Proceedings e report - Firenze University Press

NIR SPECTROSCOPIC MONITORING OF WATER ADSORPTION/DESORPTION PROCESS
3. Result and Discussion
3.1. Decomposition of NIR spectra
Variation of the NIR spectra with water adsorption onto wood was analyzed basing on three structural
forms of water molecules: free water molecules (S0), molecules with one OH group engaged in
hydrogen bonding (S1) and molecules with two OH groups engaged in hydrogen bonding (S2) [4].
Curve fitting was undertaken to separate the NIR difference spectra using OPUS (ver. 4.0, Bruker
Optik GmbH). Fig. 1 shows the difference and the decomposed spectra of water in the modern sample.
An areal integral for each component, AI(Sn), was calculated using Eq. (1), where Sn is one of the
structural form of water molecule (n=0,1,2), and DA(ν) is the difference absorbance at a wavenumber
ν.
AI( S n ) = DA(
ν ) dν
(1)
∫
3.2. Adsorption/desorption isotherm for the modern and archaeological wood
Fig. 2 shows the adsorption/desorption isotherms of the modern and archaeological wood samples.
Both samples show hysteresis loops. The equilibrium moisture content of the archaeological samples
is reduced compared to modern samples at each RH level. This is due to the decrease with ageing of
hemicellulose of which OH groups form hydrogen bonding with ambient water molecules.
Difference absorbance
0.6
0.4
0.2
0
5400
S1
S0
5200
Difference
spectrum of water
in modern wood
S2
5000
Wavenumber(cm -1 )
Fig. 1. Difference and decomposed spectra of water in
the modern hinoki wood sample.
4800
200
Moisture content (%)
20
10
0
Modern wood
Archaeological wood
Desorption
Adsorption
20 40 60 80 100
RH (%)
Fig. 2. Adsorption/desorption isotherm for the modern
and archaeological hinoki wood samples.
3.3. Spectroscopic interpretation of the mechanism of water adsorption by wood
Fig. 3 plots the variation of the areal integral (AI(Sn)) (a) and the peak wavenumber (νp(Sn)) (b) for
three water components in the NIR difference spectra of the modern sample versus RH. The
archaeological sample shows the same tendencies. For the sake of simplicity, we assumed that an areal
integral derived from the spectral decomposition is proportional to the amount of the adsorbed water
molecules. S1 and S2 components show the hysteresis loop, which is not evident for the S0 component .
The variation of AI(Sn) and νp(Sn) can be explained by classifying the RH range into following three
stages: Stage I (RH=0-40 %), Stage II (RH=40-90 %) and Stage III (RH=90-100 %). Table 1
summarizes the spectroscopic characteristics.
In Stage I, the water molecules interact with wood substance more strongly than in the other stages
since a monomolecular layer of water is formed. It is therefore suggested that most of monomolecular
layer is composed of the S2 component. The wavenumbers, νp(S0) and νp(S1) showed almost the same
value both in the adsorption and desorption processes at Stage I. This suggests that the water
molecules adsorbed in wood substance consist predominantly of two of the structural forms, namely,
S2 and S0 (or S1) components. The S0 component is very likely to exist when adjacent water molecules
are sparse so that the S0 component increased with an increase of RH at this stage. On the other hand,
the S0 component decreased gradually at the RH more than 40 % possibly because of an expansion of
the upper layers.
In Stage II, the water molecules interact with adjacent water molecules, because two or more layers
(multilayers) are formed on wood surface. The areal integral, AI(S0) decreased with an increase of RH.
This might be due to the three-dimensional (inter-layer) expansion of water molecules and the increase
of bonding force within water molecules. The shift of νp(S0) and νp(S2) to higher and lower wave
numbers, respectively, may also be caused by the inter-layer expansion of water molecules.