Abstract

The need to consider vibrational motions of molecules in bound states of a vapor–liquid system in order to calculate its thermodynamic functions (TFs) raises the question of possibly isolating them from the collective kinetic contribution and describing them. An attempt is made to do so using a molecular kinetic theory based on the cluster approach (CA) in the lattice gas model (LGM). The existence of physically bound groups of fluid particles forming associates, which is observed for rarefied vapor, is extended to the range of densities of a vapor–liquid system. An associative model of a fluid is proposed that reflects regions of rarefied and dense fluids, different types of molecular motions, and the effects of anharmonic vibrations of atoms in bound states. Starting from an isolated dimer to a dense phase, particle vibrations are calculated in a quasi-dimer model reflecting average local vibrational frequencies. In dense fluids, local density fluctuations characterize representations of associates. Under the effect of vibration anharmonism in bound associates, molecular pair interaction potentials are renormalized into many-particle potentials. The problem of calculating TFs according to correlation functions, which is done extensively in fluid theory and CA in the LGM, is discussed. The problem lies in two ways of calculating the free energy: using molecular distribution functions or thermodynamic relationships. An approach is formulated for strictly calculating TFs with the use of model free energies. A statistical sum is introduced into the theory of fluids and ensures a self-consistent description of equilibria and kinetics.

中文翻译：

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流体的关联模型和计算汽液系统热力学函数的问题

摘要

为了计算其热力学函数 (TF)，需要考虑处于汽液系统束缚态的分子的振动运动，这提出了可能将它们与集体动力学贡献分离并描述它们的问题。尝试使用基于晶格气体模型 (LGM) 中的簇方法 (CA) 的分子动力学理论来做到这一点。在稀薄蒸汽中观察到的形成缔合物的流体颗粒物理结合组的存在扩展到汽液系统的密度范围。提出了一种流体的关联模型，该模型反映了稀薄和稠密流体的区域、不同类型的分子运动以及原子在束缚态的非谐振动的影响。从孤立的二聚体到密相，在反映平均局部振动频率的准二聚体模型中计算粒子振动。在稠密流体中，局部密度波动表征了关联的表示。在束缚缔合体振动非谐的作用下，分子对相互作用势被重整化为多粒子势。讨论了根据相关函数计算 TFs 的问题，这在流体理论和 LGM 中的 CA 中广泛完成。问题在于计算自由能的两种方法：使用分子分布函数或热力学关系。制定了一种使用模型自由能来严格计算 TF 的方法。统计总和被引入流体理论，并确保平衡和动力学的自洽描述。