Chalikian Tigran V
Department of Pharmaceutical Sciences, Leslie Dan Faculty of Pharmacy, University of Toronto, 144 College Street, Toronto, Ontario M5S 3M2, Canada.
J Phys Chem B. 2008 Jan 24;112(3):911-7. doi: 10.1021/jp709578u. Epub 2008 Jan 3.
We use a statistical thermodynamic approach and a simple thermodynamic model of hydration to examine the molecular origins of the volumetric properties of solutes. In this model, solute-solvent interactions are treated as a binding reaction. The free energy of hydration of the noninteracting solute species coincides with the free energy of cavity formation, while the free energy of solute-solvent interactions is given by the binding polynomial. By differentiating the relationship for the free energy of hydration with respect to temperature and pressure, one obtains the complete set of equations describing the thermodynamic profile of hydration, including enthalpy, entropy, volume, compressibility, expansibility, and so forth. The model enables one to rigorously define in thermodynamic terms the hydration number and the related concept of hydration shell, which are both widely used as operational definitions in experimental studies. Hydration number, nh, is the effective number of water molecules solvating the solute and represents the derivative of the free energy of hydration with respect to the logarithm of water activity. One traditional way of studying hydration relies on the use of volumetric measurements. However, microscopic interpretation of macroscopic volumetric data is complicated and currently relies on empirical models that are not backed by theory. We use our derived model to link the microscopic determinants of the volumetric properties of a solute and its statistical thermodynamic parameters. In this treatment, the partial molar volume, V degrees, of a solute depends on the cavity volume, hydration number, and the properties of waters of hydration. In contrast, the partial molar isothermal compressibility, K degrees T, and expansibility, E degrees, observables, in addition to the intrinsic compressibility and expansibility of the cavity enclosing the solute, hydration number, and the properties of waters of hydration, contain previously unappreciated relaxation terms that originate from pressure- and temperature-induced perturbation of the equilibrium between the solvated solute species. If significant, the relaxation terms may bring about a new level of nonadditivity to compressibility and expansibility group contributions that goes beyond the overlap of the hydration shells of adjacent groups. We apply our theoretical results to numerical analyses of the volume and compressibility responses to changes in the distribution of solvated species of polar compounds.
我们采用统计热力学方法和一个简单的水合热力学模型来研究溶质体积性质的分子起源。在这个模型中，溶质 - 溶剂相互作用被视为一种结合反应。非相互作用溶质物种的水合自由能与空穴形成的自由能一致，而溶质 - 溶剂相互作用的自由能由结合多项式给出。通过对水合自由能与温度和压力的关系进行微分，可得到描述水合热力学概况的完整方程组，包括焓、熵、体积、压缩性、膨胀性等等。该模型使人们能够从热力学角度严格定义水合数以及相关的水合壳概念，这两个概念在实验研究中都被广泛用作操作性定义。水合数(n_h)是溶剂化溶质的水分子的有效数量，代表水合自由能相对于水活度对数的导数。一种传统的研究水合的方法依赖于体积测量。然而，对宏观体积数据的微观解释很复杂，目前依赖于没有理论支持的经验模型。我们使用推导得到的模型将溶质体积性质的微观决定因素与其统计热力学参数联系起来。在这种处理中，溶质的偏摩尔体积(V^{\circ})取决于空穴体积、水合数以及水合水的性质。相比之下，溶质的偏摩尔等温压缩性(K_T^{\circ})和膨胀性(E^{\circ})，除了包围溶质的空穴的固有压缩性和膨胀性、水合数以及水合水的性质外，还包含以前未被认识到的弛豫项，这些弛豫项源于压力和温度引起的溶剂化溶质物种之间平衡的扰动。如果这些弛豫项很显著，它们可能会给压缩性和膨胀性基团贡献带来新的非加和性水平，这种非加和性超出了相邻基团水合壳的重叠。我们将理论结果应用于对极性化合物溶剂化物种分布变化的体积和压缩性响应的数值分析。
