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Near-Infrared Spectroscopic Analysis for Classification

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Near-Infrared Spectroscopic Analysis for Classification CORE Metadata, citation and similar papers at core.ac.uk Provided by Wood and Fiber Science (E-Journal) NEAR-INFRARED SPECTROSCOPIC ANALYSIS FOR CLASSIFICATION OF WATER MOLECULES IN WOOD BY A THEORY OF WATER MIXTURES Sang-Yun Yang Graduate Research Assistant Department of Forest Sciences Seoul National University Seoul, Korea 151-921 E-mail: [email protected] Chang-Deuk Eom Research Scientist Korea Forest Research Institute Seoul, Korea E-mail: [email protected] Yeonjung Han PhD Candidate E-mail: [email protected] Yoonseong Chang Graduate Research Assistant E-mail: [email protected] Yonggun Park Department of Forest Sciences Seoul National University Seoul, Korea 151-921 E-mail: [email protected] Jun-Jae Lee Professor E-mail: [email protected] Joon-Weon Choi Associate Professor E-mail: [email protected] Hwanmyeong Yeo*{ Associate Professor Research Institute for Agriculture and Life Sciences Department of Forest Sciences Seoul National University Seoul, Korea, 151-921 E-mail: [email protected] (Received May 2013) * Corresponding author { SWST member Wood and Fiber Science, 46(2), 2014, pp. 138-147 # 2014 by the Society of Wood Science and Technology Yang et al—NIR SPECTROSCOPIC ANALYSIS FOR WOOD WATER MOLECULES 139 Abstract. This study was conducted to analyze the mechanism of moisture adsorption–desorption in wood using near-IR (NIR) spectroscopy. NIR spectra reflected from moist wood were acquired, and spectra in the range from 1800-2100 nm, which were sensitive to water variation, were decomposed into three different components according to the Buijs and Choppin theory. It is assumed that the three components represent three types of bound water: water molecules without -OH groups engaged in hydrogen bonds (S0), water molecules with one -OH group engaged in a hydrogen bond (S1), and water molecules with two -OH groups engaged in hydrogen bonds (S2). Ratios of the decomposed spectra of NIR absorbed by each type of water molecule were analyzed during changes in water adsorption– desorption states. Through this analysis, a sorption model for predicting the structural state of each water component in wood was constructed. This model may be used to explain the effect of each water component on the occurrence of hysteresis as well as the transient state between bound water and free water. Based on the model, it was concluded that the monomolecular water layer in yellow poplar wood formed below approximately 8% MC during adsorption. Additionally, the phenomenon of hysteresis was demonstrated by the difference between the ratios of the S2 components in desorption and adsorption. Keywords: Water in wood, near-infrared spectroscopy, water mixture model, Buijs-Choppin theory. INTRODUCTION desorption/adsorption process. However, this hypothesis has not been clearly demonstrated Moisture control is essential for the rational use at the molecular level because of a lack of of wood, which is a representative hygroscopic appropriate analytical techniques. material. Moisture in wood is one of the most important factors that determines its physico- In this study, near-IR (NIR) spectroscopy was chemical properties, such as specific gravity, used for interpreting the states of water in wood. size, strength, and optical and electrical proper- The NIR absorption bands of pure water are ties. To analyze and control these properties, known to appear at 960, 1063, 1450, and many studies have been conducted on water in 1912 nm (Schwanninger et al 2011). We wood (Siau 1995; Jung et al 2008). interpreted water states in the adsorption– desorption processes on the order of hydrogen In general, water within wood is classified as bonding by analyzing the NIR absorption band, free or bound. Free water is liquid-state water including the 1912-nm wavelength. found in lumens or intercellular spaces. Because this water exists in wood just by capil- lary force and is not chemically associated with MATERIALS AND METHODS the cell wall, variations in free water have little effect on physical and chemical properties of Sample Preparation wood. Conversely, water that is strongly associ- Yellow poplar (Liriodendron tulipifera) was ated with the cell wall in wood is classified as used in these experiments. The specimen was bound water. Bound water adsorbed by wood is cut into a small rectangular plate with dimen- held by intermolecular forces between water sions of 50 Â 50 Â 10 mm (radial, tangential, molecules and the major chemical components and longitudinal directions). of wood, such as cellulose, hemicellulose, and lignin. The FSP occurs when the sorption sites in wood are fully filled with water molecules. Isothermal Adsorption–Desorption Below the FSP, the physicochemical properties Experiments of wood vary significantly with changes in moisture content (Stamm 1964; Skaar 1988; To analyze water states in the adsorption– Eom et al 2013). Many researchers have posited desorption process, specimens were first dried that the reactivity of hydroxyl groups