Multicycle Wilhelmy Plate Method for Wetting Properties, Swelling and

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A multi-cycle Wilhelmy plate method for wetting properties, swelling and liquid sorption of wood Maziar Sedighi Moghaddam, Magnus Wålinder, Per Martin Claesson, and Agne Swerin Langmuir, Just Accepted Manuscript • DOI: 10.1021/la402605q • Publication Date (Web): 04 Sep 2013 Downloaded from http://pubs.acs.org on September 6, 2013

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A multi-cycle Wilhelmy plate method for wetting properties, swelling and liquid sorption of wood Maziar Sedighi Moghaddam,† Magnus E.P. Wålinder,‡ Per M. Claesson†, || and Agne Swerin,*,†, || †

SP Technical Research Institute of Sweden - Chemistry, Materials and Surfaces, Box 5607, SE-114 86 Stockholm, Sweden ‡

KTH Royal Institute of Technology, School of Architecture and the Built Environment,

Department of Civil and Architectural Engineering, Building Materials, SE-100 44 Stockholm, Sweden ||

KTH Royal Institute of Technology, School of Chemical Science and Engineering,

Department of Chemistry, Surface and Corrosion Science, SE-100 44 Stockholm, Sweden

ABSTRACT: A multi-cycle Wilhelmy plate method has been developed to investigate wetting properties, liquid sorption and swelling of porous substrates such as wood. The use of the method is exemplified by studies of wood veneers of Scots pine sapwood and heartwood, which were subjected to repeated immersion and withdrawal in a swelling liquid (water) and in a non-swelling liquid (octane). The swelling liquid changes the sample dimensions during measurements, in particular its perimeter. This, in turn, influences the force registered. A model based on a linear combination of the measured force and final change in sample perimeter is suggested, and validated to elucidate the dynamic perimeter change of wood

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veneer samples. We show that pine heartwood and pine sapwood differ in several respects in its interaction with water. Pine heartwood showed i) lower liquid uptake, ii) lower swelling, iii) higher contact angle, and iv) lower level of dissolution of surface active components (extractives) than pine sapwood. We conclude that the method is also suitable for studying wetting properties of other porous and swellable materials. The wettability results were supported by surface chemical analysis using X-ray photoelectron spectroscopy, showing higher extractives and lignin content on heartwood than on sapwood surfaces.

KEYWORDS: Wilhelmy plate method, swelling, wettability, sorption, dynamic contact angle, XPS, wood


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1. INTRODUCTION The Wilhelmy plate method is a useful technique for investigating wetting properties of heterogeneous, hygroscopic and porous materials. It has been mainly used for determining contact angles,1-10 although other wetting properties like surface energy,11-14 work of adhesion,10,15 reorientation of surface groups,16 adsorption,17 and dynamic wetting behavior15,18 have also been studied by this technique. There are, however, still opportunities to develop new approaches based on the Wilhelmy plate technique in order to study wetting properties, especially of swellable, porous and heterogeneous materials. A solid is said to swell when it absorbs a liquid under the following conditions:19 (i) its dimensions are increased, (ii) its microscopic homogeneity remains, and (iii) its cohesion is not eliminated. However, it may become soft and flexible instead of hard and brittle. The first condition means that the dimensions of a swelling material (e.g. wood or swelling polymer) are not constant during the swelling process. Conversely, the dimensional stability of materials plays an important role in their performance; for instance wood can be chemically modified in order to improve the dimensional stability.20 Different techniques can be used to study dimensional stability, and the most conventional method is based on measuring the sample dimensions before and after liquid submersion using a flatbed micrometre.20,21 Oh et al.22 introduced a computer-based dynamic swellometer to study dimensional stability of solid wood and fibreboard more precisely. Son and Gardner18 used the Wilhelmy plate method to study dimensional stability of wood veneers by comparing the perimeter before and after 24 hours immersion in water by using a non-swelling probe liquid such as octane. They concluded that the dimensional stability of thin and non-rectangular samples is more accurately determined with the Wilhelmy plate method than with the calliper method, the reason being that the Wilhelmy method allows determination of


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perimeter changes on a microscopic level. A limitation of their method is, however, its static nature. Thus, only the initial and final perimeter dimension of the sample is measurable and no dynamic or time dependent behaviour can be investigated. The idea of the present work is to modify the Wilhelmy plate method to a multi-cycle technique with the purpose of studying dynamic wettability, capillary action and liquid sorption in swellable materials. The general aim is to link to end-use properties, such as dimensional stability, and we exemplify the use of the method by investigating a set of wood samples. For most wood species one should distinguish between heartwood and sapwood. In general, heartwood, which is present in the core of a tree trunk, contains a higher amount of extractives than sapwood (the outer part of the tree trunk) and the type of extractives are also different. For instance in Scots pine, which was used in this work, the resin acids, fatty acids and pinosylvin are richer in heartwood, while sapwood has higher amounts of triglycerides.23-24 It is understood that extraction of the wood species could lead to increasing rate and degree of swelling,19 due to the hydrophobic character of many extractives that decrease the wettability. Not only the level and type of extractives, but also their location in the wood structure influences the swelling and wettability properties.25,26 In addition, the micromorphology of heartwood differs significantly from sapwood mainly due to a higher degree of so-called pit aspiration (closed pits) in heartwood, which decreases its liquid permeability (or blocks capillary uptake of liquids). Furthermore, ageing of the wood causes extractives migration and distribution on the wood surface and this will change its wetting and swelling properties (more hydrophobic).27


