Submitted to: Applied Spectroscopy
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/2/2010
Publication Date: 12/3/2010
Citation: Liu, Y., Gamble, G.R., Thibodeaux, D.P. 2010. Two-dimensional attenuated total reflection infrared correlation spectroscopy study of the desorption process of water-soaked cotton fibers. Applied Spectroscopy. 64(12):1355-1363. Interpretive Summary: Excessive rainfalls during crucial harvest season, as those in 2009 crop year, not only compromise cotton crop yield but also hurt fiber qualities. In this scenario, unpicked cotton fibers in the plants might undergo several physical and chemical changes that can affect their post-harvest qualities. The effect of water on cotton fiber structure and physical properties has been investigated considerably by various techniques, including microscopy, differential scanning calorimetry (DSC), x-ray diffractometry, 13C nuclear magnetic resonance (NMR) spectroscopy, and infrared (IR) spectroscopy. Among them, IR technique is preferred, partly due to its sensitivity to local molecular rearrangement caused by inter- and intra- molecular hydrogen bonding network inside cotton cellulose, and partly due to its potential application in assessing the cotton qualities. To facilitate the interpretation of overlapped cellulose IR spectra, two-dimensional (2D) correlation spectroscopy, a universal and modern technique of spectral analysis, has been attempted. The results provided the insights about water content-dependent intensity variations not readily accessible from one-dimensional IR spectra, and revealed remarkable differences responding to water loss between the hydrated and dehydrated fibers. The outcome provides cotton fiber and natural polymer researchers a new sight in understanding the changes of cotton fibers at rainy environment before harvesting.
Technical Abstract: Two-dimensional (2D) correlation analysis was applied to characterize the ATR spectral intensity fluctuations of native cotton fibers with various water contents. Prior to 2D analysis, the spectra were leveled to zero at the peak intensity of 1800 cm-1 and then were normalized at the peak intensity of 660 cm-1 to subjectively correct the changes resulting from water diffusion in fibers and resultant density dilution. Next, a new spectral set was subjected to principal component analysis (PCA) and two clusters of hydrated (=13.3%) and dehydrated (<13.3%) fibers were obtained. Synchronous and asynchronous 2D correlation spectra from individual ATR spectral set enhanced spectral resolution and provided insights about water content-dependent intensity variations not readily accessible from one-dimensional ATR spectra. The 2D results revealed remarkable differences corresponding to water loss between the hydrated and dehydrated fibers. Of interest were that: (1) the intensity of the 1640 cm-1 water band remains in a steady state for hydrated fibers but decreases for dehydrated fibers, (2) during the desorption process of adsorbed water, small and water-soluble carbonyl species (i.e., esters, acids, carboxylates, and proteins) begin to accumulate on the cotton surface, resulting in possible changes in the coloration and surface chemistry of native cotton fibers that were rained on prior to harvesting, (3) intensities of bands in the 1200-950 cm-1 region exhibit a more apparent intensity increase than those in the 1500-1200 cm-1 region, indicating the sensitivity of the 1200-950 cm-1 IR region to intra- and inter- molecular hydrogen bonding in fiber celluloses, and (4) the 750 cm-1 band, ascribed to unstable Ia phase in amorphous regions, might originate from the cellulose-water complex through hydrogen bonding.