Abstract:

The crosslinked reverse osmosis membrane model was established by molecular simulation. The diffusion process of water, sodium chloride, boric acid, borate ion, and D-sorbitol borate complexes in reverse osmosis membranes and solutions were studied by molecular dynamics simulation. The diffusion processes of complexes of borate and polyhydroxy compounds (D-mannitol, N-methyl D-glucamine, and sodium D-gluconate) in four types of reverse osmosis membranes were also simulated. The mean square displacement (MSD) curve of each particle could be obtained. The diffusion coefficient of each particle in the solution is greater than that in the membrane system. The diffusion coefficient of water molecule is much larger than those of solute molecules or ions. Compared with boric acid, there is no much difference in the particle volume for the borate ion, the diffusion coefficient would be influenced by the charge effect, thus, the diffusion coefficient of borate ion is an order of magnitude less than that of boric acid. Compared with monochelate complexes of D-sorbitol, the diffusion coefficient of dichelate complexes of D-sorbitol is smaller, due to the increase of molecular volume. Thus, for the boron removal enhanced by complexation in reverse osmosis seawater desalination process, the dosage of D-sorbitol should be kept at higher level, in order to increase the concentration of the dichelate complexes of D-sorbitol. For the 1,3,5-cyclohexane triformyl chloride/4-methylphenylene-diamine (HT/MMPD) membrane, the diffusion coefficient of N-methyl D-glucamine borate complexes is the lowest among the five complexes. The diffusion coefficient of sodium D-gluconate borate complexes is relative low in the type of membrane containing m-phenylenediamine (MPD) monomer, while the diffusion coefficient of D-mannitol borate complexes is relative low in the type of membrane containing MMPD monomer. The trajectories of several particles are also analyzed. Each particle takes sparse and irregular diffusion movements in the solution. In the membrane, the particles are subjected to intensive diffusion movements due to the restriction of polyamide membrane structure.

Key words: molecular dynamics simulation, reverse osmosis, molecular diffusion coefficient, membranes, boron

摘要：

利用分子模拟的方法建立了交联的反渗透膜模型，采用分子动力学模拟方法研究了水、氯化钠、硼酸、硼酸盐、山梨醇-硼酸盐络合物在反渗透膜和溶液中的扩散过程，以及其他几种硼酸盐与多羟基化合物（D-甘露醇、N-甲基D-葡糖胺和D-葡萄糖酸钠）的络合物在4种反渗透膜中的扩散过程。分析各粒子的均方位移曲线可得，各粒子在溶液中的扩散系数均大于在膜体系的扩散系数，水分子的扩散系数远大于各溶质分子和离子。与硼酸分子相比，硼酸盐离子与其体积相差不大，但受电荷效应的影响，其扩散系数小于硼酸分子一个数量级。与单络合物相比，双络合物由于分子体积增大，扩散系数更小。因此在反渗透络合强化脱硼过程中应保持过量的山梨醇添加量，以提高其双络合物浓度。在1,3,5-环己烷三甲酰氯/4-甲基间苯二胺（HT/MMPD）反渗透膜中，N-甲基D-葡糖胺-硼酸盐络合物的扩散系数是5种络合物中最低的，D-葡萄糖酸钠-硼酸盐络合物在含有间苯二胺（MPD）单体的膜类型中扩散系数较低，而D-甘露醇-硼酸盐络合物在含有MMPD单体的膜类型中扩散系数较低。由各粒子的轨迹图可知，各粒子在溶液中做较稀疏的无规则扩散运动，在膜内粒子由于受到聚酰胺膜结构的限制做较密集的扩散运动。

关键词: 分子动力学模拟, 反渗透, 分子扩散系数, 膜, 硼