Moisture Adsorption Thermodynamics of Chemically Modified Wood

The isotherms for untreated and chemically modified wood specimens were analyzed using the Hailwood-Horrobin equation. The enthalpy (ΔH), entropy (ΔS) and free energy (ΔG) changes during moisture adsorption were estimated. The ΔH, ΔS and ΔG values of untreated wood were compared with those found in literature as well as those of specimens treated with formaldehyde (formalization), acetic anhydride, glyoxal, glutaraldehyde and dimethylol dihydroxy ethyleneurea (DMDHEU). The ΔH of the untreated specimen was almost constant up to ca. 3% of moisture content differing from the results reported by others. However, the bonding energy of hydrated water molecules, which represented a large portion of the adsorbed water in the low moisture content range, should not depend on the moisture content, so that the result was undoubtedly reasonable. The decrease of the absolute ΔH for formalization and acetylation indicates the remarkable loss of the hydrated water as a result of those reactions. A peak of the absolute ΔS found at ca. 3% of moisture content except for acetylation may reflect the rearrangement of adsorption sites associated with taking up a little amount of water. In the acetylation, such rearrangement could not occur because of the introduction of bulky hydrophobic groups. Remarkable increases of both absolute ΔH and ΔS observed in glyoxal treated specimens will be due to the ester groups introduced. The ΔG values of all treated specimens became less negative than that of untreated. This indicates the formation of some bond which restricts the swelling of wood structure.