Location and state of water
The influence of acetylation on the interaction between wood and water was studied using LFNMR relaxometry. Five specimens of each sample type were initially vacuum saturated with water. For this, specimens were placed in round-bottom glass flasks equipped with rubber injection septum, vacuum (about 10–2 mbar) was maintained for 20 min prior to injection of MilliQ water through the rubber septum. Finally, atmospheric pressure was re-established.
Before LFNMR measurement, each specimen was wiped off using a moist cloth (Wettex, Freudenberg Household Products, Norrköping, Sweden) to remove free water from its surface, but without drying the specimen. The specimen was then placed in a pre-weighed NMR tube with a Teflon rod, which both sealed the tube and filled the air space above the specimen and thus limited evaporation. The tube with the specimen and the rod was then weighed again to obtain the vacuum saturated mass of the specimen. The sealed tube was then placed in a LFNMR instrument (Bruker mq20-Minispec, Bruker, Billerica, MA, USA) with a 0.47 Tesla permanent magnet in such a way that the specimen orientation was the same for all measurements. The probe temperature of the LFNMR instrument was maintained at 21 ± 0.5 °C by a water-cooling system, and each sample was allowed to equilibrate to the instrument temperature for 5 min before measurement. The spin–spin relaxation time (T2) was determined using the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence with a pulse separation (τ) of 0.1 ms, 16 000 echoes, 32 scans and a recycle delay of 5 s. The gain was set individually for each specimen and ranged between 80 and 82 dB.
The
T2 decay curves were analysed by multi-exponential decay analysis, which fits a large number of exponential decay functions to the experimentally obtained curve (Istratov and Vyvenko
1999). Each decay function is described by a characteristic time constant (relaxation time) and a pre-exponential coefficient. In this study, 200 decay functions were fitted with pre-defined, logarithmically spaced time constants that covered the time period from the first to last data point, i.e. 0.2 ms to 3.3 s. Fitting is performed with the non-negative least squares algorithm of Lawson and Hanson (
1974) to minimize the statistic
$${\chi }^{2}=\sum _{i=1}^{m}{\left({\textit{E}}_{\rm{\textit{i}}}-\sum _{n=1}^{N}{A}_{n}\rm{e}\rm{x}\rm{p}\left(\frac{-{\textit{t}}_{\rm{\textit{i}}}}{{\tau }_{\rm{\it {n}}}}\right)\right)}^{2}+\frac{1}{\alpha }\sum _{n=1}^{N-2}{\left(2{A}_{n+1}-{A}_{n}-{A}_{n+2}\right)}^{2}$$
(2)
where
m is the number of data points (16 000 echoes),
Ei and
ti are the experimental signal and time, respectively, of the
ith data point,
N is the number of fitted exponential decay functions (200),
An and τ
n are the pre-exponential coefficient and time constant, respectively, of the
nth decay function, and
α is a regularisation parameter that controls the difference between adjacent
An. The result of the analysis can be plotted as a spectrum of pre-exponential coefficients on a logarithmic scale of time constants. Typically, decay curves from LFNMR measurements on water in wood can be described by a limited number of decay functions (Araujo et al.
1992; Beck et al.
2018b; Fredriksson and Thygesen
2017). These will appear in the spectrum as peaks with a characteristic time constant at their apex. The width of the peaks and the general smoothness of the spectrum depend on the value of the regularisation parameter
α. If it is too low the spectrum is smooth and individual peaks cannot be resolved, whereas a too high value of
α will result in a spiky spectrum with multiple small peaks arising from noise in the signal. Finding an appropriate value of
α is therefore important, but it is not a trivial task (Istratov and Vyvenko
1999). A common strategy is to select a value around the threshold between the smooth and spiky domains, which can be found from a plot of
χ2 versus
α. In this study, a value of 10
9 was selected for all samples.
The amount of information that can be extracted from decay curves by multi-exponential decay analysis is limited by the inherent noise in the data. Thus, the analysis can only resolve peaks with characteristic time constants that are spaced farther apart than the resolution limit. This in turn is defined as a ratio of the time constants and depends on the signal-to-noise as (Bertero et al.
1982):
$$\delta =\frac{{\tau }_{i}}{{\tau }_{i+1}}=\rm{e}\rm{x}\rm{p}\left[\frac{{\pi }^{2}}{\rm{a}\rm{r}\rm{c}\rm{o}\rm{s}\rm{h}\left(\pi {\left(\frac{\textit{E}}{\varepsilon }\right)}^{2}\right)}\right]$$
(3)
where
δ is the resolution limit, i.e. the minimum spacing between two adjacent time constants,
τi and
τi+1 with
τi >
τi+1, and
E/
ε is the signal-to-noise ratio. In this study,
E/
ε was determined by the ratio of the initial signal
E1 in the time series to the standard deviation of the signal in the last 500 ms for each decay curve. The signal-to-noise varied in the range 357–533, which corresponds with a resolution limit
δ of 1.99–2.07. The spectra for all samples in this study had peaks with characteristic time constants spaced farther apart than the resolution limit.
