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2D NIR Sample-Sample Correlation Study of Phase Transitions of Oleic Acid
while the structure in the temperature range between 30 °C and 55 °C is composed of clusters with a less ordered structure. Above 55 °C oleic acid appears to be an isotropic liquid. These conclusions have been drawn from the studies of oleic acid by NIR, differential scanning calorimetry (DSC), density, viscosity, and self-diffusion.
Figure 9.5 shows NIR spectra in the region of 7600-6600 cm-1 of oleic acid in the pure liquid state over a temperature range of 15-80 °C.16 Bands at 7194 and 7092 cm-1 are assigned to the combination modes of the CH vibrations, while the band at 6920 cm-1 consists of at least two overlapped bands. One is due to the combination mode of CH vibrations, and the other arises from the first overtone of the OH stretching vibration of the monomer. The intensity of the band at 6920 cm-1 increases as a function of temperature, giving good evidence that the acid dimer dissociates into the monometric species even in the pure liquid state. In a plot of the absorbance at 6920 cm-1 versus temperature (not shown), one can see two break points at 30 and 55 °C. All the conclusions regarding the liquid structure were reached from the intensity change of the band at 6920 cm-1. However, the obtained temperature-dependent plot of the intensity at 6920 cm-1 did not always give convincing evidence for the existence of the two break points.
The sample-sample 2D correlation spectroscopy study applied to the whole spectral region of 7600-6600 cm-1 yields more unambiguous evidence for the break points.16 The sample-sample correlation analysis was applied to the NIR spectra in the original form, as well as those after two different kinds of spectral pretreatments. First, the spectra were offset on the higher wavenumber side (around 7500 cm-1) and adjusted with respect to the intensity of the band at
Figure 9.5 NIR spectra in the region of 7600-6600 cm-1 of oleic acid in the pure liquid state over a temperature range of 15-80 °C. (Reproduced with permission from Ref. No. 16. Copyright (2000) American Chemical Society.)
Applications of 2D Correlation Spectroscopy to Basic Molecules
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Figure 9.6 Sample-sample correlation spectrum (A) calculated from the spectra shown in Figure 9.5 and (B) and corresponding slice spectra. (Reproduced with permission from Ref. No. 16. Copyright (2000) American Chemical Society.)
7182 cm-1 measured at 15 °C. In the second pretreatment, both wavenumber sides (the 7600-7400 and 6700-6600 cm-1) were baseline corrected, and only the C-H combination bands and the band due to the first overtone of the monomer O-H group were considered. In the last case the original spectra without any pretreatment were analyzed.
It was found that the analysis based on the raw spectra gives the strongest evidence for two phase transition temperatures at 32 and 55 °C in the sample-sample correlation pattern. Figure 9.6(A) illustrates asynchronous sample-sample correlation spectra calculated from the spectra shown in Figure 9.5. Figure 9.6(B) depicts the corresponding slice spectra. Note that the slice spectra yield unambiguous evidence for the two break points at 32 and 55 °C. The sample-sample correlation can easily be applied to systems where a process similar to the monomerization of a dimer takes place.
9.3 2D NIR CORRELATION SPECTROSCOPY STUDY OF WATER
Studies of water have always been a matter of great interest because the studies are very important not only in basic science but also in a variety of applications. Water is involved in almost all kinds of substances, and the water content and the structure of water in them are often key factors in determining their functions and structure. NIR spectroscopy has been employed to investigate the water content, hydrogen bonds of water, and hydration in various fields such as agricultural and food industries, medical and pharmacological sciences, and polymer and textile industries.17,18,22-25 However, it is still not easy to understand NIR spectra of water completely because water does not exist in single species and water molecules form various cluster structures.
2D NIR Correlation Spectroscopy Study of Water
Figure 9.7 depicts NIR spectra of water in the 900-2500 nm (11 100-4000 cm-1) region.18 The three spectra were measured by using cells with the pathlength of (a) 10, (b) 1.0, and (c) 0.05 mm. An intense tail near 2500 nm is due to the fundamental of the OH stretching modes (v1, v3; see Figure 9.8). Bands near 1910, 1430, and 960nm (5235, 6900, and 10420cm-1) are assigned to v2 + v3, v1 + v3, and 2 v1 + v3 modes of water, respectively. As shown in Figure 9.7, the intensities of the water bands decrease stepwise with the decrease in the wavelength. This means that one can select the spectral region used or the pathlength of a cell when one investigates aqueous solutions using NIR spectroscopy.
NIR spectra of water are very sensitive to temperature and the ions involved. Figure 9.9(A) presents NIR spectra of water measured over the temperature range 5-85 °C at an increment of 5 °C.25 It is noted that the spectrum of water changes