Isotopic exchange in deuterated water (D2O) has been long used in experimental characterization of water-accessible fractions of cellulosics. At room temperature, D2O molecules cannot penetrate into cellulose crystals, but do penetrate into disordered areas, promoting OH → OD exchange in accessible hydroxyl groups. Such disordered regions can therefore be probed selectively by employing room temperature deuteration coupled to measurement techniques that provide isotopic contrast.
Their method combines a mid-infrared spectrometer (Frontier, PerkinElmer, United States) with H2O and D2O vapor generators (GenRH-A, Surface Measurement Systems,UK) . By acquiring time-resolved spectra during deuterium exchange, and analyzing resultant spectra with Principal Component Analysis (PCA), the method obtains OH → OD difference spectra with good repeatability and low spectral noise. Furthermore, the method allows controlling the RH of the specimen atmosphere.
The method is applicable to a wide range of cellulosic specimens . In one example, wide range of celluloses isolated from plants were characterized to investigate the effects of variable contents of xylan. The observed spectral similarities supported the existence of inherent type of polysaccharide disorder quite insensitive to the chemical identity (xylan or cellulose) of hydrated polysaccharides.
Furthermore, cotton fibers were analyzed with polarized infrared, which revealed hydrated cellulose having chains preferentially aligned with crystals but hydroxyls with more randomized orientation. In such hydrated disordered regions, polysaccharide hydroxyls dynamically explore multiple hydrogen bonding configurations.
FTIR analysis was performed in a benchtop mid-infrared Frontier spectrometer having a single bounce Universal Attenuated Total Reflectance (UATR) sample module with diamond/ZnSe crystal and polarization accessory. Both GenRH generators operated at laboratory room temperature under identical conditions of relative humidity (RH), flow, and nitrogen carrier gas. However, one generator operated with deionized water (H2O), while the other one with deuterated water (D2O).
The outlet of each humidity generator was connected to a 3-way valve so that the flow from one generator was directed to a sink while the other was directed to the specimen. The vapor environment in contact with the specimen was isolated from the surroundings by a custom-built stainless steel chamber placed on top of the UATR plate. Deuterium exchange was started by switching the vapor flow(H2O → D2O) to the specimen.
Figure 1: Photographs of Experimental Setup
The following spectra monitored the OH → OD exchange process.
Effect of relative humidity. The developed instrumentation allowed control of the RH of the specimen environment. Thus, RH effects could be explored by analyzing Avicel at 10–60% RH. Two spectral regions show major dependence with RH. The negative O H region(1600–1690 cm−1) becomes more intense at higher RH (Fig. 1),which is simply due to more H2O adsorbed by cellulose. Higher RH also generates lower spectral intensity below 930 cm−1 (Figure 2). The initial rate (see PCA scores in Figure inset) of deuterium exchange increases from 10% to 40% RH, but no significant difference is observed between 40% and 60% RH (40% and 60% scores overlap in Figure 2 inset). Since water molecules are transporters of H/D species, differences in exchange dynamics can be simply explained by the higher amounts of water adsorbed at higher RH.
Figure 2. Results of Avicel analyses performed at 10, 20, 40, and 60% relative humidity.
By analysis of representative cellulosic samples, new insights on molecular and group orientation, water transport, and hydrogen bonding were obtained with the new technique combining FTIR, deuterium exchange and the GenRH series humidity generator.
1. C Driemeier, F M. Mendes, L Y Ling, Hydrated Fractions Of Cellulosics Probed By Infrared Spectroscopy coupled With Dynamics Of Deuterium Exchange, Carbohydrate Polymers 127 (2015) 152–159
About the author:
Jan Gorgol studied Physics at Bristol University followed by a Masters at Brunel University while working with XPS & SEM at the Experimental Techniques Centre. After working extensively in surface science instrumentation globally he now is Product Manager for the GenRH series of humidity generation products at Surface Measurement Systems Ltd.