Theory of Liquids and Liquid Mixtures, their Structures and Phase Equilibria

康奈尔大学的本杰明·威多姆得到了化学理论、模型和计算方法项目的支持，该项目研究难溶物质在水中的溶液。“油和水不能混合”这句话几乎但不完全正确，油和其他物质在水中的轻微溶解往往是至关重要的。它影响化学加工工业中蒸馏和萃取分离的效率；它是制药工业药物设计的主要因素；它是水污染和其他环境问题的关注点；它是最基本的生物现象的基础。例如，人们普遍认为，蛋白质的类油成分与周围的水相对不相容，这是蛋白质折叠成适当的生物活性形式的原因。在水中的低溶解度是统称为“憎水”(字面意思为憎水)效应的一个例子。这些效应是目前理论和计算研究的目标，旨在了解它们的分子起源和后果：是什么让物质变得疏水性？除了低溶解度外，疏水性还表现在哪些方面？周围水本身的结构和其他性质如何对溶解的疏水分子的存在做出反应，以及这种分子尺度重排的物理和化学后果是什么？这项研究的主要目的是了解疏水溶质之间的溶剂诱导吸引力，特别是第二渗透维里系数中包含的吸引力的度量。由状态方程和热力学测量得到的这一系数的值与柯克伍德-布夫关系之一所要求的溶质-溶质对关联函数的积分是一致的。在最近的工作中，已经展示了如何从状态方程中提取维里系数，其参数设置为给出纯溶剂的真实性质，以及对溶质-溶剂相互作用的影响的真实解释，例如对溶质的溶解度及其在溶剂中的部分分子体积的影响。所得渗透系数与实验测得的溶解度值吻合较好。通过计算机模拟，分别得到了两组分混合物中的几个对关联函数。积分后，它们与溶剂压缩系数、溶质分分子体积和气相维里系数是一致的，但与先前得到的第二渗透维里系数不一致。这是一个严重的差异，解决这一问题很重要，因为正确理解疏水水化中的结构效应，以及更广泛地说，疏水效应取决于它。仍然存在为几对关联函数构造分析形式的问题，以检验先前的分析指示，即由于低溶解度而难以测量的溶质-溶质关联函数的幅度大大超过其他两个的幅度，使得尽管两者具有相同的形式指数衰减长度，但溶质在比溶剂分子更长的距离内保持有效的关联。剩下的一个项目是通过计算机模拟和解析地确定闭环共存曲线中溶质贫侧的溶质-溶质对关联函数，以跟踪随着两个临界溶液点之间的温度变化而从基于熵的疏溶性到基于焓的疏溶性的演变。

Benjamin Widom of Cornell University is supported by an award from the Chemical Theory, Models and Computational Methods program to study solutions, in water, of substances that are hardly soluble. The expression "oil and water don't mix" is almost but not entirely true, and the slight solubility of oils and other substances in water is often of crucial importance. It affects the efficiency of separations by distillation and extraction in the chemical process industries; it is a primary factor in drug design in the pharmaceutical industry; it is a concern in water pollution and other environmental problems; and it underlies the most fundamental biological phenomena. It is widely accepted, for example, that it is the relative incompatibility of the oil-like constituents of a protein and the surrounding water that is responsible for the protein's folding into its proper biologically active form. Low solubility in water is an example of what are known collectively as "hydrophobic" (literally water-hating) effects. These effects are the objects of the present theoretical and computational research, which is directed toward the understanding of their molecular origins and consequences: What makes a substance hydrophobic? In what other ways does hydrophobicity manifest itself in addition to low solubility? How does the structure and other properties of the surrounding water itself respond to the presence of dissolved hydrophobic molecules and what are the physical and chemical consequences of such molecular-scale rearrangements?Much of this research is directed to understanding the solvent-induced attraction between hydrophobic solutes, particularly that measure of the attraction contained in the second osmotic virial coefficient. It is desirable to reconcile the values of this coefficient as found from equations of state and thermodynamic measurements with the integral of the solute-solute pair correlation function, as required by one of the Kirkwood-Buff relations. In recent work it has been shown how the virial coefficient may be extracted from an equation of state with parameters set to give realistic properties of the pure solvent and a realistic account of those effects of the solute-solvent interaction such as on the solubility of the solute and on its partial molecular volume in the solvent. The resulting osmotic coefficient agrees well with that extracted from experimental solubility measurements. Independently, the several pair correlation functions in the two-component mixture are obtained by computer simulation. These, on being integrated, are consistent with the solvent compressibility, the solute partial molecular volume, and the gas-phase virial coefficients, yet inconsistent with the previously obtained second osmotic virial coefficient. This is a serious discrepancy, which is important to resolve, for a proper understanding of structural effects in hydrophobic hydration, and hydrophobic effects more generally, depends on it. There still remains the problem of constructing analytical forms for the several pair correlation functions to test earlier analytical indications that the amplitude of the solute-solute correlation function, which is difficult to measure because of the low solubility, greatly exceeds that of the other two so that the solutes remain effectively correlated to longer distances than do the solvent molecules despite the two having the same formal exponential decay length. A remaining project is that of determining, by computer simulation and analytically, the solute-solute pair correlation function on the solute-poor side of a closed-loop coexistence curve, to trace the evolution from entropy-based to enthalpy-based solvophobicity as the temperature varies between that of the two critical solution points.