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Knowledge of the surface chemistry is critical in order to understand the interaction of liquids with solid surfaces. XPS (X-ray Photoelectron Spectroscopy) is a highly surface sensitive (outermost 2-10 nm of surfaces) and powerful tool for chemical surface analysis. This technique has previously been used to investigate chemical changes of wood surfaces under various conditions.28-34 XPS provides quantitative data on the elements present on the sample surface. For wood samples the most important information comes from the O/C atomic ratio and the relative amount of carbon present in different binding configurations, and it is therefore possible to examine cellulose, lignin and extractives amount of wood and wood pulp surfaces.29,35-38 An increasing O/C atomic ratio and decreasing C1 component (C– C and C–H bonds) of the C1s photoelectron emission line accompany an increase of the cellulose/hemicellulose amount and a decrease of lignin and extractive amount on wood surfaces.37 Combining wettability studies with surface chemical analysis by XPS, provide valuable knowledge regarding the energetics and chemistry of wood surfaces.2,30,31,39,40 The main objective of this work is to establish a new approach to study the dynamic wetting, swelling and liquid sorption behaviour of swellable and porous materials, such as wood, based on the Wilhelmy plate method. We also aim to relate the swelling and wetting properties of pine sapwood and heartwood to their surface chemical composition. 2. MATERIALS AND METHODS 2.1. Wood veneers Veneers with dimensions of approximately 30×7×1 mm3 (in the longitudinal, radial and tangential direction respectively) were prepared from from kiln41 dried Scots pine (Pinus sylvestris L.) sapwood and heartwood boards, by splitting the wood along the fiber direction using a wood chisel. To maximize the reproducibility in the wettability measurements, veneers were cut from the same series of annual rings. Since the cross section to flat sides


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ratio of industrial wood boards are much smaller than for the veneers, and in order to prevent end grain sorption and an over representative longitudinal liquid transportation during the wettability measurements, the cross section of the immersion part of the veneers were sealed by polyurethane lacquer. One set of freshly cut veneers were directly dried and thermally treated at 104 oC for 1 h in a convection oven, and another set of veneer samples was treated at the same temperature for 3.5 h to study the effect of treatment time on the wettability properties. Some unsealed veneers were also prepared to study the effect of end-grain sorption on the wettability measurements (see supporting information). 2.2. XPS analysis XPS spectra of wood surfaces were recorded using a Kratos AXIS HS X-ray photoelectron spectrometer (Kratos Analytical, Manchester, UK) using a monochromatic Al Kα X-ray source operated at 300 W (15 kV/20 mA) for high-resolution (small electron binding energy range) carbon 1s and oxygen 1s spectra. Two samples approximately 7×7×1 mm3 were prepared from each pine sapwood and heartwood and they were analysed in one position per piece. The veneers were conditioned overnight in the pre-evacuation chamber (10-7-10-6 Pa). The analysis area was below about 1 mm2 (most of the signal emanates from an area of 700 x 300 µm2). Broad electron binding energy (survey) spectra were also recorded to detect all elements present in the surface layer. The relative surface compositions were obtained from quantification of high resolution spectra for each element. Using the Origin® software, the raw data were plotted as high-resolution carbon spectra and then fitted using Gaussian amplitude function to get peak area ratios with errors corresponding to one standard deviation. This allows obtaining the ratios of different functional carbon species and the standard deviation of these ratios from the peak area ratios and their standard error, respectively.


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2.3. Multi-cycle Wilhelmy plate method The Wilhelmy equation for porous and hygroscopic surfaces such as wood is:6 , (1) where F is the force measured by the tensiometer, P the wetted perimeter of the plate, γ the surface tension of the probe liquid, θ the liquid-solid-air contact angle, the probe liquid density, A the cross-sectional area of the plate, h the immersion depth, g the gravitational constant and (t) the force due to wicking (capillary action) and sorption of the liquid at time t. On the right-hand side of eq 1, the first term represents the wetting force, the second one the force due to the liquid uptake and the third one the buoyancy force. For nonhygroscopic and non-swellable materials that do not absorb any liquid during the measurement, the term (t) equals zero. In the case of swellable materials, the sample dimensions increase during the wettability measurement and therefore the sample perimeter (P) in eq 1 is a function of time. Furthermore, the surface tension of the liquid (γ) is also time-dependent and is changed during the measurement as a result of extractives migration and dissolution in the liquid.27 By performing a single Wilhelmy plate measurement (immersion and withdrawal in probe liquid with known surface tension), the contact angle of the surface can be determined from eq. 1.2,6 A diagram of the data collected during a two-cycle Wilhelmy plate experiment is provided in Figure 1.


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Figure 1. A two-cycle Wilhelmy plate experiment with a wood sample. The specimen is immersed and withdrawn from the probe liquid while the instrument detects the force as a function of immersion depth. Due to the hygroscopic nature of the specimen, the final force (Ff) is measured at the end of each cycle corresponding to the liquid sorbed in the specimen. The advancing, receding and final forces of the first cycle are shown by FA, FR and Ff, respectively. In this work we modify the Wilhelmy plate method to a multi-cycle method for elucidating dynamic liquid sorption and dynamic swelling behaviour of wood veneers, as schematically illustrated in Figure 2. The veneer is immersed in the grain direction to a depth of 10 mm and is then withdrawn to 5 mm above the liquid surface. This process is repeated during several cycles. Complete withdrawal of the veneer out of the probe liquid in each cycle allows the detection of the final force of the corresponding cycle. The veneer perimeter is increased after each single cycle due to liquid swelling of the wood cell wall.


