptoundecanoic acid monolayers. / Luque, Antonio M.; Cuesta, Angel; Calvente, Juan Jose; Andreu, Rafael.

In: Electrochemistry Communications, Vol. 45, 08.2014, p. 13-16.

Research output: Contribution to journalArticle

Luque, Antonio M. ; Cuesta, Angel ; Calvente, Juan Jose ; Andreu, Rafael. / Potentiostatic infrared titration of 11-Mercaptoundecanoic acid monolayers. In: Electrochemistry Communications. 2014 ; Vol. 45. pp. 13-16.
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abstract = "In situ IR difference spectra of 11-mercaptoundecanoic acid monolayers deposited on gold 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 in 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. 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.",
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AB - In situ IR difference spectra of 11-mercaptoundecanoic acid monolayers deposited on gold 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 in 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. 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.

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