Kinoshita Masahiro, Harano Yuichi, Akiyama Ryo
International Innovation Center, Kyoto University, Uji, Kyoto 611-0011, Japan.
J Chem Phys. 2006 Dec 28;125(24):244504. doi: 10.1063/1.2403873.
The changes in excess thermodynamic quantities upon the contact of two solutes immersed in a solvent are analyzed using the radial-symmetric and three-dimensional versions of the integral equation theory. A simple model mimicking a solute in water is employed. The solute-solute interaction energy is not included in the calculations. Under the isochoric condition, the solute contact always leads to a positive entropy change irrespective of the solute solvophobicity or solvophilicity. The energy change is negative for solvophobic solutes while it is positive for solvophilic ones. Under the isobaric condition, the contact of solvophobic solutes results in system-volume compression but that of solvophilic ones gives rise to expansion. Effects of the compression and expansion on the changes in enthalpy and entropy are enlarged with rising temperature. When the solute solvophobicity is sufficiently high, the entropy change (multiplied by the absolute temperature) can become negative due to the compression, except at low temperatures with the result of an even larger, negative enthalpy change. The expansion in the case of solvophilic solutes leads to a large, positive entropy change accompanied by an even larger, positive enthalpy change. The changes in enthalpy and entropy are strongly dependent on the temperature. However, the changes in enthalpy and entropy are largely cancelled out and the temperature dependency of the free-energy change is much weaker. The authors also discuss possible relevance to the enthalpy-entropy compensation experimentally known for a variety of physicochemical processes in aqueous solution such as protein folding.
利用积分方程理论的径向对称和三维版本，分析了浸入溶剂中的两种溶质接触时过量热力学量的变化。采用了一个模拟水中溶质的简单模型。计算中未包括溶质-溶质相互作用能。在等容条件下，溶质接触总是导致正的熵变，而与溶质的疏溶剂性或亲溶剂性无关。对于疏溶剂性溶质，能量变化为负，而对于亲溶剂性溶质，能量变化为正。在等压条件下，疏溶剂性溶质的接触导致系统体积压缩，而亲溶剂性溶质的接触则导致膨胀。随着温度升高，压缩和膨胀对焓变和熵变的影响增大。当溶质的疏溶剂性足够高时，由于压缩，熵变（乘以绝对温度）可能变为负，除非在低温下，结果是更大的负焓变。亲溶剂性溶质情况下的膨胀导致大的正熵变，同时伴有更大的正焓变。焓变和熵变强烈依赖于温度。然而，焓变和熵变在很大程度上相互抵消，自由能变化的温度依赖性要弱得多。作者还讨论了与水溶液中各种物理化学过程（如蛋白质折叠）实验已知的焓-熵补偿的可能相关性。
