The influence of aliphatic amines on the potential of the structural transition (hex) → (1 × 1) for Au (100) electrode
The impact of the aliphatic amines on the potential (E T) of the structural transition (hex) → (1 × 1) for Au (100) electrode was investigated. This potential was shifted to more negative values as the concentration of a given amine increased. As E T depends linearly on the log of the bulk concentration for each amine it was suggested to use this relationship for quantitative determination of the amines. Moreover, it was shown that the presence of amines reduced the stability range of the (hex) structure in the following sequence methylamine < ethylamine < propylamine < butylamine due to their increasing adsorbability. Furthermore, for a given concentration the introduction of each subsequent –CH2– group into the carbon chain of an amine is accompanied by the reduction of the stability range of ca. 23 mV.
KeywordsSingle crystal Au(100) electrode Surface reconstruction Aliphatic amines
At present short-chain aliphatic amines can be often met in the aquatic environment which is a consequence of their widespread use in production of polymers, dyes, pharmaceuticals, corrosion inhibitors, fuel additives, and explosives [1, 2, 3].
Aliphatic amines are also common components of biological systems as they are degradation products of organic materials such as amino acids and proteins . Most amines are toxic and can react with nitrogen-containing compounds to form nitrosamines, which are carcinogenic substances. Therefore, to reduce the risk of the exposure to them, their detection and monitoring have become of great interest for the natural environment protection .
However, in general only chromatographic techniques have been used for determination of these compounds because of their little, if any, ultraviolet or visible light absorption and fluorescence . Recently, also an attempt was made to use electrochemical methods with employment of polycrystalline gold electrode but the results were unsatisfactory [7, 8].
Unexpectedly, in the course of our electrochemical study of the reconstruction phenomenon at Au(100) electrode in the presence of amines, we obtained some data that could be useful for their quantitative determination.
The Au (100) surface undergoes a reconstruction of its outermost atomic layer from the (1 × 1) bulk termination structure to a rotated hexagonal (hex) structure, which has an atomic density that is ~25% greater than that of the bulk-terminated (1 × 1) surface layer. The reconstruction of Au (100) has been studied intensively by electrochemical measurements , in ultra high vacuum condition , scanning tunnelling microscopy  and by in situ surface X-ray scattering . This phenomenon has been also the subject of theoretical consideration where the density functional theory  has been applied.
As follows from the hitherto data, a flame-annealed Au(100) electrode is reconstructed and stable in electrochemical environment. The reconstruction is lifted due to adsorption of anions and organic molecules and the surface structural transition (hex) → (1 × 1) takes place at the so-called transition potential (E T) yielding the unreconstructed surface. Usually the occurrence of surface reconstruction has a marked impact on electrochemical processes, e.g., adsorption and phase transition in organic adlayer [12, 13, 14].
As follows from the general approach to the stability of the reconstructed Au (100) electrode in the presence of organic compounds these adsorbates alone do not necessarily lift the reconstruction as have been revealed for, e.g., coumarin  or propanal . As evident from the energy diagram (see, e.g., Fig. 3 in ), the reason for this is that the difference in adsorption energy of the adsorbate on the (hex) and (1 × 1) structure has to exceed the difference in surface energy for both structures. On the other hand, the organic molecules for which the lifting of the reconstruction of the Au(100) electrode was documented [17, 18, 19] are mainly heterocycles with one or two nitrogen atoms in the ring.
In order to pursue the investigations, in the present article we have chosen another type of molecules containing nitrogen, i.e., aliphatic amines in order not only to probe but mainly to explain the impact of molecular structure or more precisely the length of the carbon chain in the amine molecule on the potential of structural transition at the Au (100)-(hex) electrode. These short-chain amines were methyl- (Me), ethyl- (Et), propyl- (Pr), and butylamine (Bu), respectively. Finally, we propose to apply the concentration dependence of E T obtained for the above-mentioned amines in their quantitative determination.
