Piñeiro Manuel M, Cerdeiriña Claudio A, Medeiros Milton
Departamento de Fisica Aplicada, Universidad de Vigo, As Lagoas s/n, Ourense 32004, Spain.
J Chem Phys. 2008 Jul 7;129(1):014511. doi: 10.1063/1.2943317.
On the basis of NpT Monte Carlo simulations, a detailed analysis on the microscopic origins of some specific features of thermodynamic response functions of fluids is performed. Specifically, the residual isobaric heat capacity C(p) (res), the isobaric thermal expansivity alpha(p), and the isothermal compressibility kappa(T) for Lennard-Jones methane and optimized potential for liquid simulations (OPLS) methanol have been determined via standard techniques. For the former, data along the liquid, gas, and supercritical regions are presented, while a wide temperature range at a single supercritical pressure is covered for the latter. They have been obtained by computing the various pairwise fluctuations contributing to each property. Attention is mainly focused on isothermal and isobaric maxima found for both C(p) (res) and alpha(p), which have been rationalized at a molecular level using qualitative arguments. It is encountered that maxima emerge as a natural consequence of the destruction of fluid structure as temperature is increased or as pressure is decreased. The results for Lennard-Jones methane reveal the competition of energetic and volumetric effects, while those for OPLS methanol evidence that hydrogen-bonding is dominant as energetic effects are concerned. Further discussion on previous results and alternative approaches using equations of state as well as on closely related topics such as "maxima and critical phenomena" is included.
基于NpT蒙特卡罗模拟，对流体热力学响应函数某些特定特征的微观起源进行了详细分析。具体而言，通过标准技术确定了 Lennard-Jones 甲烷以及用于液体模拟的优化势（OPLS）甲醇的剩余等压热容C(p)(res)、等压热膨胀系数α(p)和等温压缩系数κ(T)。对于前者，给出了沿液体、气体和超临界区域的数据，而对于后者，则涵盖了在单一超临界压力下的宽温度范围。它们是通过计算对每种性质有贡献的各种成对涨落而获得的。注意力主要集中在C(p)(res)和α(p)中发现的等温及等压最大值，已使用定性论据在分子水平对其进行了合理化解释。可以看到，随着温度升高或压力降低，流体结构遭到破坏，最大值作为自然结果出现。Lennard-Jones 甲烷的结果揭示了能量效应和体积效应的竞争，而OPLS甲醇的结果表明，就能量效应而言，氢键起主导作用。文中还包括了对先前结果以及使用状态方程的替代方法的进一步讨论，以及对诸如“最大值和临界现象”等密切相关主题的讨论。
