Concretely, assuming a diffusion coefficient of 10 -7 cm /s, as is the case for Na + in acetylated wood in our study (Fig. ), the ion would penetrate approximately 0.4 mm after an hour of uptake, compared with 1.2 mm for unacetylated wood. Iron, which already diffuses much more slowly due to the strong interaction with the carboxyl groups, is further slowed as the fraction of bulk water is reduced with increasing acetylation (Fig. ).