Mechanism of NaCl effect on diffusion of water molecules in Cunninghamia lanceolata

  Objective  The existence of NaCl can change the diffusion behavior of water molecules in wood, making water molecules “easy to enter” or “difficult to diffuse”, thus affecting the moisture absorption and equilibrium moisture content of wood. It is of great significance to study the mechanism of its influence on the protection of salty wooden cultural relics.

  Method  Taking Cunninghamia lanceolata as the research object, the samples with different salt content were made, and the moisture absorption and drying experiments were carried out to explore the macro effect of salt on water diffusion in Cunninghamia lanceolata. The molecular model of Cunninghamia lanceolata was established. The molecular dynamics (MD) simulation method was used to study the process of water molecules entering into the two models with and without salt, and the diffusion behavior of water molecules in the two models. The difference between the two models was analyzed by equilibrium configuration and relative concentration distribution. Based on the mean square displacement, adsorption energy, radial distribution function, hydrogen bond and other parameters, the interaction relationship between water molecules and cellulose, hemicellulose, lignin and other main components of Chinese fir in the two models were analyzed, and the microscopic mechanism of water diffusion affected by salt was qualitatively explained.

  Result  The results showed that the moisture absorption rate of Cunninghamia lanceolata increased with the increase of salt content. When the water content was lower than 80%, the drying rate decreased with the increase of salt content. MD simulation results showed that water molecules were easier to enter the molecular model of Cunninghamia lanceolata containing NaCl, but NaCl can inhibit the diffusion of water molecules in the molecular model of Cunninghamia lanceolata. The results of the interaction analysis among the molecules in the model showed that NaCl can reduce the interaction strength between water molecules and cellulose molecules, hemicellulose molecules, but the interaction strength between lignin and water molecules was enhanced, and the overall performance was that the interaction strength between Cunninghamia lanceolata molecules and water molecules was reduced.

  Conclusion  NaCl can reduce the binding effect of Cunninghamia lanceolata on water molecules, but its strong adsorption on water molecules leads to the phenomenon of “easy entry” and “difficult diffusion” of water molecules.