Abstract The idea that cubic EoS's are very primitive and limited models, quite extended at present among researchers working on fluid properties and phase equilibria, has different roots, including some limitations observed specifically for classic and popular equations like Peng-Robinson (PR) or Soave-Redlich-Kwong (SRK). These are two-parameter models, i.e. they have only an attractive and a repulsive parameter to characterize each molecule, while other models like SAFT but also cubic –and still for non-associating molecules-introduce also a third parameter related somehow to the molecular structure or shape. One of the alluded limitations, actually a very clear one, is the complete failure in describing the non-ideality in nearly athermal mixtures, like those composed of n-alkanes with different chain lengths: SRK and PR predict positive deviations from ideality, which increase with the system asymmetry, while experimental measurements show exactly the opposite, i.e. increasing negative deviations from ideality. This provides an excellent opportunity to try to clarify whether such failure is due to the cubic nature of these classic models or to their two-parameter character and/or to the classic van der Waals one-fluid (vdW1f) mixing rules typically used. With that motivation, in this work we used models representing three different categories, in a completely predictive way: a two-parameter cubic EoS (PR), a three-parameter cubic EoS (RKPR) and a three-parameter SAFT EoS (PC-SAFT). Their predictions of infinite dilution activity coefficients were analyzed and compared, in contrast to available data for different mixtures of n-butane to n-octane as the lighter compound and paraffins ranging from C16 to C36 as the heavier, in both extremes of dilution. The obtained results, and their analysis, allowed us to extract very clear conclusions which were not present in the literature so far, regarding the importance of a third parameter in any type of EoS.

中文翻译：

---

从状态方程预测的几乎无热混合物中的活度系数：当缺少第三个参数时，不要责怪三次！

摘要 三次 EoS 是非常原始和有限的模型的想法，目前在研究流体性质和相平衡的研究人员中相当广泛，具有不同的根源，包括一些专门针对经典和流行方程（如 Peng-Robinson (PR) 或 Soave）观察到的局限性。 -Redlich-Kwong (SRK)。这些是双参数模型，即它们只有一个吸引参数和一个排斥参数来表征每个分子，而其他模型（如 SAFT 以及三次模型——仍然是非缔合分子）也引入了与分子结构有关的第三个参数或形状。提到的限制之一，实际上是一个非常明确的限制，是完全没有描述几乎无热混合物的非理想性，例如由不同链长的正烷烃组成的混合物：SRK 和PR 预测与理想的正偏差，随着系统不对称性而增加，而实验测量显示正好相反，即增加与理想的负偏差。这提供了一个极好的机会来试图澄清这种失败是由于这些经典模型的三次性质还是由于它们的双参数特性和/或通常使用的经典范德华单一流体 (vdW1f) 混合规则。出于这种动机，在这项工作中，我们以完全预测的方式使用了代表三个不同类别的模型：双参数三次 EoS (PR)、三参数三次 EoS (RKPR) 和三参数 SAFT EoS (PC-安全）。他们对无限稀释活度系数的预测进行了分析和比较，与正丁烷到正辛烷的不同混合物作为较轻的化合物和从 C16 到 C36 的链烷烃作为较重的两种极端稀释的可用数据形成对比。获得的结果及其分析使我们能够得出迄今为止文献中没有的非常明确的结论，即关于第三个参数在任何类型的 EoS 中的重要性。