Quantitative characterization of the subsurface acid–base properties of wood by XPS and Fowkes theory

Abstract

The subsurface acid–base properties of pine wood have been quantitatively characterized in terms of the C1s peak displayed by the X-ray photoelectron spectroscopy (XPS) and the Fowkes acid–base theory. Using the ratio of acidity to basicity, A/B, to characterize the pine wood, it is found that the pine wood is strongly basic and weakly acidic as expected from previous wetting experiments. It is found, however, that the wetting method gives an acid/base ratio which is lower than that determined by XPS. The difference is suggested to be due to the probing depth. The wetting result corresponds to the outermost surface of wood while the XPS value reflects the subsurface of wood. The surface is known to be more easily oxidized than the subsurface. Furthermore, based on a comparison in which the acid/base ratio is plotted as a function of the oxygen/carbon ratio for pine wood and some hardwoods reported in the literature, the softwood seems to have a lower A/B ratio than the hardwood. The reason is that the softwood has a stronger basicity and a weaker acidity than the hardwood.