Mechanism of NaCl effect on diffusion of water molecules in Cunninghamia lanceolata
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摘要:目的 NaCl的存在会改变木材中水分子的扩散行为，使水分子“易进入”或“难扩散”，从而影响了木材的吸湿性和平衡含水率。研究其影响机理对含盐木质文物的保护具有重要意义。方法 以杉木为研究对象，制作了不同含盐率的杉木试件，开展吸湿与干燥试验，探究盐分对杉木中水分扩散的宏观影响规律；建立了杉木的分子模型，采用分子动力学（MD）模拟方法分别研究了水分子进入含盐与不含盐两种模型的过程，以及水分子在两种模型中的扩散行为。通过平衡构型、相对浓度分布等参数分析水分子进入两种模型结果的差异；通过均方位移、吸附能、径向分布函数、氢键等参数分析两种模型中的水分子与纤维素分子、半纤维素分子、木质素分子等杉木主要组成成分之间的相互作用关系，定性解释了盐分影响杉木中水分扩散的微观机理。结果 试验结果表明杉木试件的吸湿速率随着含盐率的升高而增大；在含水率低于80%范围内，干燥速率随含盐率的升高而减小。MD模拟结果表明，水分子更容易进入含有NaCl的杉木分子模型中，但NaCl会抑制水分子在杉木分子模型中的扩散。模型中各分子之间的相互作用分析结果显示，NaCl与水分子间有很强的吸附作用，但会降低水分子与杉木之间作用强度。结论 NaCl使杉木对水分子的束缚作用降低，但其对水分子的强吸附作用导致了水分子“易进入”与“难扩散”现象。Abstract: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.
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Keywords:
- NaCl /
- water molecular diffusion /
- Cunninghamia lanceolata /
- molecular dynamics
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图 1 吸湿与干燥速率随含水率的变化关系
Figure 1. Curves of moisture absorption and drying rate with moisture content
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图 7 径向分布函数曲线
g(r)为径向分布函数。g(r) is ridial distribution function.
Figure 7. Radial distribution function curves
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图 8 氢键判定几何标准示意图
原子D代表施主，以化学键与氢原子H相连，原子A代表受主，与氢原子形成氢键，R为施主与受主之间距离，β为施主、氢原子和受主形成的夹角。The atom D represents the donor, which can form a chemical bond with a hydrogen atom H. The atom A represents the acceptor, which can form a hydrogen bond with a hydrogen atom H. R is the distance between the donor and the acceptor, and β is the angle formed by the donor, hydrogen atom and the acceptor.
Figure 8. Schematic diagram of geometric standard for hydrogen bond determination
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图 9 氢键数目统计
NC-W为纤维素与水分子之间氢键数，NH-W为半纤维素与水分子之间氢键数，NL-W为木质素与水分子之间氢键数，NTotal为杉木与水分子间总氢键数。 NC-W is the number of hydrogen bonds between cellulose and water molecules, NH-W is the number of hydrogen bonds between hemicellulose and water molecules, NL-W is the number of hydrogen bonds between lignin and water molecules, and NTotal is the total number of hydrogen bonds between Cunninghamia lanceolate and water molecules.
Figure 9. Statistics of the number of hydrogen bonds
表 1 杉木与水分子间的吸附能
Table 1 Adsorption energy between Cunninghamia lanceolate and water molecules
kcal/mol 能量
Energy杉木分子模型
Molecular model of
Cunninghamia lanceolate (P-C)含盐杉木分子模型
Salty molecular model of
Cunninghamia lanceolate (Y-C)杉木–水
Cunninghamia
lanceolate-water杉木–水
Cunninghamia
lanceolate-water含盐杉木–水
Salty Cunninghamia
lanceolate-water总吸附能
Total adsorption energy (ETotal)−1 314.79 −1 111.25 −1 830.58 范德华力吸附能
van der Waals adsorption energy (EVdW)99.51 89.01 186.624 静电力吸附能
Electrostatic adsorption energy (EEle)−1 348.915 −1 189.26 −2 006.05 
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