On the size dependence of hydrophobic hydration
On the size dependence of hydrophobic hydration
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DOI：
10.1039/a805733h
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发表时间：
1998-11-21
期刊：
影响因子：
--
通讯作者：
Graziano, G
中科院分区：
文献类型：
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作者：
Graziano, G
在室温下，惰性气体在水中的溶解度随其尺寸的增大而增大，而气态脂肪烃在水中的溶解度随其尺寸的增大而减小。这一令人费解的实验观察是水作为溶剂的一个独特特征。对这一现象没有真正的解释，尽管有人认为这是非极性分子周围形成笼状结构的证据。在这篇文章中，我们证明了用Lee疏水水化理论可以很好地再现实验数据。这一理论的一个基本要素是证明了非极性溶质水化壳层中氢键的纯粹结构重组是一个补偿过程。溶解度取决于两个相反的因素的平衡：由于溶剂中空穴的产生而排除的体积熵变化，以及直接的溶质-溶剂范德华相互作用。空穴形成的工作占主导地位，决定了非极性化合物在水中的较差溶解度。然而，对于稀有气体，随着硬球直径的增加，范德华相互作用的绝对值增加得比空穴产生的功更快，从而提高了溶解度。相反，对于脂肪族碳氢化合物，随着硬球直径的增加，范德华相互作用的绝对值增加的速度慢于空穴形成功的增加，从而降低了溶解度。因此，可以解释水化吉布斯能的实验数据，而不会引起非极性溶质水化壳层中水结构的增强。
The solubility in water of noble gases, at room temperature, increases with their size, whereas that of gaseous aliphatic hydrocarbons decreases on increasing their size. This puzzling experimental observation is a unique feature of water as solvent. No real explanation of the phenomenon exists, even though it has been suggested that it is evidence of clathrate-type structure formation around nonpolar molecules. In this paper, we show that the experimental data can be reproduced well by means of Lee's theory of hydrophobic hydration. A fundamental ingredient of this theory is the demonstration that the purely structural reorganization of H-bonds in the hydration shell of a nonpolar solute is a compensating process. The solubility is determined by the balance of two contrasting factors: the excluded volume entropy change due to cavity creation in the solvent, and the direct solute-solvent van der Waals interactions. The work of cavity creation is dominant, determining the poor solubility of nonpolar compounds in water. However, for noble gases, on increasing the hard-sphere diameter, the van der Waals interactions increase, in absolute value, more rapidly than the work of cavity creation, enhancing the solubility. On the contrary, for aliphatic hydrocarbons, on increasing the hard-sphere diameter the van der Waals interactions increase, in absolute value, less rapidly than the work of cavity creation, lowering the solubility. The experimental data of hydration Gibbs energies, therefore, can be accounted for without invoking an enhancement of water structure in the hydration shell of a nonpolar solute.