Spectroelectrochemical Studies of CTAB Adsorbed on Gold Surfaces in Perchloric Acid

Abstract

The behaviour of CTAB adsorbed on polycrystalline gold electrodes has been studied using a combination of spectroelectrochemical methods. The results indicate that the formation of the layer is the consequence of the precipitation of the CTAB micelles on the electrode surface as bromide ions, which stabilize the micelles, are replaced by perchlorate anions. This process leads to the formation of CTA⁺ layers in which perchlorate ions are intercalated, in which the adlayer suffers a continuous rearrangement that leads to the formation of micro-dominions of different types of hydrogen-bonded water populations throughout the adlayer. After prolonged cycling, a stable situation is reached. Under these conditions, water molecules permeate through the adlayer toward the electrode surface at potentials positive of the potential of zero charge, due to the repulsion between the CTA⁺ layer and the positive charge of the electrode.

Figure 1

(A) ATR absorbance spectra of 3.3 × 10^–4 M (black), 10^–3 M (red) and 0.01 M (blue) aqueous CTAB solutions. (B)ATR absorbance spectrum of CTAB precipitated from a 10^–3 M aqueous solution. Pure water was used as the reference spectrum in all cases. The vertical lines show the main bands in the alkane stretching region. 50 interferograms were taken for each spectrum.

Figure 2

Voltammetric profiles of (A) unmodified polycrystalline Au, and (B,C) CTAB-modified polycrystalline Au in 0.1 M HClO₄. The evolution upon continuous cycling with a positive potential limit of 1.2 V is illustrated in (B), while (C) shows the profile when the positive potential limit is increased to 1.7 V and the subsequent profile if the limit is set again to 1.2 V immediately after the excursion up to 1.7 V and back to 0 V. ν = 50 mV s^–1.

Figure 3

Potential-dependent ATR-SEIRA spectra (E_ref = 0 V) of CTAB adsorbed on a gold film chemically deposited on the Si prism in 0.1 M HClO₄ before any voltametric cycle (A) and after a cyclic voltammogram between 0 and 1.7 V (B). The CH_x stretching region is plotted on the right. Reference spectra were taken at 0 V in each series of spectra.

Figure 4

Potential-dependent ATR-SEIRA spectra in 0.1 M HClO₄ of adsorbed CTAB adsorbed on a gold film chemically deposited on Si recorded after 100 cycles between 0 and 1.2 V at 20 mV s^–1. The reference spectrum was recorded at 0.1 V.

Figure 5

ATR-SEIRA spectra of CTAB on Au in 0.1 M HClO₄ collected during continuous cycling between 0 and 1.2 V. Scan rate: 20 mV s^–1. Spectra were collected at intervals of 0.6 s (i.e., each spectrum covers an interval of 12 mV), but are shown at intervals of 56 s for the sake of clarity. Resolution: 4.0 cm^–1. Black lines show the growth of the water bands. Reference spectrum was taken at 0 V.

Figure 6

Selected ATR-SEIRA spectra (A,B) of CTAB on Au in 0.1 M HClO₄ during a cycle voltammogram between 0 and 1.2 V. (A) shows the positive and (B) the negative sweep. Scan rate: 20 mV s^–1. (C) Dependence of the integrated intensity of selected bands in the ATR-SEIRA spectra of CTAB on Au on the applied potential. Spectra were collected at intervals of 0.6 s (i.e., each spectrum covers an interval of 12 mV). Resolution: 4.0 cm^–1. Reference spectrum was taken at 0 V.

Figure 7

Selected ATR-SEIRA spectra of CTAB on Au in 0.1 M HClO₄ recorded during a cyclic voltammogram at 10 mV s^–1 between 0 and 1.2 V in the positive (A) and negative (B) sweep after 100 cycles at 50 mV s^–1. Spectra were collected at intervals of 0.2 s (i.e., each spectrum spans over 10 mV). Resolution: 4.0 cm^–1. The reference spectrum was the spectrum at 0 V of this series.

Figure 8

Potential-dependent SER spectra of CTAB-modified Au on the high (A) and mid-low (B) frequency region.