ATR prism

Arachidic acid monolayers were prepared from a benzene solution on the water subphase of pH5.8(pure water) and 12.6(adjusted by addition of NaOH) at Tsp of 303 K below Tm(=328 K) of the monolayer [31]. The ionic dissociation state of hydrophilic group was estimated on the basis of the stretching vibrations of carbonyl and carboxylate groups by Fourier transform-infrared attenuated total reflection, FT-IR ATR measurements. 70 arachidic acid monolayers were transferred on germanium ATR prism, resulting in the formation of the multi-layered film. Transfer on the prism was carried out at surface pressures of 25 or 28 mN-nr1. Infrared absorption measurements revealed that almost carboxylic groups of arachidic acid molecules did not dissociate on the water subphase of pH5.8, whereas all carboxylic groups dissociated as carboxylate ions on the water subphase of pH 12.6. 

Fig. 1.9 For in situ IR vibrational characterization of an electrochemical interface the silicon electrode in the double O-ring cell has to be shaped as an ATR prism.

A special O-ring cell design is needed for in situ infrared (IR) vibrational characterization of an electrochemical interface. The absorption of one monolayer (i.e. <1015 cm 2 vibrators) can be measured if the silicon electrode is shaped as an attenuated total reflection (ATR) prism, which allows for working in a multiple-in-ternal-reflection geometry. A set-up as shown in Fig. 1.9 enhances the vibrational signal proportional to the number of reflections and restricts the equivalent path in the electrolyte to a value close to the product of the number of reflections by the penetration depth of the IR radiation in the electrolyte, which is typically a tenth of the wavelength. The best compromise in terms of sensitivity often leads to about ten reflections [Oz2]. 

We have used FTIR-ATR extensively to study SA monolayers (e.g., 1), and multilayers of alkyltrichlorosilanes (11, 39,40). Figure 8a presents the bulk (CC14 solution) IR spectrum of 1, while Figures 8b, and 8c, present the grazing angle and ATR spectra of a monolayer of 1 on aluminized silicon, and silicon ATR prism, respectively. 

Figu re 10.1 Schematic diagram of the optical arrangement using an inverted ATR prism. 

In ATR, a beam of infrared light is totally reflected inside a specially cut infrared transparent material that has a high index of refraction. Typical materials used for ATR prisms are Ge, Si, and ZnSe. Because the index of refraction differs between the polymer and the prism, an evanescent wave penetrates the polymer if it intimately contacts the prism. The infrared radiation will interact with molecular vibrations in the same manner as in conventional infrared spectroscopy. The amplitude of the evanescent wave decays exponentially from the surface, so the depth of penetration is arbitrarily taken as the point where the amplitude decays to 1/e (37%) of its initial value. The depth of penetration depends on the ratio of the refractive indices between the polymer and the prism, the angle of incidence, and frequency of radiation in the following manner (Ishida, 1987) ... 

Although ATR has been used to quantify the variation in composition at the surface in TPEs (Sung and Hu, 1980), a related utility is its ability to monitor in situ processes such as reaction injection molding (RIM) (Ishida and Scott, 1986) and protein adsorption onto a polyurethane substrate (Pitt and Cooper, 1986). In the latter, the effect of shear rate on the kinetics of protein adsorption and desorption from phosphate-buffered saline (PBS) was studied in a specially designed flow cell. A very thin film of the commercial MDI-ED-PTMO polyurethane Biomer was cast from solution onto a Ge ATR prism. The thickness of the film was less than the penetration depth so the protein concentration could be monitored after the infrared absorption of the polymer... 

The application of IR spectroscopy to organic sorbates has been primarily limited to gas-phase a orption kinetics and gas-phase catalysis (21). The usefrilness of IR for investigating sorption/desorption processes in situ, has been demonstrated for the selectivity of conformer adsorption at mineral surfaces (22). Strong absorption of H O vibrational modes by IR radiation has been a major hindrance in the application of IR spectroscopy to study organic sorption at aqueous-mineral interfaces. Attenuated total reflectance-IR (ATR-IR) spectroscopy and the use of DjO in the aqueous phase minimizes the water absorption problem. Figure 3 details the ATR setup used in this study. Clay pastes were loaded into Teflon plaques and clamped to both sides of a vertical ATR prism. Silicon sealant around the edges of the plaques prevented water evaporation during extended data collection times (up to 2 days). The area of the D O... 

Figure 3.6. Dispersion curves of surface poiariton (soiid iine) at interface of inactive and active media characterized by ei and sz, respecrtiveiy. Short-dashed iine dispersion iine of incident photon in inactive medium with iong-dashed iine in ATR prism with spr > j-i.

Figure 10.7 (a) A plot of log (IR band intensity) for on-top hydrogen versus electrode potential and (b) a plot of log (current density) for the HER versus log (IR band intensity). These plots have been used in the analysis of the HER mechanism (see text). Data recorded for polycrystalline Pt (on the ATR prism) in 0.5 M H2SO.f ( ) or 3 M H2SO.f (o). Reprinted from Ref [15] with permission from Elsevier. 

Fig. 2.12 Schematic diagram of an ATR prism, d denotes the evanescent wave penetration depth into the sample. ni and n2 are the indexes of refraction of the prism and sample, respectively. X is the light wavelength and 9 its the angle of incidence...

Common materials for ATR prisms are ZnSe, diamond, Ge, and Si. The choice of crystal will depend on the spectral range, chemical compatibility with the sample, and pressure between sample and prism to be used for the measurements. 

Figure 2.13 Schematic representation of an ATR prism showing the principle of the internal reflection sampling technique. See text for detaUs.

Figure 9.28 (A) Schematic representation of PDMS microfluidic device on ZnSe ATR prism and mixing setup. (B) ATR FT-IR images (a-c) show the abundance of DjO,...

The materials most commonly employed as the ATR prisms are KRS-5, germanium, zinc selenide, silicon, silver bromide, synthetic sapphire and diamond (the latter being used mainly in the far infrared region). In Table 1 are shown the typical depths of penetration of the infrared beam for germanium and KRS-5 of different geometries, when the sample used is polytehylene. 

Experimentally, certain precautions are needed to produce good quality ATP. spectra. The surfaces of the ATR prism should be clean and highly polished dul1 surfaces will result in loss of optical throughput. The sample under study should make intimate and uniform contact with the crystal, since the depth of penetration depends on these factors as well. It is particularly important to maintain the uniformity of contact if further data processing, such as absorbance subtraction, and quantitative measurements are to be made from the ATR spectra. Reproducible contact between the crystal and the sample can be ensured by employing a torque wrench set at a constant value to press the sample against the crystal. From the above discussion, it is also obvious that the sample should be pliable, and have a smooth surface. 

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