IR external reflectance

For many years, IRRAS has been successfully applied to the study of thin films adsorbates on metal surfaces [36], In the case of monolayers deposited on metal surfaces, an IR external reflection spectrum is obtained by reflecting the incoming radiation from the three-phase ambient-adsorbate-substrate system, measuring the reflected intensity as a function of wavelength, and then ratioing... 

Fig. 5. IR external reflection absorption spectra of a 7 1 DPPC-d62 DPPG+ 5 wt.% SP-B/C monolayer film collected at surface pressures from 6 to 60mN/m. (A) CH2 stretching region between 3000 and 2800 cm-1. (B) CD2 stretching region between 2250 and 2050 cm-1. Taken from Ref. [65] with permission from Biophysical Society.

The external reflection of infrared radiation can be used to characterize the thickness and orientation of adsorbates on metal surfaces. Buontempo and Rice [153-155] have recently extended this technique to molecules at dielectric surfaces, including Langmuir monolayers at the air-water interface. Analysis of the dichroic ratio, the ratio of reflectivity parallel to the plane of incidence (p-polarization) to that perpendicular to it (.r-polarization) allows evaluation of the molecular orientation in terms of a tilt angle and rotation around the backbone [153]. An example of the p-polarized reflection spectrum for stearyl alcohol is shown in Fig. IV-13. Unfortunately, quantitative analysis of the experimental measurements of the antisymmetric CH2 stretch for heneicosanol [153,155] stearly alcohol [154] and tetracosanoic [156] monolayers is made difflcult by the scatter in the IR peak heights. 

In 1960, Harrick demonstrated that, for transparent substrates, absorption spectra of adsorbed layers could be obtained using internal reflection [42]. By cutting the sample in a specific trapezoidal shape, the IR beam can be made to enter tlirough one end, bounce internally a number of times from the flat parallel edges, and exit the other end without any losses, leading to high adsorption coeflScients for the species adsorbed on the external surfaces of the plate (Irigher than in the case of external reflection) [24]. This is the basis for the ATR teclmique. 

External reflectance. The most commonly applied in situ IR techniques involve the external reflectance approach. These methods seek to minimise the strong solvent absorption by simply pressing a reflective working electrode against the IR transparent window of the electrochemical cell. The result is a thin layer of electrolyte trapped between electrode and window usually 1 to 50 pm. A typical thin layer cell is shown in Figure 2.40. 

Figure 2.40 Schematic representation of the external reflectance cell design commonly employed in in situ IR experiments, if the working electrode is a semiconductor, then the semiconductor/ electrolyte interface can be studied under illumination with, for example, UV light by directing the beam perpendicular to the IR beam, as shown.

Before in situ external reflectance FTIR can be employed quantitatively to the study of near-electrode processes, one final experimental problem must be overcome the determination of the thickness of the thin layer between electrode and window. This is a fundamental aspect of the application of this increasingly important technique, marking an obstacle that must be overcome if it is to attain its true potential, due to the dearth of extinction coefficients in the IR available in the literature. In the study of adsorbed species this determination is unimportant, as the extinction coefficients of the absorption bands of the surface species can be determined via coulometry. 

A majority of traditional NIR measurements are made on solid materials and these involve reflectance measurements, notably via diffuse reflectance. Likewise, in the mid-IR not all spectral measurements involve the transmission of radiation. Such measurements include internal reflectance (also known as attenuated total reflectance, ATR), external reflectance (front surface, mirror -style or specular reflectance), bulk diffuse reflectance (less common in the mid-IR compared to NIR), and photoacoustic determinations. Photoacoustic detection has been applied to trace-level gas measurements and commercial instruments are available based on this mode of detection. It is important to note that the photoacoustic spectrum is a direct measurement of infrared absorption. While most infrared spectra are either directly or indirectly correlated... 

Figure 3. External reflection IR spectrum of an 10-A film of poly (acrylic acid) on native-oxide-covered, evaporated aluminum. The jagged line is the unsmoothed spectrum at 2 cm 1 resolution. The major peak assignments are 1740 cm 1, unionized carboxylic acid, C = O stretch and 1620 cm 1, the carboxylate ion asymmetric stretch 1475 cm 1, CH2 bending. (Reproduced, with permission, from Ref.

Reflectance. Both internal and external reflectance spectroscopy are relatively simple experiments to perform. Commercially available attachments for standard UV-visible spectrometers can be used. For films with strong electronic transitions reasonable spectra can be obtained. The theory for external and internal reflectance is the same as that for the IR and can be found elsewhere (2, 37). The techniques have not been very popular in their applications to surface analysis. The major reason appears to be... 

With the development of the external reflectance IR technique for observing monolayers in-situ at the A/W interface, we now have the ability, for the first time, to directly compare the structure of the monolayer film at the A/W interface with the monolayer transferred to a solid substrate. In order to determine whether these transfer artifacts occur for the DPPC monolayer, we have studied the structure of DPPC when transferred to Ge ATR crystals. Figure 6 is the pressure-area curve of the DPPC monolayer on which are indicated the points at which film transfer was made. Specific surface pressures of transfer were chosen in order to insure that transferred monolayers were studied in the LE, LE-LC and LC-SC regions, to provide a basis of comparison with the in-situ monolayers. 

Modern methods of vibrational analysis have shown themselves to be unexpectedly powerful tools to study two-dimensional monomolecular films at gas/liquid interfaces. In particular, current work with external reflection-absorbance infrared spectroscopy has been able to derive detailed conformational and orientational information concerning the nature of the monolayer film. The LE-LC first order phase transition as seen by IR involves a conformational gauche-trans isomerization of the hydrocarbon chains a second transition in the acyl chains is seen at low molecular areas that may be related to a solid-solid type hydrocarbon phase change. Orientations and tilt angles of the hydrocarbon chains are able to be calculated from the polarized external reflectance spectra. These calculations find that the lipid acyl chains are relatively unoriented (or possibly randomly oriented) at low-to-intermediate surface pressures, while the orientation at high surface pressures is similar to that of the solid (gel phase) bulk lipid. 

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