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Figure 2. Schematic illustration of the multi-cycle Wilhelmy plate method. The sample is intermittently immersed in and withdrawn from the probe liquid. The veneer dimension is increased during the experiment as a consequence of swelling. The arrow length does not depict actual values.

By linear regression of the advancing, receding and final data registered in the multi-cycle Wilhelmy curves (Figure 1) to zero depth (h=0), the advancing force , the receding force ( and the final force ( can be obtained for each cycle. The intercepts are given by: , cos, (2) , cos, , (3) where θA,n and θR,n are the advancing and receding contact angle at cycle n, respectively, and , is the final force at cycle n which is the same as the term in eq 1. For wood surfaces in which the surface is completely wetted by water after the first immersion (first advancing curve in Figure 1), advancing contact angles of subsequent cycles 1 and receding contact angles of all cycles equal zero. Therefore, eq 2 and 3 can be simplified to: ,

1

(4)


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, , (5) The liquid mass up-take of the wood veneer after cycle n is calculated as: !" #

%$ %

'( )'* '*

+ 100

-.,( '*

+ 100 (6)

where / is the veneer weight after cycle n and /0 is the weight of oven dried veneer. Wettability measurements were performed using a Sigma 70 tensiometer from KSV Instruments. The test velocity was 12 mm min-1.2,6 Veneers were immersed into ultrapure water (prepared by a MilliQ RO unit giving a resistivity of >18 MΩ·cm) and n-octane (1 99%) from Alfa Aesar, as swelling and non-swelling liquids, respectively. The surface tension of water and octane were measured to be 72.0 6 0.2 mN/m and 21.4 6 0.1 mN/m respectively. Experiments were performed using 20 cycles in water and 10 cycles in octane. Due to dissolution of extractives during the wettability study, the water was changed after each multi-cycle experiment.27 The surface tension of the water phase was measured before and after each multi-cycle experiment. Freshly cleaned glassware was used for each measurement. Measurement conditions were 22-23 oC and 35±5 %RH.

2.4. Perimeter model Since some liquids such as water can swell the wood cell wall and increase the dimension of wood sample, the perimeter (P) of the sample is not constant during multi-cycle Wilhelmy plate measurement. It has been suggested to use a non-swelling liquid like octane to determine P in eq 4 and 5.6 In measuring the final perimeter (Pf), the veneer was subsequently immersed in octane after a multi-cycle experiment in water. The perimeter of dry veneer (P0) was similarly measured by immersion in octane. Considering that just the initial and the final perimeter can be measured by octane immersion, we propose to calculate the perimeter for cycle n as: ); , ,);

∆= ∆-.

(7)


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where and ); are the veneer parameters after cycle n and (n-1), respectively, , and ,); are final forces for cycle n and (n-1), respectively, ∆ and ∆ the total perimeter change and final force difference between the last cycle and first cycle, respectively. This model assumes that there is a linear relation between the change in the weight of the sample and the sample dimension (i.e. perimeter). The perimeter model was evaluated by performing shorter series, using 5, 10 and 15 cycles. An octane immersion measurement was done after these multi-cycle tests to determine the perimeter values P5, P10 and P15. The perimeter results were then compared to those calculated from eq 7 and after each individual cycle calculated as: Change of perimeter (%)

=( )=> =>

+ 100 (8)

3. RESULTS AND DISCUSSION

3.1. Surface Characterization-XPS High-resolution XPS spectra of the C1s region are shown in Figure 3. The high-resolution spectra provide information of the binding state of the carbon atoms in the surface region of the wood samples. The chemical shifts for the carbon (C1s) peak from wood and pulp surfaces can be classified into four groups C1-C4.42 As seen in Figure 3, the pine sapwood has more C2-carbon than pine heartwood, whereas unoxidised carbons (C1-carbon) are more abundant in heartwood. The chemical surface composition of pine sapwood and heartwood expressed in atomic % in Table 1 is obtained from quantification of high resolution spectra for each element. The chemical composition of cellulose, hemicellulose, lignin and a typical extractive (oleic acid) are also given in Table 136 and applied here in a similar manner as described elsewhere.29,36,38,42,43


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Figure 3. XPS high resolution C1s spectra of pine sapwood and heartwood (C1: C-C, C=C, C-H at 285.0 eV; C2: C-O, C-O-C at 286.7 eV; C3: C=O, O-C-O at 288.2 eV; C4: O-C=O, C(=O)OH at 289.4 eV). Table 1. Atomic percents, C1, C2, C3, C4 distribution and O/C ratio of pine sapwood and heartwood samples and wood components. Sample O1 s C1 s C1 C2 36 --45.4 54.6 83 Cellulose Hemicellulose 44.8 55.2 --78 (Arabinoglucuronoxylan)36 Lignin36 24.8 75.2 49 49 Extractives 9.9 90.1 94 --(oleic acid)36 Sapwood1 26.1 73.4 51.9±1.5 35.3±1.2 Sapwood2 28.0 71.2 43.1±1.5 43.5±1.7 Heartwood1 18.4 81.0 68.8±1.7 23.7±0.8 Heartwood2 17.2 82.5 68.4±3.0 22.9±1.3 Functional groups composition C1-C4 according to Figure 3

C3

C4

O/C

17

---

0.83

19

3

0.81

2

---

0.33

---

6

0.11

8.0±2.1 9.7±2.5 4.7±1.8 4.7±3.2

4.7±1.8 3.7±1.9 2.8±1.5 3.9±2.8

0.36 0.39 0.23 0.21

Mainly C and O were detected on the wood surfaces. Trace amounts of Ba, N and S (< 0.5 atomic %) were also found on the surface of some samples. The amount of C1 carbon decreases in the order: extractives >lignin >cellulose=hemicellulose. Thus, a higher relative amount of unoxidized carbon (C1) corresponds to a lower O/C atomic ratio, which can be related to increased content of lignin and extractives on the surface. Theoretically, cellulose is devoid of C1 carbon due to its


