Study of Lewis Acid-Base Properties of Wood by Contact Angle Analysis

Summary In the present work, an attempt is made to study the Lewis acid-base properties of wood by contact angle analysis. Using contact angle data for extracted, non-extracted, fresh and aged pine wood veneer samples, the Lewis acid-base contribution to the work of adhesion (W a AB ) and the Lewis acid-base components of the surface free energy of the wood samples were determined by basic wetting theory, and by the so-called van Oss-Chaudhury-Good (vOCG) model. Results show, on the one hand, that W a AB between water and the non-extracted wood decreases with increasing aging time. This indicates that extractives migrate from the interior of wood to its exterior surface during the aging, creating a hydrophobic wood surface. On the other hand, W a AB between the non-extracted wood and ethylene glycol tends to increase with longer aging times. No such trends are observed for extracted samples. If water is considered to be predominantly Lewis acidic, and ethylene glycol predominantly Lewis basic, the changes in W a AB with longer aging times, indicate that the acidity and the basicity of non-extracted wood in these cases increases and decreases, respectively. This is presumably due to the presence of the extractives, and to reorientation of functional groups and oxidation at the wood/extractives-air interface. The acid-base characterization by the vOCG model also indicates that the acidity and the basicity of the non-extracted wood increases and decreases, respectively, with increasing aging time. However, there seems to be a similar trend even for extracted wood, presumably due to reorientation of functional groups at the wood-air interface. In the vOCG model, considerably higher base/acid ratios are obtained when using probe liquid parameters according to van Oss compared to those obtained by using liquid parameters according to Della Volpe and Siboni.