Characteristics of water transport in relation to microscopic structure in Nafion membranes
Characteristics of water transport in relation to microscopic structure in Nafion membranes
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Nafion 膜中水传输特性与微观结构的关系
DOI：
10.1039/ft9969200663
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发表时间：
1996
期刊：
影响因子：
--
通讯作者：
T. Okada
中科院分区：
文献类型：
--
作者：
G. Xie;T. Okada
用流动电位法和阻抗测量法研究了Nafion膜、水分子和具有亲水性和疏水性的有机脂肪族铵离子之间的相互作用。结果表明，对于与脂肪族氯化铵溶液平衡的膜，膜的水迁移系数随着阳离子摩尔体积的增加而线性增加，与其疏水性无关，表明水分子主要通过阳离子的泵送作用与疏水阳离子一起携带。通过与亲水性碱金属阳离子的结果比较，提出了Nafion膜内伴随阳离子迁移的水迁移模型。迁移的水分子归因于被转移阳离子的水合水分子，它们与自由溶液中几乎相同，以及由于阳离子在膜内狭窄的通道中的体积排斥效应，阳离子在流体力学中“泵送”的水分子与水合阳离子的大小成正比。结果还表明，这种脂肪族阳离子的加入导致了膜的水分含量下降。此外，大多数阳离子的水迁移系数都大于或至少接近相应膜的含水率。与亲水性无机阳离子一样，这些疏水性有机阳离子似乎也优先通过Nafion膜中的亲水区域移动，当膜与这些疏水有机阳离子平衡时，膜中只有部分交换位置参与了离子传导，其余的交换位置是不活跃的，不能贡献膜的离子电导。膜中连接离子-水团簇的通道的有效直径约为0.67-0.83 nm。
The interactions among Nafion membranes, water molecules and organic aliphatic ammonium cations which have both hydrophilic and hydrophobic parts have been studied using the streaming-potential method and the impedance-measurement method. The results showed that, for membranes equilibrated with an aliphatic ammonium chloride solution, the water-transference coefficients of the membranes increased linearly with increasing molar volume of the cations, independently of their hydrophobicity, indicating that the water molecules are carried along with the hydrophobic cations predominantly by the pumping effect of the cations. By comparison with the results for hydrophilic alkali-metal cations, a model of water transport accompanying cation transport inside the Nafion membranes was proposed. The transported water molecules are attributed to the hydrated water molecules of the transferred cations, which are almost the same as in free solutions, and to the water molecules hydrodynamically ‘pumped’ by the cations, which are proportional to the hydrated cation size, owing to the cation's volume exclusion effect in the narrow channel inside the membrane. The results also revealed that the incorporation of such aliphatic cations caused a decrease in water content of the membrane. Furthermore, the water-transference coefficients of most of the cations probed were found to be larger than, or at least close to, the water content of the corresponding membranes. It seems that even these hydrophobic organic cations move preferentially through hydrophilic domains in the Nafion membranes as hydrophilic inorganic cations do, and only some of the exchange sites in the membrane take part in the ionic conduction and the rest of the exchange sites are not active and cannot contribute to the ionic conductance of the membrane when the membrane is equilibrated with these hydrophobic organic cations. The effective diameter of the channel connecting ion–water clusters in the membrane was estimated to be ca. 0.67–0.83 nm.