Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221, USA.
J Chem Phys. 2013 Jul 28;139(4):044504. doi: 10.1063/1.4814070.
The Quasichemical Theory (QCT) involves a length-scale organization of solvation thermodynamics. The QCT has been employed in studies of solutes ranging in size from small molecules and ions to proteins. There are three contributions to the QCT free energy: (1) an inner-shell term that includes the direct solute-solvent chemical interactions, (2) an outer-shell packing term that is the free energy to dig out a cavity in the solvent, and (3) an outer-shell long-ranged term that includes all interactions of the solute with the solvent conditional on an empty inner shell. The present study utilizes a regularizing generalization of the QCT and classical simulations to compute these three contributions to the ion hydration free energy out to large cavity radii for eight ions in the alkali halide series. The accuracy of simple approximations for the long-ranged term is also examined. The inner-shell contribution exhibits ion specificity for cavity sizes less than 4-5 Å, followed by a common length scale of 6.15 Å at which its value equals the bulk hydration free energy for all eight of the ions. The 6.15 Å length scale is closely approximated by the distance at which the revised scaled-particle theory packing contribution matches the magnitude of a simple Born estimate for the long-ranged term.
准化学理论（QCT）涉及溶剂化热力学的长度尺度组织。QCT 已被用于研究大小从小分子和离子到蛋白质的溶质。QCT 自由能有三个贡献：（1）一个包含直接溶质-溶剂化学相互作用的内壳项，（2）一个用于挖掘溶剂中空腔的外壳堆积项，（3）一个包括溶质与溶剂所有相互作用的外壳远程项，条件是内壳为空。本研究利用 QCT 和经典模拟的正则化推广，计算了碱卤系列中八种离子在大空腔半径下的离子水化自由能的这三个贡献。还检查了远程项的简单近似的准确性。内壳贡献对于小于 4-5Å 的腔尺寸表现出离子特异性，然后是 6.15Å 的共同长度尺度，对于所有八种离子，其值等于体相水合自由能。6.15Å 的长度尺度非常接近修正后的比例粒子理论堆积贡献与简单 Born 估计远程项大小相匹配的距离。