The working electrode was an Au(100) single crystal disc, 4 mm in diameter and 4 mm thick, and oriented to <1° (MaTeck, Jülich). Before each experiment the electrode was prepared according to the well-known procedure for preparation of well-ordered gold surfaces . Namely, the crystal was annealed for 3 min in a Bunsen burner flame, then cooled down to room temperature in a stream of nitrogen. Contact with the electrolyte was achieved by the hanging-meniscus method . The counter electrode was a Pt wire. The reference electrode was a saturated calomel electrode (SCE) and throughout this work all potentials are reported versus SCE. The voltammetric measurements were performed using an AUTOLAB system (Eco Chemie). All the voltammograms were obtained at a scan rate of 10 mV s−1.
The supporting electrolyte solution (2 × 10−2 M NaClO4) was prepared from doubly recrystallised NaClO4 (analytical grade, Merck). Amines were of Aldrich analytical grade quality. All solutions were prepared from Milli-Q water.
All electrolytes were deaerated (for 50 min) by nitrogen. Nitrogen was allowed to flow over the solution at all times. All experiments were carried out at room temperature 25 ± 1 °C.
3 Results and discussion
We mentioned in passing that the subsequent increase in the density of the current found to occur for positive values of potential greater than E T is connected with the oxidation of Pr. The overlapping of the reconstruction peak and the oxidation wave, however, does not obscure the observation that the charge under the peak decreases with increasing bulk concentration of Pr. Nevertheless, due to the coincidence of both processes we had to perform our measurements in solutions containing not less than 10−4 M of Me, Et, Pr, and 5 × 10−4 M of Bu.
Finally, it is worthwhile to mention a few interesting observation following from a detailed study of the concentration dependence of the potential of the structural transition (hex) → (1 × 1) for the amines considered. This concentration dependence depicted in Fig. 4 shows the transition potentials, as derived from CV curves for thermally prepared Au (100)-(hex) in various solutions, as a function of the logarithm of the amines concentration. As can be seen for each amine E T changes linearly with log of the amine concentration similarly as already observed for thiourea but in contrast to pyridine and pyrazine for which this relationship was nonlinear . The slope and correlation coefficient calculated for the lines depicted in Fig. 4 are −0.121 ± 0.001 V (R 2 = 0.9995, n = 6), −0.118 ± 0.0006 V (R 2 = 0.9998, n = 6), −0.119 ± 0.002 V (R 2 = 0.994, n = 6) and −0.116 ± 0.002 V (R 2 = 0.999, n = 5) per decade for Me, Et, Pr, and Bu, respectively. Thus, taking into account both the linear relationship between E T and log of concentration and the well-known reproducibility and stability of the measurements with single crystal electrodes [22, 23, 24], the reconstructed Au(100) electrode could have potential application for quantitative determination of aliphatic amines in the above-mentioned concentration range.
The observed relationship (Fig. 4) is presented by a series of not only straight but also nearly parallel lines. The parallelism of the lines shows a qualitatively similar impact of the amines studied on the stability range of the reconstructed surface. In addition, this parallelism is associated with almost equal distance separating the lines. As follows from Fig. 4, the intersect is shifted towards more negative values by about 23 mV as Me is replaced by Et and then subsequently by Pr and Bu, respectively. A linear reduction of the stability range as results of increasing carbon chain of the amine could therefore be postulated. In other words for a given concentration the introduction of each subsequent –CH2– group into the carbon chain of the amine is accompanied by the reduction of the stability range by ca. 23 mV.
The influence of aliphatic amines molecules on the potential of structural transition (hex) → (1 × 1) at Au (100) electrode has been investigated over a wide concentration range. The data obtained indicate that adsorption of these organic compounds lifts the reconstruction and in consequence shifts the potential of structural transition in the negative direction. It was shown that the presence of amines in the supporting electrolyte reduces the stability range of the (hex) structure in the following sequence Me < Et < Pr < Bu due to their increasing adsorbability.
for each amine under study E T changes linearly with log c
for a given amine concentration, gradual introduction of –CH2– group into the carbon chain of the amine, is accompanied by reduction of the stability range by ca. 23 mV.
This former [i.e., (i)] finding was suggested to be tested as possible method for quantitative determination of the aliphatic amines under consideration.
Financial support from A. Mickiewicz University, Faculty of Chemistry is greatly appreciated.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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