Analytical Sciences

Temperature Effect on the Selective Hydration of Sodium Ion in Nitrobenzene

Toshiyuki OSAKAI,*  Akiko TOKURA,* Hajime OGAWA,* Hiroki HOTTA,* Masaru KAWAKAMI,** and Kazuyuki AKASAKA***

*Department of Chemistry, Faculty of Science, Kobe University, Nada, Kobe 657-8501, Japan
**Department of Molecular Science, Graduate School of Science and Technology, Kobe University, Nada, Kobe 657-8501, Japan
***Department of Biotechnological Science, School of Biology-Oriented Science and Technology, Kinki University, Uchita-cho, Wakayama 649-6493, Japan

The effect of the temperature on the co-extraction of water molecules with Na⁺ from water to nitrobenzene (NB) in the presence of dipicrylaminate ion has been studied. The number (n) of water molecules co-extracted with a Na⁺ ion, as measured by the Karl Fischer method, increased from 3.1 to 5.2 with increasing temperature (6 - 65°C). This observation is in apparent contradiction to the expectation from simple thermodynamics because hydration is generally an entropically unfavorable process. Additional ¹H NMR experiments for the selective hydration of Na⁺ in deuterated NB have confirmed that the association constants of water with Na⁺ indeed decrease with increasing temperature. On the other hand, however, it has been shown that water solubility into NB substantially increases with temperature. We conclude that the latter effect overwhelms the former unfavorable entropy effect, which results in a net increase of the n-value, as observed.

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