The strength of the acetyl-ion interaction clearly cannot compete with the strength of the electrostatic attraction between the positively charged cation and the negatively charged carboxyl group. Indeed, there are approximately 28 times more acetyl groups at 18% WPG compared to the relatively rare carboxyl groups, and so individually each cation-acetyl group interaction is relatively weak. However, since there are so many of these acetyl groups to interact with, and cations interact more strongly acetyl groups than hydroxyl groups (Fig. ), wood acetylation increases the potential for ions to be held up as they diffuse through the material. Indeed, there are by construction an equal number of acetyl groups in the fully acetylated system as there are hydroxyl groups in the unacetylated system, thus the ∼5x change in the peak heights between hydroxyl and acetyl systems highlights this strengthened interaction.