A Study on Water Adsorption onto Microcrystalline Cellulose by Near-Infrared Spectroscopy with Two-Dimensional Correlation Spectroscopy and Principal Component Analysis

Abstract

Water adsorption onto microcrystalline cellulose (MCC) in the moisture content (M_c) range of 0.2–13.4 wt % was investigated by near-infrared (NIR) spectroscopy. In order to distinguish heavily overlapping O–H stretching bands in the NIR region due to MCC and water, principal component analysis (PCA) and generalized two-dimensional correlation spectroscopy (2DCOS) were applied to the obtained spectra. The NIR spectra in four adsorption stages separated by PCA were analyzed by 2DCOS. For the low M_c range of 0.2–3.1 wt %, a decrease in the free or weakly hydrogen-bonded (H-bonded) MCC OH band, increases in the H-bonded MCC OH bands, and increases in the adsorbed water OH bands are observed. These results suggest that the inter- and intrachain H-bonds of MCC are formed by monomeric water molecule adsorption. In the M_c range of 3.8–7.1 wt %, spectral changes in the NIR spectra reveal that the aggregation of water molecules starts at the surface of MCC. For the high M_c range of 8.1–13.4 wt %, the NIR results suggest that the formation of bulk water occurs. It is revealed from the present study that approximately 3–7 wt % of adsorbed water is responsible for the stabilization of the H-bond network in MCC at the cellulose–water surface.