A possible way to explicitly account for different molecular geometries with an equation of state

Different molecular geometries are determinant to condensed phases structures, and to the thermodynamic behavior. To take into consideration the nonsphericity in the development of equations of state, molecules have been taken either as chains of spherical segments or sets of nonspherical segments. In this work, we model molecules as single anisotropic particles, introducing a general Hard Nonspherical Segment (HNS) reference term, which can, in principle, be used within the framework of the Statistical Associating Fluid Theory (SAFT), once coupled with an appropriate association term. The resulting model explicitly accounts for different molecular geometries. Variants of the HNS reference term for ellipsoids, cylinders, and spherocylinders were applied to calculate vapor-liquid equilibrium of carbon dioxide, benzene, toluene, n-perfluoroalkanes (CF4, C2F6, C3F8), and n-alkanes (from methane to octane). We have shown that, in general, our approach of modeling molecules as nonspherical particles yield better results than a comparable equation of state for which molecules are taken as chains of spherical segments. Furthermore, the fitted parameters lead to physically sound geometrical models; for instance, the oblate geometry of the aromatic rings was inferred from vapor-liquid equilibrium data upon fitting the model parameters. We show that the adjusted aspect ratios of the ellipsoidal and spherocylindrical particles can be correlated with the acentric factor of their respective molecules, adding evidence that the degree of nonsphericity of molecules is indeed somehow captured by the acentric factor. State-of-the-art equations of state such as PC-SAFT and SAFT-VR Mie still give better results, and the level of complexity in their formulation may justify it. Therefore, we believe that the proposed methodology with nonspherical segments is promising, but has space for further improvement. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Modeling molecules as single anisotropic particles leads to better results compared to using molecules as chains of spherical segments in equation of state. Adjusting parameters results in physically sound geometrical models, such as inferring the oblate geometry of aromatic rings from vapor-liquid equilibrium data. In addition, the adjusted aspect ratios of ellipsoidal and spherocylindrical particles can be correlated with the acentric factor of their respective molecules, showing that the degree of nonsphericity of molecules can be captured by the acentric factor.