Fachbereich Physik, Freie Universität Berlin, 14195 Berlin, Germany.
J Chem Phys. 2013 Mar 21;138(11):115101. doi: 10.1063/1.4794153.
The solvation thermodynamics and in particular the solvation heat capacity of polar and charged solutes in water is studied using atomistic molecular dynamics simulations. As ionic solutes we consider a F(-) and a Na(+) ion, as an example for a polar molecule with vanishing net charge we take a SPC/E water molecule. The partial charges of all three solutes are varied in a wide range by a scaling factor. Using a recently introduced method for the accurate determination of the solvation free energy of polar solutes, we determine the free energy, entropy, enthalpy, and heat capacity of the three different solutes as a function of temperature and partial solute charge. We find that the sum of the solvation heat capacities of the Na(+) and F(-) ions is negative, in agreement with experimental observations, but our results uncover a pronounced difference in the heat capacity between positively and negatively charged groups. While the solvation heat capacity ΔC(p) stays positive and even increases slightly upon charging the Na(+) ion, it decreases upon charging the F(-) ion and becomes negative beyond an ion charge of q = -0.3e. On the other hand, the heat capacity of the overall charge-neutral polar solute derived from a SPC/E water molecule is positive for all charge scaling factors considered by us. This means that the heat capacity of a wide class of polar solutes with vanishing net charge is positive. The common ascription of negative heat capacities to polar chemical groups might arise from the neglect of non-additive interaction effects between polar and apolar groups. The reason behind this non-additivity is suggested to be related to the second solvation shell that significantly affects the solvation thermodynamics and due to its large spatial extent induces quite long-ranged interactions between solvated molecular parts and groups.
本文使用原子分子动力学模拟研究了极性和带电溶质在水中的溶剂化热力学，特别是溶剂化热容量。我们选择 F(-)和 Na(+)离子作为离子溶质的例子，选择 SPC/E 水分子作为带有零净电荷的极性分子的例子。所有三种溶质的部分电荷通过缩放因子在很宽的范围内变化。使用最近引入的一种精确确定极性溶质溶剂化自由能的方法，我们确定了三种不同溶质的自由能、熵、焓和热容量随温度和部分溶质电荷的函数关系。我们发现，Na(+)和 F(-)离子的溶剂化热容量之和为负，这与实验观察结果一致，但我们的结果揭示了带正电荷和带负电荷基团之间的热容存在明显差异。虽然 Na(+)离子的电荷增加时，溶剂化热容量 ΔC(p)保持正值甚至略有增加，但 F(-)离子的电荷增加时，它会减小，当离子电荷 q = -0.3e 时，它会变为负值。另一方面，我们考虑的所有电荷缩放因子下，源自 SPC/E 水分子的整体电中性极性溶质的热容均为正值。这意味着，具有零净电荷的广泛类别的极性溶质的热容为正值。将负热容归因于极性化学基团可能源于忽略了极性和非极性基团之间的非加和相互作用效应。这种非加和性的原因与显著影响溶剂化热力学的第二溶剂化壳有关，并且由于其大的空间延伸，在溶剂化分子部分和基团之间诱导相当长程的相互作用。
