van Westen Thijs, Gross Joachim
Institute of Thermodynamics and Thermal Process Engineering, University of Stuttgart, Pfaffenwaldring 9, D-70569 Stuttgart, Germany.
J Chem Phys. 2021 Dec 28;155(24):244501. doi: 10.1063/5.0073572.
We develop a simplification of our recently proposed uf-theory for describing the thermodynamics of simple fluids and fluids comprising short chain molecules. In its original form, the uf-theory interpolates the Helmholtz energy between a first-order f-expansion and first-order u-expansion as (effective) lower and upper bounds. We here replace the f-bound by a new, tighter (effective) lower bound. The resulting equation of state interpolates between a first-order u-expansion at high densities and another first-order u-expansion that is modified to recover the exact second virial coefficient at low densities. The theory merely requires the Helmholtz energy of the reference fluid, the first-order u-perturbation term, and the total perturbation contribution to the second virial coefficient as input. The revised theory-referred to as uv-theory-is thus simpler than the uf-theory but leads to similar accuracy, as we show for fluids with intermolecular pair interactions governed by a Mie potential. The uv-theory is thereby easier to extend to fluid mixtures and provides more flexibility in extending the model to non-spherical or chain-like molecules. The usefulness of the uv-theory for developing equation-of-state models of non-spherical molecules is here exemplified by developing an equation of state for Lennard-Jones dimers.
我们对最近提出的用于描述简单流体和包含短链分子的流体热力学的uf理论进行了简化。在其原始形式中，uf理论在一阶f展开和一阶u展开之间对亥姆霍兹自由能进行插值，将其作为（有效的）下限和上限。我们在此用一个新的、更紧的（有效）下限取代f边界。由此产生的状态方程在高密度下的一阶u展开和另一个在低密度下经修正以恢复精确第二维里系数的一阶u展开之间进行插值。该理论仅需要参考流体的亥姆霍兹自由能、一阶u微扰项以及对第二维里系数的总微扰贡献作为输入。正如我们针对由米氏势控制分子间对相互作用的流体所展示的那样，修订后的理论——称为uv理论——比uf理论更简单，但具有相似的精度。uv理论因此更易于扩展到流体混合物，并且在将模型扩展到非球形或链状分子方面提供了更大的灵活性。通过开发 Lennard-Jones 二聚体的状态方程，这里举例说明了uv理论在开发非球形分子状态方程模型方面的有用性。
