Transformation of the Near-Infrared Bands of Cellulose Surface Hydroxyls under the Influence of Adsorbed Water Molecules

Abstract

Near-infrared absorption spectra of sulfite cellulose with a varied adsorbed water content were studied. Analysis of the spectra has been made on the basis of previously published relations and results of the cellulose surface hydroxyls' interaction with absorbed water. Spectra of the OH overtone region of 1.3–1.65 μm were deconvoluted to components of cellulose volume and surface hydroxyls as well as to those of absorbed water. Besides the 1.53–1.55 μm component of “bound” water, the 1.42–1.44 μm component of “free” water starts to grow as the surface coverage exceeds 1.5. Comparison of the peak positions of these water components with those of liquid water suggests that bound water is constituted of hydrogen-bonded molecules. The 1.42–1.44 μm component of free water implies nonhydrogen-bonded and partially hydrogen-bonded molecules. The whole 1.363 μm band and a part of the 1.424 μm band of surface hydroxyls turn into the 1.52–1.53 and 1.57–1.59 μm peaks, respectively, after attachment of H2O molecules. The red shift of the surface bands by approximately 780 cm−1 caused by the adsorbate is consistent with a shift of the fundamental OH band of silica gel. The possible chemical, morphological, and physical causes of the two 1.363 and 1.424 m surface hydroxyl bands are discussed.