# Infrared Spectroscopy as Molecular Probe of the Macroscopic Metal-Liquid Interface
## 3 Results  
The pairs of spectra recorded with and without an aluminum plate are displayed in Figure 2. Note that the spectral window between 1800 and 2800 cm\({}^{-1}\) does not contain any appreciable signals, and was therefore omitted. The broad and strong hydroxyl (OH) stretching band of water dominates the high wavenumber region, i.e., 2800 to 3800 cm\({}^{-1}\). This band is commonly deconvolved into several sub-bands, indicating the different hydrogen-bonding states of water molecules [29; 30]: The lower the wavenumber, the stronger the hydrogen bonding [31]. However, even with the naked eye, systematic differences between the three cases can be observed. Pure water (without Al plate) has the strongest hydrogen bonding (HB) network, as indicated by a maximum at the low wavenumber side of the band. The citric acid solution has the weakest HB network, and the sodium sulfate solution is in between the two. The fingerprint region of the spectra shows a common peak between 1630 and 1640 cm\({}^{-1}\), owing to the OH bending vibration of water. The spectrum of the sulfate solution additionally exhibits a strong S = O stretching band at 1091 cm\({}^{-1}\). The citric acid solution shows a multitude of peaks in the fingerprint region. The most dominant ones appear at 1713 and 1222 cm\({}^{-1}\), and can be assigned to the carboxylic acid groups [32].  
Figure 1: Schematic of the experimental procedure. IRE = internal reflection element; F = force.  
When the Al plate is added, all three spectra change significantly. The intensity of the OH stretching band is reduced by about 20-30%. At first glance, this reduction appears small when we argue that we move from studying the bulk fluid to the interface. However, the working principle of ATR needs to be kept in mind here. The penetration depth \(d_{p}\) of the evanescent field is commonly described as  
\[d_{p} = \frac{\lambda}{2\pi\cdot n_{IRE}\cdot\left(\sin^{2}\alpha-\left( \frac{n_{sample}}{n_{IRE}}\right)^{2}\right)^{1/2}} \tag{1}\]  
with the wavelength \(\lambda\), the reflection angle \(\alpha\), and the refractive indices of the IRE and the sample, \(n_{IRE}\) and \(n_{sample}\), respectively. When we assume that the liquid wets the diamond surface even in the presence of the Al plate, this penetration depth will remain the same. So, the key is to get the metal so close to the IRE surface that the probed molecules represent the thin interfacial layer. This is why a force needs to be applied to the metal plate in order to press it onto the IRE. The effective path length, on the other hand, determines the absolute absorbance, but it is an auxiliary parameter that cannot be measured. It represents the absorption path that would be necessary in a transmission experiment to observe the same attenuation of the radiation [17; 33].  
The shape of the OH stretching band alters such that the high-wavenumber wing becomes dominant in all three cases. Figure 3 illustrates the absorbance normalized bands and their differences in order to emphasize this effect. This change suggests a weakening of the HB network in the presence of the Al surface. The Al plate exhibits a thin oxide layer and the O\({}^{2-}\) ions can act as  
Figure 2: Fingerprint and hydroxyl (OH) stretching region of the IR spectra recorded with and without aluminum plate. (**A**): water; (**B**): aqueous sodium sulfate solution; and, (**C**): aqueous citric acid solution.  
HB acceptors. Moreover, the Al\({}^{3+}\) ions can interact with water oxygen atoms via Coulomb forces. However, the interactions at the surface are weaker than the hydrogen bonds in fully tetrahedrally coordinated bulk water. The change in the molecular interactions is further supported by the peaks in the fingerprint region. The OH bending mode slightly shifts towards higher wavenumber. The same behavior can be noted for the sulfate and the carboxylic acid peaks.