Adsorbed Water in the Cell Wall

The adsorbed water is held quite tenaciously. The amount of heat needed to remove the adsorbed water from wood is greater than that needed to remove the absorbed water. This can be measured quite simply by immersing oven-dry ground-wood particles in a known quantity of water and measuring the temperature rise (Stamm, 1964). The heat released on completely wetting a gram of oven-dry 

For most woods the initial differential heat of sorption (at 0% moisture content) is approximately 1250 J g . This equates to 22.5 kJ mol (18 g of water in one mol) which is typical for hydrogen bonding. By the time a complete monolayer has formed, by about 4-5% moisture content, the differential heat of wetting is around half the initial value (9-10 kJ moF ). This value would be amongst the weakest hydrogen bond values reported. The first water molecules to be adsorbed on the oven-dry ground-wood have a complete choice of accessible cell wall hydroxyls and 

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If fresh green wood is heated in water one would expect the amount of adsorbed water in the cell walls to deerease, the volume of the swollen cell walls to contract and the wood to shrink, even though it is still green. In practice the matter is more complicated due to the presenee of growth stresses in wood that are partially relieved at high temperatures (Yokota and Tarkow, 1962). 

In measurements of the dielectric relaxation of water adsorbed on acetylated wood, a large change in the activation enthalpy and entropy of dielectric relaxation was found to occur at 6 % moisture content (Zhao etal., 1994), this presumably being attributable to the onset of formation of capillary water in the cell wall. 

By eontrast adsorption is different. Adsorbed water within the cell wall remains even at very low vapour pressures indicating that the attractive force between the adsorbent (in this case wood) and the adsorbate (water) is much greater than the... 

The actual adsorption process is not fully understood. In partieular there is a dearth of experimental detail on the aetual distribution of the adsorbed water within the cell wall at different moisture eontents. At low relative humidities the water molecules penetrate the non-erystalline regions of the cell wall in the proeess of... 

One might anticipate that if the water in the cell wall were to be frozen and then the ice were to be subliminated off there should be no liquid capillary tension, no cell wall shrinkage and it should be possible to create a porous cell wall. However, sublimating the water molecules from the cell wall at -20°C does not prevent collapse of the internal pore structure (Merchant, 1957). This implies that the cell wall water is not actually frozen at this temperature the cell wall still shrinks and very little internal surface is created. Indeed there is evidence (Tarkow, 1971) that at least some adsorbed water does not lose the mobility characteristic of the liquid phase until very low temperatures (<-80°C). Of course water in the lumens behaves like bulk water and freezes at a temperature between -0.1°C and -2.0°C, depending on the concentration of dissolved sugars in the sap. 

In green wood, the cell walls are saturated, whereas some cell cavities are completely filled and others may be completely empty. Moisture in the cell walls is called bound, hygroscopic, or adsorbed water. Moisture in the cell cavities is called free or capillary water. The distinction is made because, under ordinary conditions, the removal of the free water has litde or no effect on many wood properties. On the other hand, the removal of the cell wall water has a pronounced effect. 

This analysis is a little simplistic (Alince, 1989) in that it treats the water and the cell wall as two distinct phases rather than as a solid solution and, besides, the adsorbed water might encourage a realignment of the cell wall material itself which may then pack more closely, i.e. the cell wall material could equally well be densified rather than just the water. 

Water in wood ean exist as either absorbed (also called free water) in the cell lumens and intereellular spaees or as adsorbed (also called bound water) within the cell walls. When wood dries water first evaporates from the lumens and intercellular spaees. The fibre saturation point is defined as the moisture content at which all the absorbed water has been removed but at which the cell walls are still fully saturated. This oeeurs at a moisture eontent of 25 to 35%. In most instances it is adequate to presume the fibre saturation point to be 30% moisture content. 

Figure 3.4. Water adsorbed in the cell wall of Picea mariana (Stone and Scallan, 1968). The amount of adsorbed water that is accessible to a polymer molecule increases as the size of the polymer molecule decreases. During pulping the cell wall is opened up with both pore volume and pore size distribution increasing with the degree of delignification. The yield is the ratio of the weight of oven-dry fibre remaining after pulping to the initial weight of oven-dry wood.

Moisture may be present in lignocellulosic fibers in the cell voids or lutnina (free water) and adsorbed to the cellulose and hemicellulose in the cell wall (bound water). 

The ready accumulation of plutonium by seaweeds, concentration factors of 10s have been observed (173), is presumably the result of plutonium uptake by the sul-phated polysaccharides which compose part of the cell wall. It is possible that plutonium could concentrate on manganese nodules in the deep ocean since it has been shown that manganese dioxide adsorbs plutonium from water (174). 

Maximum Shrinking and Swelling of the Cell Wall. When dry wood is immersed in water the cell wall swells in proportion to the volume of water adsorbed. If it is assumed that the sorbed water has the same density as free liquid water, the percent swelling Su of the cell wall can be approximated by Equation 5. 

The concept of the fibre saturation point cannot be over emphasized. Its importance lies in the fact that the manner in which water is held is different in the adsorbed and absorbed states. The fact that the absorbed water can be removed before stripping off the adsorbed water indicates that a distinction can be made between these two states of sorption. For example, when green wood is dried there is no appreciable change in its mechanical properties until the fibre saturation point is reached. Below this moisture content they increase almost linearly with any further decrease in moisture content. Furthermore, wood only shrinks when the adsorbed water is removed from the cell walls. 

The amount of moisture adsorbed by the cell walls is directly related to the humidity of the surrounding air. Relative humidity or relative vapour pressure is defined as the ratio of the amount of water vapour in the atmosphere to the amount... 

Another approach treats the cell wall water as a solution phenomenon, with the water of hydration accounting for the first 4-6% of moisture adsorbed or the last moisture lost during desorption (essentially the same 4-6% that accounts for the monolayer in the BET analysis). All further adsorbed cell wall water is the water of solution. In a similar vein Harley and Avramidis (1993) calculate that where moisture contents exceed 20-25% the water of solution forms clusters of 10 or more water molecules that corresponds to a hydraulic spheroid about 0.6 nm in diameter. 

Wood only shrinks when water is lost from the cell walls and it shrinks by an amount that is proportional to the moisture lost below fibre saturation point. To a first approximation the volumetric shrinkage is proportional to the number of water molecules that are adsorbed within the cell wall, and that in turn is related to the number of accessible hydroxyls on the cellulose, hemicelluloses and lignin, and to the amount of cell wall material, i.e. the basic density of the wood (Figure 4.2). 

The cell wall of wood can be considered to consist of a non-crystalline matrix of lignin and hemicelluloses in which strong, stiff cellulosic microfibrils are embedded. The crystalline microfibrils exhibit no tendency either to adsorb moisture or to change in length or cross-section. On the other hand the non-crystalline isotropic matrix can lose and gain water and shows a considerable tendency to shrink and swell. In isolation one would expect the matrix to shrink or swell equally in all directions, that is Ox = Oy = = tto and Ovoi = 3ao i.e. ao is the isotropic shrinkage in... 

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