Surface energy determinations of wood: Comparison of methods and wood species

The Lifshitz-van der Waals, acid-base, and total surface free energies of various wood species were calculated from contact angle measurements. For spruce (Picea abies) and meranti (Shorea spp.) the following three methods were compared: capillary rise in wood powder columns (based on the Washburn equation), dynamic contact angle measurements (according to the Wilhelmy-plate principle), and sessile drop measurements along and across the grain of the wood. The capillary rise method was limited to nonswelling solvents, which means that only the Lifshitz-van der Waals component could be measured. With the dynamic contact angle measurement, the wettability during the first immersion was decreased compared to that of the sessile drop. This was probably due to reduced capillary penetration, but with the second immersion the presence of an adsorbed solvent layer increased the wettability and hence affected the surface energy data. The sessile drop measurements were highly dependent on the direction of measurement. Increasing the wood moisture content decreased the Lifshitz-van der Waals component and increased the basic surface energy parameter of the wood. All of the wood species tec;ted were characterized as having low-energy surfaces with a dominant Lifshitz-van der Waals component. Measurement of acid and base parameters of wood surfaces seemed not to be very reliable because of its strong dependence on the measuring conditions. With respect to this, it should be noted that thermodynamic equilibrium conditions assumed by Young's equation are generally not fulfilled with wood surface a because of chemical heterogeneity, surface roughness, and the adsorption of the test solvent.