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polysaccharide structure. Moreover, extractives generally contain no C2-carbon, while almost 50% of the carbon bonds in lignin and the main carbon bonds in cellulose/hemicellulose fall into category C2. In addition, although there is no C3-carbon in lignin and extractives, 17-19% of the carbon bonds in cellulose and hemicellulose are C3carbon. These differences provide an opportunity to determine cellulose, lignin, and extractive contents of wood surfaces by means of XPS analysis. The data of Table 1 demonstrates significant chemical differences between pine sapwood and heartwood. Firstly, the pine heartwood surface has lower O/C ratio and higher C1 content than pine sapwood, demonstrating that the surface amounts of lignin and extractives are higher. It has been widely understood that heartwood cell walls hold a larger amount of extractives than sapwood cell walls.23,44 Secondly, pine sapwood contains higher atomic ratio of C2 and C3-carbons compared to pine heartwood (Table 1). This observation suggests that lignin might be more abundant in pine heartwood cell walls.29 Variation of the XPS results for the two sapwood samples (Table 1) reveals a heterogeneous chemical composition of the sapwood surface, which is consistent with the wettability results discussed below. It is informative to present the XPS data by plotting the C1 content vs. the O/C ratio,29,38,43 see Figure 4 which contains our data for pine sapwood and heartwood as well as data for the main wood components. If it was possible to do XPS analysis without any conditioning in vacuum, the sapwood and heartwood points (●) would be shifted closer to the cellulose/hemicellulose points. However, the XPS conditioning leads inevitably to some extractive migration to the wood surface.45 We note that the wettability of the samples conditioned for the wettability and those conditioned in the XPS instrument were similar, and thus the XPS data is expected to reflect


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the chemistry of the surfaces used in the wettability studies. Combining the data of Table 1 in Figure 4 shows that Scots pine heartwood contains a larger amount of lignin and extractives than Scots pine sapwood.

Figure 4. The relative amount of C1 aliphatic carbon as a function of the O/C atomic ratio for pine sapwood and heartwood samples. Values for cellulose, hemicellulose, fresh Scots pine, lignin and extractives (■) are calculated from literature data.36 3.2. Multi-cycle Wilhelmy plate experiments – basic considerations Typical multi-cycle Wilhelmy plate curves for Scots pine sapwood and heartwood in water are shown in Figure 5. A number of features can be observed from these plots. Firstly, from the change in force after removing the sample from the liquid it is seen that sorption increases with time (i.e. with number of cycles) for both samples due to filling the voids of the wood cell structure and due to bulk sorption in the cell walls by water, the latter sorption resulting in swelling of the wood. However, different sorption rate and water uptake are noticed for pine sapwood and heartwood, see discussion in more detail later. Secondly, from the more irregular shape of the first advancing curves of sapwood, indicating


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more local variations in the contact angle, we conclude that the surfaces of sapwood samples are more heterogeneous than heartwood surfaces, which is consistent with the XPS results. This appears to be consistent for essentially all sapwood and heartwood samples investigated. Thirdly, by linear regression of the first advancing curve to zero depth (h=0), the advancing contact angle (CAadv.) of the veneers can be calculated to be 65±7° and 83±3° for pine sapwood and heartwood, respectively (see supporting information for more details). Thus, pine heartwood is significantly more hydrophobic than pine sapwood as also reported by Metsä-Kortelainen et al.46 Another interesting feature of the multi-cycle experiments can be seen at the right end of the Wilhelmy plots in Figure 5 when the sample is approaching the end of the immersion. For both sapwood and heartwood the first couple of cycles on this part of the curve clearly “dips” to almost the same level as for the first cycle, probably due to the fact that the water-line comes close to a nonwetted region. In the later cycles this part of the curve does not “dip” as much for the sapwood samples as compared with the heartwood samples, clearly due to the fact that the heartwood is less affected by the water sorption (less hydrophilic) than the sapwood.


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Figure 5. Typical curves of multi-cycle Wilhelmy plate experiments on pine sapwood and heartwood veneers in water. The curves display the force (F) per perimeter (P) as a function of immersion depth (h) during a 20-cycle measurement. Liquid uptake increases with increasing cycle number. The veneers were oven dried with moisture content of close to zero.

Figure 6. Wilhelmy plate experiments using pine sapwood (the same sample as in Figure 5) in octane. (a) 10 cycles on oven dried veneer (moisture content ≈ 0 %) and (b) 2 cycles on water saturated veneer after 20 cycles in water. Due to the water penetration into voids before measurement, no initial wicking is observed in the wet veneer curve (plot b). 3.3. Multi-cycle Wilhelmy plate experiments – sorption Typical Wilhelmy plate measurements using octane and a wood veneer are provided in Figure 6. For the dry veneer (Figure 6a), one interesting feature is the hysteresis between advancing and receding curves in the first 5 mm of the first cycle. This hysteresis is not observed for the water saturated veneer (Figure 6b) and confirms the initial octane up-take (wicking) that has been suggested for immersion of dry wood samples in octane.6 For the water saturated veneer, the voids of the wood structure are already filled by water and since octane is a non-swelling liquid, no significant wicking occurs. This difference also rationalizes


