PM FTIRRAS Studies of Potential-Controlled Transformations of a Monolayer and a Bilayer of 4-Pentadecylpyridine, a Model Surfactant, Adsorbed on a Au(111) Electrode Surface

Abstract

Polarization modulation Fourier transform infrared reflection-absorption spectroscopy (PM FTIRRAS) has been combined with electrochemistry to monitor the potential-induced transformations of a monolayer and a bilayer formed by 4-pentadecylpyridine (C15-4Py), a model amphiphilic compound, at a Au(111) electrode surface. The optical constants for a solution of randomly oriented C15-4Py molecules have been determined and used to calculate the integrated band intensities for a monolayer and bilayer of randomly oriented molecules. The orientation of the hydrophobic and hydrophilic parts of the molecule with respect to the surface normal was determined from the ratio of the intensity of experimental to calculated bands. The CH stretch region of the spectra was used to determine the tilt angle between hydrocarbon chains and the surface normal. The tilt angle varied with potential. The minimum value of the tilt angle was 16°f or the monolayer and 27°for the bilayer. In the monolayer, the headgroup of the C15-4Py molecule lies almost flat on the Au(111) surface. At negative potentials, the angle between the plane of the pyridine ring and the surface normal is 68°. As applied potential becomes more positive, the pyridine group stands up gradually and the tilt angle decreases to 63°. In the bilayer, the C2 axis of the pyridine group in the leaflet turned to the electrolyte solution is tilted at ∼70°with respect to the normal and this tilt angle changes little with potential. In the leaflet turned to the electrode, the angle between the C2 axis of the headgroup and the surface normal changes from 64°to 54°by moving from negative to positive potentials. The PM IRRAS data also show that in the bilayer, the plane of the pyridine group rotates with respect to the C 2 axis when the electrode potential changes. In the monolayer, the tilt angle of the C2 axis changes but the plane of the pyridine moiety does not rotate with potential.

Application of Polarization Modulation Infrared Reflection Absorption Spectrosocpy Under Electrochemical Control for Structural Studies of Biomimetic Assemblies

“…In the past 30 years IRRAS has developed into a powerful analytical tool to probe films adsorbed at the electrode|electrolyte interface [18,22,29,[49][50][51]. When IR-RAS is combined with electrochemical experiments, the following considerations have to be taken into account: ( ) a selection of a mirror for the IR radiation, which simultaneously serves as a working electrode, (ii) a strong absorption of the IR radiation by the electrolyte (usually aqueous solution) and (iii) the use of an IR optical window which is required to prevent electrolyte leakage.…”

“…The beam convergence was set to ± 5%; comparable with the beam convergence in commercially available IR spectrometers. The wavenumber of the incoming IR radiation was set to 2900 cm −1 ( = 3.44 μm), corresponding to absorption modes of CH groups, the most often analyzed spectral region in organic molecules adsorbed on solid surfaces [12,32,51,[58][59][60][61]. Figure 2 shows the three-dimensional plot of the MSEFS as a function of the angle of incidence and electrolyte layer thickness.…”

“…The IR absorption spectra from molecules adsorbed on the electrode surface can be obtained when calculated experimental parameters can be applied to a precisely machined spectroelectrochemical cell. Only such spectra provide structural details of the electrified interface [17,21,29,51].…”

“…In polarization modulation infrared reflection absorption spectroscopy (PM IR-RAS) an incident IR beam is modulated between the s-and p-polarization and thus the resulting reflection-absorption spectrum of species adsorbed on the metal|air interface is almost insensitive to the absorption of water vapour and CO 2 from the atmosphere [45,51,74,81]. Russel et al for the first time applied PM IRRAS to the electrochemical interface [74].…”

“…The second order Bessel function at which the dephasing was performed. This function contributes to the intensity of the measured spectrum and has to be subtracted [47,51]. In the first in situ PM IRRAS study the potential-driven changes in a monolayer of CO molecules adsorbed on a platinum electrode were investigated [45,74].…”