Effects of sodium chloride on mechanical properties in amorphous polymers of waterlogged archaeological wood: Insights from molecular dynamics simulations

Sodium chloride (NaCl) is the main soluble salt component of marine waterlogged archaeological wood, and it causes deterioration of wood fibres. In this study, a molecular dynamics simulation of three types of archaeological wood from the Quanzhou Song Shipwreck was employed to investigate the impact of NaCl on the mechanical properties of amorphous components consisting of amorphous cellulose, hemicellulose, and lignin. The findings revealed that water molecules bound to the fibres enhanced the mechanical performance of the woody fibres. However, NaCl affected the stability and mechanical properties of the amorphous components. By using molecular dynamics simulations, 24 sets of amorphous fibre models, consisting of fir, pine, and camphorwood with salt concentrations ranging from 0 % to 7 %, were constructed to further analyse the influence of salt content on the mechanical properties of the amorphous components. The results showed that all three species of archaeological wood exhibited similar trends, with the adsorption energy and radial distribution function between wood fibres and water molecules decreasing with increasing NaCl concentration, leading to the gradual rupture of hydrogen bonds in water molecules. Consequently, more water molecules engage in ion hydration, resulting in an increase in the amount of free water within the amorphous regions. This, in turn, degrades the mechanical properties of the amorphous components. This study contributes significantly to a comprehensive understanding of the mechanical properties of marine archaeological wood and is important for decision-making in studies related to the preservation and evaluation of marine waterlogged wooden artefacts.