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the high liquid uptake of dry veneer compared to water saturated veneer in octane as observed by the change in force after removal of the sample from the liquid (Figure 6). A comparison between multi-cycle experiments with water (Figure 5a) and octane (Figure 6a) shows that octane has a different sorption rate profile than water. This is clearly demonstrated in Figure 7, which includes the sorption results for pine sapwood and heartwood in a swelling liquid (water) and a non-swelling liquid (octane). Octane, which has a zero contact angle on wood surfaces, spreads and penetrates the voids very quickly. This is revealed by the high liquid content after the initial cycles for both sapwood and heartwood. Thus, the void filling process occurs fast for octane. Water sorption occurs due to void filling, bulk sorption and liquid diffusion in the cell-wall resulting in swelling. This fact leads to a sorption process that approaches equilibrium slower than the sorption of the non-swelling liquid octane. The initial liquid up-take is dominated by void filling, and this process occurs nearly as fast for water as for octane. The additional water uptake that occurs for longer times is dominated by the slower bulk sorption and swelling of the cell-walls. In particular, we note that the rate of cell-wall swelling is significantly higher for pine sapwood than heartwood. This slower water up-take of pine heartwood has been reported by floating tests at longer time.47 The difference can be attributed to the differences in extractive content and type of extractives in pine sapwood and heartwood.


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Figure 7. Liquid mass as a function of cycle number for pine sapwood and heartwood in water and octane. The results ware obtained by extrapolation of the final force (Ff) data to zero depth in multi cycle Wilhelmy experiments. 3.4. Multi-cycle Wilhelmy plate experiments – dimensional stability The apparent perimeter change of sapwood and heartwood veneers as a function of cycle number (time) is given in Figure 8 based on octane immersion experiments and based on calculations using eq 7. As an example, for a 20-cycle experiment the initial (P0) and final (P20) perimeters were obtained by octane immersion, while the other perimeter data (P1-P19) were calculated from the perimeter model. Data based on the model are average values of 10 measurements with standard deviation shown by the patterened areas in Figure 8. To test the accuracy of the perimeter model, cycling was done for five, ten and fifteen times followed by octane immersion to determine the final perimeter and the results for five replicates are shown in Figure 8.


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Considering the heterogeneous nature of wood samples, even though the veneers are prepared from the same board and band of annual rings, we find a satisfactory agreement between the two methods used for determining the perimeter change. Thus, the model (eq 7) is found to be suitable for relating the liquid uptake to the perimeter change in swellable materials such as wood, and likely provides a suitable approach for determining the dimensional stability of other swelling materials. It is important to consider that it is not possible to completely distinguish between "void filling water" and "swellable water" especially in initial cycles (Figure 7). On the other hand the perimeter model relates the perimeter change to all liquid up-take, not just to "swellable water". We note that the model is more valid for cycle 2 and onwards where swelling occurs more than void filling. Furthermore, as illustrated in Figure 8, pine sapwood and heartwood show different dimensional stability. After 20 cycles, the perimeter of the sapwood veneers increases about 8 %, while this amount for the heartwood veneers is less than 4 %.


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Figure 8. Apparent perimeter change of pine sapwood and heartwood veneers during multicycle Wilhelmy experiments. All data were calculated using the perimeter model (eq 7) with the exception of final perimeter (P20) which was determined by a single immersion in octane after 20 cycles in water. Octane immersion data of intermediate cycles (C5, C10, C15) are also shown. The standard deviation of the octane immersion and the model results are illustrated by error bars and filled patterned areas, respectively. 3.2. Multi-cycle Wilhelmy plate experiments – dissolution of extractives It is important to point out that the surface tension of the water can change during wood wettability measurements as demonstrated during contact angle measurement by the sessile drop method48 and by the Wilhelmy plate method.27 Figure 9 shows the surface tension reduction during multi-cycle Wilhelmy plate experiments for pine sapwood and heartwood. For the sapwood samples the water surface tension decreases by increasing cycle number, demonstrating accummulation of surface active extractives at the air-liquid interface. The surface tension reduction reaches a plateau level of about 4 mN/m after 1015 cycles. In contrast, no surface tension reduction is observed for heartwood veneers. Combining this with the XPS results and the effect of thermal treatment time (supporting information) demonstrates that although pine heartwood contains more extractives than pine sapwood, they are less prone to migrate to the air-water interface which may be due to that most of the extractives in heartwood are present in the cell walls or due to that they are less mobile and have aged to a different oxidation state. The surface tension change will not affect the perimeter results, since the tensiometer data used for the calculations is the amount of liquid uptake and not affected by the surface tension (eq 7).


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The wettability results can be related to the surface chemical composition as evaluated by XPS. Isolated cellulose is hydrophilic with a water contact angle of 20-30°.49-51 Most extractives are hydrophobic and by migration during ageing or experiments they will render the wood surface less hydrophilic.27,35,48 The water contact angle of lignin is in between that of cellulose and extractives with reported values in the range 55-77°.52-53 Hence, a wood species with a higher amount of lignin and extractives, such as pine heartwood, is expected and observed to have a relatively large water contact angle. The XPS results showing more extractives on the surface of pine heartwood than on sapwood is thus consistent with the wettability data showing higher water contact angle on pine heartwood than on pine sapwood.

