Variability in surface polarity of wood by means of AFM adhesion force mapping

• Polarity of wood surfaces was measured by means of AFM adhesion force mapping.
• Microstructural variability in polarity of wood can be imaged by AFM at atmospheric conditions.
• Differences in polarity of freshly cut and native surfaces correlate with chemical heterogeneity.
• Surface ageing of wood originates almost exclusively from changes in freshly cut secondary cell walls S2.
• Native inner lumen surfaces hardly contribute to surface ageing.

The adhesion force between polar AFM tips and freshly cut wood surfaces was studied using AFM adhesion force mapping. Clear differences in polarity were found at microstructural level between freshly cut cell walls and native inner cell surfaces. Distinctly higher polarity in freshly cut cell walls is attributed to an abundance of accessible hydrophilic carbohydrate cell wall polymers compared to native cell surfaces. In good agreement with macroscopic studies of wood surface inactivation, the polarity of freshly cut cell walls showed a significant decrease with increasing surface age. Since such a decrease was observed to a much lesser degree in native inner surfaces compared to freshly cut cells it is proposed that predominantly the inactivation of freshly cut cell walls is responsible for overall surface inactivation phenomena in wood. It is concluded that the wood surface is heterogeneous in polarity, and that only the primarily hydrophilic wood surface regions are subject to inactivation, whereas surface chemistry of inner cell wall surfaces is very little affected by surface age.

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