in the cell in an oven. Using the schedule in Table 1, speci- wall and dipole interactions between water mol- mens were humidified at 12 different RH condi- ecules play a significant role in the moisture tions in the adsorption process (0-100% RH) and 140 WOOD AND FIBER SCIENCE, APRIL 2014, V. 46(2) Table 1. Temperature and RH conditions used in the isothermal adsorption–desorption experiment. Temperature (C) RH (%) 25015304050607080909599100 in the desorption process (100-15% RH) until spectrum, which is specially absorbed to or reaching equilibrium moisture content (EMC) reflected from pure water, consists of four for each except 100% RH. Because it is difficult major absorption bands at 961, 1063, 1450, and to achieve 0% MC in a thermohygrostat, speci- 1912 nm (Maeda et al 1995). Similarly, water in mens were dried in a desiccator. wood also absorbs NIR spectrum around the four major absorption bands. To analyze the depen- After completing the adsorption process, the dence of the state of water in wood on moisture thermohygrostat maintained 100% RH to supply content, NIR absorption spectra at each moisture supersaturated moist air for several hours. content were decomposed. Although the spec- Supersaturated air has greater vapor pressure trometer acquired NIR spectra from 1000 to than saturated water vapor. We assumed that 2400 nm, only the water absorption combination residual water vapors in the supersaturated air band from 1800 to 2100 nm was extracted were transformed to liquid water droplets on the for analysis. wood specimen surface. The desorption process was performed by reversing this procedure. Generally, in absorbance spectra, intensity means the amount of electromagnetic wave energy that was absorbed by water molecules. Acquisition of Near-IR Spectra Each ratio of energy absorbed at the four major For measuring NIR spectra, an optical probe-type absorption bands by three different water com- NIR spectrometer (NIR Quest 256-2.5; Ocean ponents (S0, S1, and S2) is the same. However, Optics, Dunedin, FL) was used in this study. The in this study, the highest intensity absorption optical probe, connected to a tungsten–halogen band among those bands was chosen and used lamp (20 W) light source, was composed of for classifying the type of water components, seven optical fibers. The outer six optical fibers because a high-intensity absorption band has illuminated the surface of the specimen with NIR more data and bigger variance than a low- rays, and the central optical fiber delivered the intensity absorption band. Spectra in the range from reflected rays to the spectrometer. 1800 to 2100 nm, which was the highest NIR absorbance intensity (Fig 1b), was decomposed NIR reflectance spectra were acquired at the for classifying the type of water components in same position of the specimen when the speci- this study. men reached EMC during the adsorption– desorption processes. For decreasing variations To analyze the structure of water in wood, we in moisture content on the surface of the speci- had to choose a model of water structure men caused by factors such as evaporation, NIR because there has been controversy between spectra were acquired in a thermohygrostat. To two liquid water structure theories: the mixture enhance the signal-to-noise ratio, 15 scans were and continuum models (Falk and Ford 1966). averaged. NIR spectra were acquired at a spec- The continuum model describes liquid water as a tral resolution of 6.3 nm. complete water molecule network fully hydrogen- bonded with different hydrogen bond energies and geometries. This model explains that the Chemometric Analysis of Near-IR Spectra average strength of hydrogen bonds in water is In this study, we analyzed the states of water in weaker than that in ice because of random distor- wood using NIR spectra, monitoring variations tion and elongation. In contrast, the mixture in the water absorption band of wood. An NIR model describes liquid water as an equilibrium Yang et al—NIR SPECTROSCOPIC ANALYSIS FOR WOOD WATER MOLECULES 141 mixture composed of different numbers of hydro- gen bonds per molecule. This model explains that each water molecule has a different hydrogen bonding energy. Additionally, the proportion of each number of hydrogen bonds at particular temperature is maintained at an equilibrium state in this model. Although these two models still conflict with each other, we applied a water mix- ture model that is adjusted to the classical adsorp- tion model of wood; this concept has been researched extensively in water structure analysis (Maeda et al 1995; Segtnan et al 2001). Buijs and Choppin (1963) proposed that water molecules could be classified into three catego- ries: water molecules without hydrogen bonds (S0), water molecules with one -OH group engaged in a hydrogen bond (S1), and molecules with two -OH groups engaged in hydrogen bonds (S2). In fact, a water molecule is able to engage in four hydrogen bonds. However, because the hydrogen bonds involving the free orbitals of oxygen merely affect the vibrational energy of the water molecule, water molecules could be defined using just three categories.
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