Figure 9. Surface tension reduction of the water phase after multi-cycle Wilhelmy experiments with pine sapwood and heartwood veneer samples. From the sorption and perimeter change results, it is found that pine heartwood takes up less water and has higher dimensional stability than pine sapwood. These properties are not


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only affected by the surface chemical composition, but also by the structure and extractives composition of the wood bulk. The first step in the sorption process starts with the wetting and spreading of the liquid on the wood surface. Since heartwood is more hydrophobic than sapwood due to more extractives and lignin, this step takes longer time for the heartwood samples. The next regime is associated with liquid penetration followed by void filling. This process occurs faster in sapwood due to more hydrophilic components in its structure. Finally, at the same time and for longer times the liquid will swell the wood cell-wall and increase the dimension of the veneer. As a result of the detailed morphological difference, e.g. the aspiration of bordered pits and having more closed pits, heartwood shows a lower rate of swelling as well. Heartwood compared to sapwood contains more extractives located in the wood cell wall than in the capillary structure, resulting in lower rate and extent of swelling. 4- CONCLUSIONS A dynamic wettability technique based on a multi-cycle Wilhelmy plate method is suggested for liquid penetration, sorption and dimensional stability of swelling materials. We applied the methodology to wood samples and performed wettability measurements in swelling liquid (water) and non-swelling liquid (octane). The data revealed lower contact angle, higher liquid uptake, higher swelling, less dimensional stability and higher water contamination during the measurement for pine sapwood than pine heartwood due to different structure and chemical composition (level of extractives and lignin). A perimeter model is suggested to obtain the perimeter changes with time based on a linear combination of the sample perimeter changes and measured wetting force. The results of this model were found to be consistent with data obtained using a completely wetting and non-swelling liquid to determine the final perimeter. Thus, we suggest that this model can be applied to


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study dynamic perimeter changes of swelling substrates. The wettability results for sapwood and heartwood samples were consistent with their different surface chemical compositions showing more extractives and lignin on pine heartwood surfaces. ASSOCIATED CONTENT Supporting Information The effect of veneer moisture content and end sealing of the veneers on sorption behavior and thermal treatment time on contact angle results. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Acknowledgement Financial support of this work was provided by the Nils and Dorthi Tröedsson Foundation for Scientific Research within a sustainable wood modification PhD project for MSM and an adjunct professorship for AS at KTH. REFERENCES (1) Casilla, R. C.; Chow, S.; Steiner, P. R., An immersion technique for studying wood wettability. Wood Science and Technology 1981, 15 (1), 31–43. (2) Gardner, D. J.; Generalla, N. C.; Gunnells, D. W.; Wolcott, M. P., Dynamic wettability of wood. Langmuir 1991, 7 (11), 2498-2502. (3) Tretinnikov, O. N.; Ikada, Y., Dynamic Wetting and Contact Angle Hysteresis of Polymer Surfaces Studied with the Modified Wilhelmy Balance Method. Langmuir 1994, 10 (5), 16061614.


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(4) Teasdale, P. R.; Wallace, G. G., In situ characterization of conducting polymers by measuring dynamic contact angles with Wilhelmy's plate technique. Reactive Polymers 1995, 24 (3), 157-164. (5) Mantanis, G. I.; Young, R. A., Wetting of wood. Wood Science and Technology 1997, 31(5), 339-353. (6) Wålinder, M. E. P.; Ström, G., Measurement of wood wettability by the Wilhelmy method. Part 2. Determination of apparent contact angles. Holzforschung 2001, 55 (1), 3341. (7) Pétrissans, M.; Gérardin, P.; El Bakali, I.; Serraj, M., Wettability of heat-treated wood. Holzforschung 2003, 57 (3), 301-307. (8) Garcia, R. A.; Cloutier, A.; Riedl, B., Chemical modification and wetting of medium density fibreboard panels produced from fibres treated with maleated polypropylene wax. Wood Science and Technology 2006, 40 (5), 402-416. (9) Wang, S.; Zhang, Y.; Xing, C., Effect of drying method on the surface wettability of wood strands. Holz als Roh- und Werkstoff 2007, 65 (6), 437-442. (10) Bryne, L. E.; Wålinder., M. E. P., Ageing of modified wood. Part 1: Wetting properties of acetylated, furfurylated, and thermally modified wood. Holzforschung 2010, 64 (3), 295– 304. (11) Shen, Q.; Nylund, J; Rosenholm, J. B., Estimation of the Surface Energy and Acid-Base Properties of Wood by Means of Wetting Method. Holzforschung 1998, 52 (5), 521-529. (12) de Meijer, M.; Haemers, S.; Cobben, W.; Militz, H., Surface Energy Determinations of Wood: Comparison of Methods and Wood Species. Langmuir 2000, 16 (24), 9352-9359. (13) Wålinder, M. E. P., Study of Lewis Acid-Base Properties of Wood by Contact Angle Analysis. Holzforschung 2002, 56 (4), 363–371.