Based on these continuous
T2 distributions, the influence of the modifications on water in different locations within the wood structure was evaluated. Since the
T2 is related to the physical environment (surface-to-volume ratio of pores) (Menon et al.
1987) water within cell walls can be separated from water outside of cell walls and capillary water in differently sized voids within the wood structure can be distinguished (Almeida et al.
2007; Araujo et al.
1992; Fredriksson and Thygesen
2017; Labbé et al.
2006; Thygesen and Elder
2008). The
T2 at the maximum intensity of each peak was evaluated, and in those cases where a certain water population was represented by more than one peak (e.g. water in small macro voids), the
T2 was calculated as the exponentially weighted average of the
T2 of the individual peaks. The moisture content represented by each peak,
ωi, was evaluated as (Telkki et al.
2013):
$${\omega }_{i}={\omega }_{\rm{t}\rm{o}\rm{t}}\frac{{S}_{i}}{{S}_{\rm{t}\rm{o}\rm{t}}}$$
(4)
where
Si (–) is the sum of pre-exponential coefficients related to peak
i,
Stot (–) is the sum of all pre-exponential coefficients in the spectrum, and
ωtot (g g
−1) is the total moisture content of the wood specimen determined as:
$${\omega }_{\rm{t}\rm{o}\rm{t}}=\frac{{m}_{\rm{v}\rm{a}\rm{c}}-{m}_{\rm{d}\rm{r}\rm{y}}}{{m}_{\rm{d}\rm{r}\rm{y},0}}$$
(5)
where
mvac (g) is the vacuum saturated mass after modification,
mdry (g) is the dry mass after modification and
mdry,0 (g) is the dry mass before modification which can be calculated from:
$${m}_{\rm{d}\rm{r}\rm{y},0}=\frac{{m}_{\rm{d}\rm{r}\rm{y}}}{1+{R}_{\rm{m}\rm{o}\rm{d}}}$$
(6)
For the uniform modifications, the average mass increase given in Table
1 was used. However, for the interface modifications,
Rmod values for the individual specimens were available and used instead (see Table S2). The moisture content was thus always based on the dry mass before modification,
mdry,0, which enabled comparison between specimens. Note that no correction was made for the pyridine controls where a mass loss was seen. The assignment of peaks to cell wall water was consistently made including only the peak with the shortest
T2, except for uniform modification C where this peak was split in two for two specimens (see Figure S9). The peak with the longest
T2 was assigned to water in cell lumina and all small peaks between the cell wall water peak and the peak representing water in cell lumina were assigned to water in small macro voids.
Moisture exclusion efficiency in the hygroscopic moisture range
The efficiency of acetylation in reducing the cell wall moisture content was evaluated in the hygroscopic moisture range using two sorption balances (DVS Advantage, Surface Measurement Systems Ltd., London, UK) which monitor the mass of a sample with a resolution of 0.1 μg and where different relative humidity (RH) levels are generated by mixing dry and water saturated nitrogen gas (see e.g. Williams (
1995)). One specimen (5 × 5 × 10 mm
3) of each specimen type (Table
1) was vacuum saturated with deionised water. Before each measurement, a sample was prepared by cutting thin pieces with a razor blade from one on these specimens. Excess water was removed on a moist cloth before placing the pieces in the sample pan. For each measurement, the vacuum saturated sample was exposed to the following RH levels: 95, 80, 65, 50, 35, 0, 35, 50, 65, 80, and 95%, i.e. both desorption and absorption isotherms were measured. The change between RH levels was automated using the following predefined hold times: at 35, 50 and 65% RH was 12 h, the duration at 80% was 24 h while the duration at 95% RH was 60 h in desorption and 24 h in absorption. The total duration of one measurement was thus 10 days. At the end of each measurement, the sample was dried at 60 °C and 0% RH for 8 h using the built-in preheater with succeeding temperature stabilisation at 20 °C and 0% RH for 2 h before the dry mass was taken. The dry masses of each sample ranged between 7 and 11 mg. Predefined hold times were selected instead of a mass stability criterion, which is commonly used, because of uncertainties with the latter (Glass et al.
2018,
2017). However, if the sorption kinetics is different for different sample types this could give apparent differences between sample types since they have equilibrated to different extents. Therefore, the degree of equilibrium at each RH step was evaluated as the change in moisture content over time (
dω/
dt) for the last two hours at each step (Table
2). According to Glass et al. (
2018),
dω/
dt < 3 μg g
−1 min
−1 calculated over two hours gives an average error in moisture content of less than 0.003 g g
−1. As seen in Table
2, the errors in equilibrium moisture content caused by lack of equilibrium should thus be small in this study. Also, the difference in degree of equilibrium was rather between absorption and desorption than between sample types.
