Cylindrical internal reflection technique

Final justification for using terms such as inner- or outer-sphere awaits direct spectroscopic confirmation. Electron Spin Resonance, Mossbauer, and Fourier Transform Infrared-Cylindrical Internal Reflection Spectroscopic techniques are being used to establish the structure of surface complexes (see, e.g., McBride, Ambe et al., and Zeltner et al., this volume). The potential for using EXAFS (extended x-ray absorption fine structure) to establish the type of surface complex for Pb + adsorbing onto goethite is currently being undertaken in our laboratory. 

Titration calorimetry and cylindrical internal reflection-Fourier transform infrared (CIR-FTIR) spectroscopy are two techniques which have seldom been applied to study reactions at the solid-liquid interface. In this paper, we describe these two techniques and their application to the investigation of salicylate ion adsorption in aqueous goethite (a-FeOOH) suspensions from pH 4 to 7. Evidence suggests that salicylate adsorbs on goethite by forming a chelate structure in which each salicylate ion replaces two hydroxyls attached to a single iron atom at the surface. 

To put things into perspective, we. can broadly classify these analytical methods into bulk, dry surface, and in situ interfacial techniques. This chapter focuses on the last category, illustrating two in situ techniques used to study anion binding at the goethite (a-FeOOH)-water interface titration calorimetry and cylindrical internal reflection-Fourier transform infrared (CIR-FTIR) spectroscopy. In fact, CIR-FTIR could prove to be extremely powerful, since it allows direct spectroscopic observation of ions adsorbed at the mineral-water interface. 

The ATR technique is now routinely used for IR spectroscopy as it allows measurement of spectra for a variety of sample types with minimal preparation. The crystals employed are generally prismatic in shape, allowing contact of a flat surface with the sample. The ATR method was first adapted for HP IR spectroscopy by Moser [29-33], who realised that a conventional autoclave could easily be adapted for in situ IR spectroscopy by fitting an ATR crystal of cylindrical cross section. The technique developed by Moser is thus known as cylindrical internal reflectance (CIR) spectroscopy and high pressure cells based upon the CIR method have been commercialised by Spectra-Tech. A typical CIR cell is illustrated in Figure 3.8. 

An infrared spectrum is a plot of percent radiation absorbed versus the frequency of the incident radiation given in wavenumbers (cm ) or in wave length ( xm). A variation of this method, diffuse reflectance spectroscopy, is used for samples with poor transmittance, e.g. cubic hematite crystals. Increased resolution and sensitivity as well as more rapid collection of data is provided by Fourier-transform-IR (FTIR), which averages a large number of spectra. Another IR technique makes use of attenuated total reflectance FTIR (ATR-FTIR) often using a cylindrical internal reflectance cell (CIR) (e.g. Tejedor-Tejedor Anderson, 1986). ATR enables wet systems and adsorbing species to be studied in situ. 

ATR studies of the biocorrosion of submerged copper surfaces have been reported. The IRE of a cylindrical internal reflectance cell (CIRCLE) was coated with a thin copper layer via a vacuum deposition technique (105). The copper layer reduces the sampling depth of the radiation outward from the surface of the IRE. Therefore, the intensity of the water bending band will vary with copper layer thicknesses of 4.1 nm or less. The copper layers were shown to be stable to exposure to water alone, but the presence of acidic polysaccahrides in the water caused a reduction in the copper layer thicknesses (106.107). The adsorption of a model compound, Gum Arabic, onto the coated IRE was detected by increases in the C-O stretching band of the pyranose units near 1050 cm"1 (106). 

Cylindrical internal reflectance (CIR) techniques have been applied to humic and fulvic acids that were size fractionated by using hollow-fiber ultrafiltration methods with cutoffs of 0.1pm and 100,000, 30,000,... 

Cylindrical internal reflectance infrared spectroscopy presents many advantages over conventional infrared techniques for the study of aquatic humic and fulvic materials. Samples can be studied in their natural state and in the aqueous environment from which they are isolated. Sample alterations due to drying and exposure to high pressures in the pellet forming process are avoided. In addition. 

The techniques of Cylindrical Internal Reflectance (CIR) and Diffuse Reflectance spectroscopies are described herein. The CIR phenomenon was employed in three different apparatus. Two different high pressure CIR cells were used to study reactions homogeneously catalyzed by [y-HW2(CO)10]-. Low temperature reactions of Mo and W complexes were studied using an ambient pressure CIR cell. The diffuse reflectance technique was employed to study powdered samples of Ru carbonyl complexes supported on A1203. 

A technique which has proven useful for our studies is that of cylindrical internal reflectance (CIR), coupled with a Fourier transform infrared spectrometer. In this study, an IBM-85 FTIR equipped with either a DTGS (deuterated triglycine sulfate) or MCT (mercury-cadmium-tellurium) detector was used. The infrared radiation is focused by concave mirrors onto the 45° conical ends of a transmitting crystal (Figure 1). The crystal may be made of any material which is optically transparent, has a high mechanical strength and high index of refraction, and is resistant to thermal shock and chemical attack. Suitable materials include ZnS, ZnSe,... 

As discussed in Sections 2.4 and 2.5.4, the spectral contrast can be increased using MIR-OTEs. This approach has been used, for example, in the ATR studies of adsorption of lipid bilayers to Au [481], p-nitrobenzoic acid on Ag and Au [482] and electrochemical reactions on Au [434], Cu [483], steel [484], Pt [485], and iron [486] electrodes. The ATR technique with a Si MIRE covered by Ag and Au films was applied to the study of thiocyanate adsorption on silver and gold [487]. Enhanced SNR has been reported for the MIR in situ spectra of proteins adsorbed onto a Cu-coated cylindrical internal reflection element [488]. Since the optical path of the beam through a MIR OTE strongly depends on the wavelength, optical schemes with a fixed incident beam are not applicable in this case [362],... 

The combination of these two powerful characterisation techniques can provide the ability simultaneously to characterise the molecular distribution and to identify and quantify IR-active functional groups in the distribution. The main problem in linking such technologies is the choice of solvent or eluent used to carry the polymer through the SEC system. Many of the typical SEC solvents have strong absorption bands in wide regions of the infrared spectrum. Developments have centred around the use of low-volume cylindrical internal-reflection flow cells for use in the FTIR system. However, their use is still restricted in many cases to IR-friendly SEC solvents. 

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