Moisture-sensitivity is an inherent feature of cellulosic materials from wood to nanocelluloses, affecting their physical properties and performance in many applications. Moreover, the accessibility of the nanoscale components of plant cell walls to water govern their reactivity and processability in treatments taking place in aqueous environments. Currently, the limited understanding on the interactions between water and the nanostructure of plant-based materials restricts their exploitation in full. Utilizing new methods to study the cell wall structure and the location of water in it may produce information that helps to optimize accessibility and to tailor the properties of cellulose-based materials.
We investigated the water-accessibility of nanoscale pores within the fibrillar structures of Norway spruce (Picea abies) cell walls by using a combination of small-angle neutron scattering (SANS) and Fourier-transform infra-red (FT-IR) spectroscopy. SANS has different sensitivity to hydrogenated and deuterated water (H₂O/D₂O), which allows determining the location of water in the hierarchical cell wall structure non-destructively and in real-time. We used this capability to follow the exchange of water at different levels of the hierarchical cell wall structure in wood samples transferred from a solvent with 65% H₂O/35% D₂O to 100% D₂O. FT-IR spectroscopy, on the other hand, was used to study the time-dependent re-exchange of OD groups to OH in wood samples transferred from liquid D₂O to H₂O. In addition, the effects of drying on the nanoscale structure and water-accessibility of the pores were addressed by comparing the SANS data and the kinetics of water exchange between never-dried and dried/re-wetted wood samples.
Our SANS results showed that water in the interfibrillar pores was exchanged within tens of minutes, with indications of a slightly faster rate at the lengthscale above 5 nm than below. Based on the FT-IR data, the re-exchange of OD groups to OH was considerably slower, requiring several hours or days. No significant effects of drying were observed in any of the experiments, indicating that in the light of the current methodology, the nanostructure of wood was recovered by immersing in liquid water for a sufficiently long time. The results demonstrate the possibilities of SANS in addressing the location and kinetics of water in hierarchical plant cell walls and offer new insights to understand wood-water interactions.

Dr. Paavo Penttilä

Academy of Finland Research Fellow, Aalto-yliopisto

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