Table 2
Degree of equilibrium at the different RH levels expressed as dω/dt in μg g−1 min−1. Note that the values at 0% RH is for the 0% RH step between desorption and absorption while the dry mass used to calculate the moisture content was determined by drying at elevated temperature (see method description in the text for details)
Pyridine controls | 5.4 | 2.4 | 3.8 | 3.6 | 3.5 | 0.4 | 0.9 | 2.8 | 0.0 | 0.6 | 2.3 |
Untreated | 3.4 | 2.4 | 3.9 | 3.7 | 2.8 | 0.2 | 0.4 | 1.1 | 1.8 | 0.6 | 2.1 |
Interface mod. 1 | 1.2 | 2.6 | 4.2 | 3.3 | 2.6 | 0.2 | 0.4 | 0.9 | 1.5 | 0.7 | 1.5 |
Interface mod. 2 | 2.0 | 2.5 | 4.0 | 3.2 | 2.3 | 0.2 | 0.2 | 1.1 | 1.6 | 0.9 | 1.2 |
Uniform mod. A | 0.7 | 2.9 | 4.8 | 3.2 | 2.5 | 0.1 | 0.2 | 1.4 | 1.3 | 1.5 | 1.7 |
Uniform mod. B | 0.8 | 3.5 | 3.9 | 2.5 | 1.9 | 0.1 | 0.2 | 0.8 | 1.3 | 1.8 | 2.1 |
Uniform mod. C | 1.1 | 2.8 | 4.2 | 2.7 | 1.7 | 0.1 | 0.1 | 0.7 | 1.4 | 1.7 | 1.2 |
The equilibrium moisture content at each RH level was evaluated using Eq.
5, but with the equilibrium mass at each RH level instead of the vacuum saturated mass. Note that the masses were corrected for the mass gain obtained by the modification (Table
1) so that all moisture contents were based on dry mass before modification,
mdry,0, in order to enable comparison of moisture contents between specimen types. Otherwise, the increased dry mass obtained by the modification would give a seemingly higher reduction in moisture content than what was actually the case. The mass increase of the individual specimen was used for the interface modification (Table S2) while the average of the whole batch was used for the uniformly modified specimens (Table
1). Note that no correction was made for the pyridine controls where a mass loss was seen. From these moisture contents, the moisture exclusion efficiency was determined as:
$${\xi }_{\omega }=\frac{{\omega }_{\text{untr}}-{\omega }_{\text{mod}}}{{\omega }_{\text{untr}}}$$
(7)
where
ξω (-) is the moisture exclusion efficiency,
ωmod (g g
−1) is the equilibrium moisture content of the modified wood material (or pyridine controls) at a certain relative humidity level, and
ωuntr (g g
−1) is the equilibrium moisture content of the untreated material at the same relative humidity level. These moisture contents were, as previously described, also based on the dry mass before modification,
mdry,0.
Hydroxyl accessibility
The hydroxyl accessibility was determined by deuterium exchange in a sorption balance (DVS Advantage, Surface Measurement Systems Ltd., London, UK). These measurements were made both for earlywood and latewood separately and for samples consisting of both earlywood and latewood. A sample corresponding to a dry mass in the range of 8–11 mg was prepared by cutting thin pieces from a 5 × 5 × 10 mm
3 specimen using a razor blade. For the measurements where both earlywood and latewood were included, five replicates were used and for the measurements which were made on earlywood and latewood separately, 4 replicates were used. In total, measurements were thus made on 91 samples. Each measurement was performed as follows: the sample was placed in the sorption balance and was dried at 60 °C and 0% RH for 6 h using the preheater in the instrument. Ramps of 10 min were used to increase/decrease the temperature to/from 60 °C. After drying, an additional 2 h period of temperature stabilisation at 20 °C and 0% RH followed. The sample was then exposed to deuterium oxide vapour (95% RH) for 10 h after which the sample was again dried using the same procedure as for the initial drying. Since the exposure to deuterium oxide vapour causes an exchange from hydrogen to deuterium for the accessible hydroxyl groups, the mass obtained after the second drying is higher than the initially determined dry mass. From this change in dry mass the hydroxyl accessibility was determined by (Thybring et al.
2020b):
$$c_{{{\text{acc}}}} = \frac{{\Delta m_{{{\text{dry}}}} }}{{{\text{}}\Delta {\textit{M}}_{{{\text{hydrogen}}}} m_{{{\text{dry}}}} {\text{}}({1 + \textit{R}}_{{{\text{mod}}}} )}}$$
(8)
where
cacc (mol g
−1) is the hydroxyl accessibility, Δ
mdry (g) is the difference in dry mass before and after deuteration, Δ
Mhydrogen (g mol
−1) is molar mass difference (1.006 g mol
−1) between deuterium (
2H or D) and protium (
1H), and
mdry (g) is the dry mass and
Rmod (g g
−1) is the mass increase caused by the modification (Table
1). The correction of the dry mass by (1 +
Rmod) was used to enable comparison between specimen types. In this way, the hydroxyl accessibility is related to the dry mass before modification,
m0,dry, for all specimen types. Here, the average mass increase after modification was taken for all modifications. No correction was, however, made for mass loss of the pyridine controls.