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(14) Wang, X.; Ederth, T.; Inganäs, O., In Situ Wilhelmy Balance Surface Energy Determination of Poly(3-hexylthiophene) and Poly(3,4-ethylenedioxythiophene) during Electrochemical Doping−Dedoping. Langmuir 2006, 22 (22), 9287-9294. (15) Pu, G.; Severtson, S. J., Characterization of Dynamic Stick-and-Break Wetting Behavior for Various Liquids on the Surface of a Highly Viscoelastic Polymer. Langmuir 2008, 24 (9), 4685-4692. (16) Wang, J. H.; Claesson, P. M.; Parker, J. L.; Yasuda, H., Dynamic Contact Angles and Contact Angle Hysteresis of Plasma Polymers. Langmuir 1994, 10 (10), 3887-3897. (17) Eriksson, L. G. T.; Claesson, P. M.; Eriksson, J. C.; Yaminsky, V. V., Equilibrium Wetting Studies of Cationic Surfactant Adsorption on Mica: 1. Mono- and Bilayer Adsorption of CTAB. Journal of Colloid and Interface Science 1996, 181 (2), 476-489. (18) Son, J.; Gardner, D. J., Dimensional stability measurements of thin wood veneers using the wilhelmy plate technique. Wood and Fiber Science 2004, 36 (1), 98-106. (19) Mantanis, G. I.; Young, R. A.; Rowell, R. M., Swelling of wood. Wood Science and Technology 1994, 28 (2), 119-134. (20) Rowell, R.; Imamura, Y.; Kawai, S.; Norimoto, M., Dimensional Stability, Decay Resistance, and Mechanical Properties of Veneer-Faced Low-Density Particleboards Made From Acetylated Wood. Wood and Fiber Science 1989, 21 (1), 67-79. (21) Youngquist, J.; Krzysik, A.; Rowell, R., Dimensional Stability of Acetylated Aspen Flakeboard. Wood and Fiber Science 1986, 18 (1), 90-98. (22) Oh, Y. S.; Sites, L. S.; Sellers, JR. T.; Nickolas, D. D., Computerized dynamic swellometer evaluation of oriented strand products. Forest Products Journal 2000, 50 (3), 35-38. (23) Back, E. L., The locations and morphology of resin components in the wood. In Pitch Control, Wood Resins and Deresination, Tappi Press Atlanta, 2000, p 43.


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(24) Nuopponen, M.; Willför, S.; Jääskeläinen, A. S.; Vuorinen, T., A UV resonance Raman (UVRR) spectroscopic study on the extractable compounds in Scots pine (Pinus sylvestris) wood: Part II. Hydrophilic compounds. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2004, 60 (13), 2963-2968. (25) Stamm, A.J.; Loughborough, W. K., Variation in shrinking and swelling of wood. Transactions of the American Society of Mechanical Engineers 1942, 64, 379-386. (26) Mantanis, G. I.; Young, R. A.; Rowell, R. M., Swelling of wood. Part III. Effect of temperature and extractives on rate and maximum swelling. Holzforschung 1995, 49, 239248. (41) Wålinder, M. E. P.; Johansson, I., Measurement of wood wettability by the Wilhelmy method. Part 1. Contamination of probe liquids by extractives. Holzforschung 2001, 55 (1), 21-32. (28) Hon, D. N. S., ESCA study of oxidized wood surfaces. Journal of Applied Polymer Science 1984, 29 (9), 2777-2784. (29) Rautkari, L.; Hänninen, T.; Johansson, L. S.; Hughes, M., A study by X-ray photoelectron spectroscopy (XPS) of the chemistry of the surface of Scots pine (Pinus sylvestris L.) modified by friction. Holzforschung 2012, 66 (1), 93-96. (30) Mohammed-Ziegler, I.; Hórvölgyi, Z.; Tóth, A.; Forsling, W.; Holmgren, A., Wettability and spectroscopic characterization of silylated wood samples. Polymers for Advanced Technologies 2006, 17 (11-12), 932-939. (31) Bryne, L. E.; Lausmaa, J.; Ernstsson, M.; Englund, F.; Wålinder, M. E. P., Ageing of modified wood. Part 2: Determination of surface composition of acetylated, furfurylated, and thermally modified wood by XPS and ToF-SIMS. Holzforschung 2010, 64 (3), 305-313.


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(32) Sernek, M.; Kamke, F. A.; Glasser, W. G., Comparative analysis of inactivated wood surface. Holzforschung 2004, 58, 22-31. (33) Popescu, C.-M.; Tibirna, C.-M.; Vasile, C., XPS characterization of naturally aged wood. Applied Surface Science 2009, 256 (5), 1355-1360. (34) Inari, G. N.; Petrissans, M.; Lambert, J.; Ehrhardt, J. J.; Gérardin, P., XPS characterization of wood chemical composition after heat-treatment. Surface and Interface Analysis 2006, 38 (10), 1336-1342. (35) Ström, G; Carlsson, G., Wettability of kraft pulps-effect of surface composition and oxygen plasma treatment. Journal of Adhesion Science and Technology 1992, 6 (6), 745-761. (36) Laine, J.; Stenius, P.; Carlsson, G.; Ström, G., Surface characterization of unbleached kraft pulps by means of ESCA. Cellulose 1994, 1 (2), 145-160. (37) Gustafsson, J.; Ciovica, L.; Peltonen, J., The ultrastructure of spruce kraft pulps studied by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Polymer 2003, 44 (3), 661-670. (38) Johansson, L.-S.; Campbell, J. M.; Fardim, P.; Hultén, A. H.; Boisvert, J.-P.; Ernstsson, M., An XPS round robin investigation on analysis of wood pulp fibres and filter paper. Surface Science 2005, 584 (1), 126-132. (39) Gindl, M.; Reiterer, A.; Sinn, G.; Stanzl-Tschegg, S. E., Effects of surface ageing on wettability, surface chemistry, and adhesion of wood. Holz als Roh- und Werkstoff 2004, 62 (4), 273-280. (40) Englund, F.; Bryne, L. E.; Ernstsson, M.; Lausmaa, J.; Wålinder, M., Spectroscopic studies of surface chemical composition and wettability of modified wood. Wood Material Science and Engineering 2009, 4 (1-2), 80-85.


