Ohki Takumi, Harada Makoto, Okada Tetsuo
Department of Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan.
J Phys Chem B. 2007 Jun 28;111(25):7245-52. doi: 10.1021/jp071666j. Epub 2007 Jun 1.
Solvation of ions in concentrated aqueous poly(ethylene glycol) (PEG) has been studied from thermodynamic and structural viewpoints using ion-transfer voltammetry at the interface between aqueous and nitrobenzene phases and X-ray absorption fine structure (XAFS). Systematic changes in the ion-transfer potential from water to aqueous PEG have been confirmed for several ions relative to the corresponding potential of tetraethylammonium ion (Et4N+), which is almost independent of PEG concentration. The results obtained for alkali cations strongly suggest the involvement of their complexation with PEG even in relatively diluted PEG solutions. It has been implied that the solvation circumstances of Br- and ClO4- are drastically altered when the PEG concentration becomes higher than particular critical values (e.g., 30-50% PEG200), where free water molecules are diminished because of the hydration of PEG. XAFS measurements have also been performed for K+ and Br- to get direct evidence for these findings. Although the spectra at the K K-edge clearly indicate the presence of a PEG complex of K+ in relatively diluted PEG solutions ( approximately 33% PEG200), an obvious increase in its ion-transfer potential has been detected at lower PEG concentrations, indicating that complexes formed at the interface rather than in bulk solution are transferred into an organic phase. Br- is fully hydrated in 0-50% PEG solutions, whereas some water molecules are replaced by PEG when the PEG concentration increases. Increasing the PEG concentration causes decreases in the coordination number from 6 in water to 2-3 in neat PEG. Thus, the present approach not only has elucidated the structural and thermodynamic aspects of ionic solvation in aqueous PEG but also has provided the information of the hydration of PEG.
从热力学和结构角度，利用水相和硝基苯相界面处的离子转移伏安法以及X射线吸收精细结构（XAFS），研究了离子在浓聚乙二醇（PEG）水溶液中的溶剂化作用。相对于几乎与PEG浓度无关的四乙铵离子（Et4N+）的相应电位，已证实几种离子从水到PEG水溶液的离子转移电位存在系统性变化。碱金属阳离子的研究结果强烈表明，即使在相对稀释的PEG溶液中，它们也会与PEG发生络合。这意味着当PEG浓度高于特定临界值（例如30 - 50% PEG200）时，Br-和ClO4-的溶剂化情况会发生剧烈变化，此时由于PEG的水合作用，自由水分子减少。还对K+和Br-进行了XAFS测量，以直接证明这些发现。尽管在K K边的光谱清楚地表明在相对稀释的PEG溶液（约33% PEG200）中存在K+的PEG络合物，但在较低PEG浓度下检测到其离子转移电位明显增加，这表明在界面而非本体溶液中形成的络合物被转移到有机相中。在0 - 50% PEG溶液中，Br-完全水合，而当PEG浓度增加时，一些水分子被PEG取代。PEG浓度的增加导致配位数从水中的6降至纯PEG中的2 - 3。因此，本方法不仅阐明了离子在PEG水溶液中溶剂化的结构和热力学方面，还提供了PEG水合作用的信息。
