Adsorptive and capillary condensed water in biological material

Abstract

Biological material such as wood is a typical hygroscopic material with porous structure. It exhibits the characteristic behaviour of swelling accompanied by remarkable changes in mechanical or physical properties (for example, modulus of elasticity, dielectric constant etc.) when adsorbing or desorbing water. However, the circumstances of water in wood have never been thoroughly investigated [1±3]. We tried to clarify this issue by sophisticated analysis of the near infrared re ectance spectra where the moisture content of a sample was varied gradually. The near infrared spectrum is resolved into three component bands (S0, S1 and S2) according to the hydrogen bonding formation of water molecules [4, 5]. Plural hysteresis loops appear in an adsorption±desorption isotherm related to the areal intensity of the difference-near-infrared spectra of S0, S1 and S2. They clearly re ect the presence of capillary condensed water as distinguished from adsorptive water, variation of which with relative vapour pressure depends on the component bands. The adsorption±desorption mechanism can be appreciated in consideration of the hydrogen bonding formation of water molecules. Accounting for the hysteresis loop in the adsorption±desorption isotherm can give clues to the above problem. The loop is not necessarily closed at relative vapour pressure (P=P0) of 0.3 or less, at which, theoretically, capillary condensed water should not exist. Thus, the idea of intercalation is considered. The water, which adsorbs the adsorption sites originating from the rupture of hydrogen bonding of potentially sorptive hydroxyl groups in amorphous regions of wood, is mainly related to the adsorption±desorption phenomena, i.e. intercalation contributes to the hysteresis loop. However, the problem is not so simple. Capillary condensed water is also believed to arise above P=P0 0:9 [6, 7], judging implicitly from the absence of heat of adsorption and swelling near P=P0 ˆ 1. However, the presence of pores of diameter 3±8 nm or tens of nanometres in the cell wall has been revealed [8±10]. Applying Kelvin's law to a pore of diameter 3 nm, capillary condensed water exists at P=P0 0:5, which is signi®cantly lower than 0.9. Conclusively, we cannot grasp the essence of the adsorption±desorption mechanism until we ®nd an interpretation that is not inconsistent with the presence of adsorptive water related to the swelling of the sample and capillary condensed water determined by the pore size distribution in the cell wall. Now, we propose a new explanation for this problem using the mixture model of water, in which water molecules can be resolved into three components: free water molecules (S0), molecules with one OH engaged in hydrogen bonding (S1) and molecules with two OH engaged in hydrogen bondings (S2) [4, 5]. Near infrared spectroscopy [11, 12] was adopted for detecting the three components. The differences in absorbance between the spectra obtained at each step of moisture content of Sitka spruce and from the oven-dried state were found, then the simplex method [13] was applied to separate the difference spectrum, assuming that each component had Lorentz distribution. Fig. 1 shows the difference-near-infrared spectrum of water in Sitka spruce from P=P0 ˆ 0:55 to P=P0 ˆ 0:03 (solid line) and resolution in the three component bands (broken lines) at 25 8C. The sample dimensions were 30 mm 3 30 mm 3 2 mm. This study deals with analysis of the absorption band around 1930 nm, which is the combination band i2 ‡ i3 in water [5]. An integrating sphere was utilized to collect the diffusely re ected light from the sample. Fig. 2 indicates the variation of areal intensity of the near infrared spectra for the sum of the three component bands and for the three components individually with relative vapour pressure (P=P0). Each areal intensity was calculated within the range