Manchester Interdisciplinary Biocentre, The University of Manchester, 131 Princess Street, Manchester M1 7DN, UK.
J Phys Condens Matter. 2010 Jul 21;22(28):284108. doi: 10.1088/0953-8984/22/28/284108. Epub 2010 Jun 21.
An equation for the chemical potential of a dilute aqueous solution of noble gases is derived in terms of energies, force and torque magnitudes, and solute and water coordination numbers, quantities which are all measured from an equilibrium molecular dynamics simulation. Also derived are equations for the Gibbs free energy, enthalpy and entropy of hydration for the Henry's law process, the Ostwald process, and a third proposed process going from an arbitrary concentration in the gas phase to the equivalent mole fraction in aqueous solution which has simpler expressions for the enthalpy and entropy changes. Good agreement with experimental hydration free energies is obtained in the TIP4P and SPC/E water models although the solute's force field appears to affect the enthalpies and entropies obtained. In contrast to other methods, the approach gives a complete breakdown of the entropy for every degree of freedom and makes possible a direct structural interpretation of the well-known entropy loss accompanying the hydrophobic hydration of small non-polar molecules under ambient conditions. The noble-gas solutes experience only a small reduction in their vibrational entropy, with larger solutes experiencing a greater loss. The vibrational and librational entropy components of water actually increase but only marginally, negating any idea of water confinement. The term that contributes the most to the hydrophobic entropy loss is found to be water's orientational term which quantifies the number of orientational minima per water molecule and how many ways the whole hydrogen-bond network can form. These findings help resolve contradictory deductions from experiments that water structure around non-polar solutes is similar to bulk water in some ways but different in others. That the entropy loss lies in water's rotational entropy contrasts with other claims that it largely lies in water's translational entropy, but this apparent discrepancy arises because of different coordinate definitions and reference frames used to define the entropy terms.
从平衡分子动力学模拟中测量的能量、力和扭矩大小以及溶质和水的配位数，推导出稀水溶液中稀有气体化学势的方程。还推导出亨利定律过程、奥斯特瓦尔德过程和第三个提议过程的吉布斯自由能、焓和水合熵的方程，该过程从气相中的任意浓度到水溶液中的等效摩尔分数。在 TIP4P 和 SPC/E 水模型中，与实验水合自由能得到很好的一致，尽管溶质的力场似乎影响得到的焓和熵。与其他方法相比，该方法为每个自由度提供了熵的完整分解，并使对小非极性分子在环境条件下的疏水性水合伴随的已知熵损失的直接结构解释成为可能。稀有气体溶质的振动熵仅略有减少，较大的溶质损失更大。水的振动和旋转熵分量实际上增加，但只是略有增加，否定了任何关于水限制的想法。对疏水性熵损失贡献最大的是水的取向项，它量化了每个水分子的取向极小值数量以及整个氢键网络可以形成的方式数量。这些发现有助于解决实验中得出的相互矛盾的推论，即非极性溶质周围的水结构在某些方面与体相水相似，但在其他方面则不同。熵损失在于水的旋转熵，这与其他声称它主要在于水的平移熵的说法形成对比，但这种明显的差异是由于用于定义熵项的不同坐标定义和参考系引起的。