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(41) Keey, R. B.; Langrish, T. A.G; Walker, J. C. F., Kiln-drying of lumber (Springer Series in Wood Science) Springer, Berlin, 1999. (42) Dorris, G. M.; Gray, D. G., The surface analysis of paper and wood fibres by ESCA. II. Surface composition of mechanical pulps. cellulose chemistry and technology 1978, 12, 721734. (43) Johansson, L.-S.; Campbell, J. M.; Koljonen, K.; Stenius, P., Evaluation of surface lignin on cellulose fibers with XPS. Applied Surface Science 1999, 144–145 (0), 92-95. (44) Taylor, A.; Gartner, B.; Morrell, J., Heartwood Formation and Natural Durability—A Review. Wood and Fiber Science 2002, 34 (4), 587-611. (45) Nguila Inari, G.; Pétrissans, M.; Dumarcay, S.; Lambert, J.; Ehrhardt, J. J.; Šernek, M.; Gérardin, P., Limitation of XPS for analysis of wood species containing high amounts of lipophilic extractives. Wood Science and Technology 2011, 45 (2), 369-382. (46) Metsä-Kortelainen, S.; Viitanen, H., Wettability of sapwood and heartwood of thermally modified Norway spruce and Scots pine. European Journal of Wood and Wood Products 2012, 70 (1-3), 135-139. (47) Metsä-Kortelainen, S.; Antikainen, T.; Viitaniemi, P., The water absorption of sapwood and heartwood of Scots pine and Norway spruce heat-treated at 170 °C, 190 °C, 210 °C and 230 °C. Holz als Roh- und Werkstoff 2006, 64 (3), 192-197. (48) Nussbaum, R. M.; Sterley, M., The effect of wood extractive content on glue adhesion and surface wettability of wood. Wood and Fiber Science 2002, 34 (1), 57-71. (49) Luner, P.; Sandell, M., The wetting of cellulose and wood hemicelluloses. Journal of Polymer Science Part C: Polymer Symposia 1969, 28 (1), 115-142. (50) Hodgson, K.; Berg, J., Dynamic Wettability Properties of Single Wood Pulp Fibers and Their Relationship to Absorbency. Wood and Fiber Science 1988, 20 (1), 3-17.


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(51) Holmberg, M.; Berg, J.; Stemme, S.; Ödberg, L.; Rasmusson, J.; Claesson, P., Surface Force Studies of Langmuir–Blodgett Cellulose Films. Journal of Colloid and Interface Science 1997, 186 (2), 369-381. (52) Lee, S.B; Luner, P., The wetting and interfaeial properties of lignins. Tappi journal 1972, 55, 116-121. (53) Laschimke, R., Investigation of the wetting behaviour of natural lignin - a contribution to the cohesion theory of water transport in plants. Thermochimica Acta 1989, 151 (0), 3556.


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table of contents


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Figure 1. A two-cycle Wilhelmy plate experiment with a wood sample. The specimen is immersed and withdrawn from the probe liquid while the instrument detects the force as a function of immersion depth. Due to the hygroscopic nature of the specimen, the final force (Ff) is measured at the end of each cycle corresponding to the liquid sorbed in the specimen. The advancing, receding and final forces of the first cycle are shown by FA, FR and Ff, respectively. 247x188mm (72 x 72 DPI)


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Figure 2. Schematic illustration of the multi-cycle Wilhelmy plate method. The sample is intermittently immersed in and withdrawn from the probe liquid. The veneer dimension is increased during the experiment as a consequence of swelling. The arrow length does not depict actual values. 213x121mm (150 x 150 DPI)


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Figure 3. XPS high resolution C1s spectra of pine sapwood and heartwood (C1: C-C, C=C, C-H at 285.0 eV; C2: C-O, C-O-C at 286.7 eV; C3: C=O, O-C-O at 288.2 eV; C4: O-C=O, C(=O)OH at 289.4 eV). 418x192mm (72 x 72 DPI)


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Figure 4. The relative amount of C1 aliphatic carbon as a function of the O/C atomic ratio for pine sapwood and heartwood samples. Values for cellulose, hemicellulose, fresh Scots pine, lignin and extractives (■) are calculated from literature data.35 233x177mm (72 x 72 DPI)


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Figure 5. Typical curves of multi-cycle Wilhelmy plate experiments on pine sapwood and heartwood veneers in water. The curves display the force (F) per perimeter (P) as a function of immersion depth (h) during a 20-cycle measurement. Liquid uptake increases with increasing cycle number. The veneers were oven dried with moisture content of close to zero. 411x193mm (72 x 72 DPI)


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Figure 6. Wilhelmy plate experiments using pine sapwood (the same sample as in Figure 5) in octane. (a) 10 cycles on oven dried veneer (moisture content ≈ 0 %) and (b) 2 cycles on water saturated veneer after 20 cycles in water. Due to the water penetration into voids before measurement, no initial wicking is observed in the wet veneer curve (plot b). 371x173mm (72 x 72 DPI)


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Figure 7. Liquid mass as a function of cycle number for pine sapwood and heartwood in water and octane. The results ware obtained by extrapolation of the final force (Ff) data to zero depth in multi cycle Wilhelmy experiments. 246x188mm (72 x 72 DPI)


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Figure 8. Apparent perimeter change of pine sapwood and heartwood veneers during multi-cycle Wilhelmy experiments. All data were calculated using the perimeter model (eq 7) with the exception of final perimeter (P20) which was determined by a single immersion in octane after 20 cycles in water. Octane immersion data of intermediate cycles (C5, C10, C15) are also shown. The standard deviation of the octane immersion and the model results are illustrated by error bars and filled patterned areas, respectively. 222x166mm (72 x 72 DPI)


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Figure 9. Surface tension reduction of the water phase after multi-cycle Wilhelmy experiments with pine sapwood and heartwood veneer samples. 288x201mm (300 x 300 DPI)


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Multicycle Wilhelmy Plate Method for Wetting Properties, Swelling and

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