dos Ramos María Carolina, Blas Felipe J
Departamento de Física Aplicada, Facultad de Ciencias Experimentales, Universidad de Huelva, 21071 Huelva, Spain.
J Phys Chem B. 2005 Jun 23;109(24):12145-53. doi: 10.1021/jp0507142.
A modification of the statistical associating fluid theory, the so-called Soft-SAFT equation of state, is proposed to predict the excess thermodynamic properties of binary mixtures of n-alkanes. n-Alkane molecules are modeled as fully flexible Lennard-Jones chains. This molecular model accounts for the most important microscopic features of real chainlike molecules: attractive and repulsive interactions between different chemical groups and the connectivity of the segments that form the molecules. In this work we consider an additional microscopic effect that can profoundly affect certain thermodynamic properties, namely, the conformational changes when two different n-alkane molecules are mixed. We propose, following the work of Vega and co-workers [J. Chem. Phys. 1999, 111, 3192], a simple model to account for the conformational changes in molecules. The resulting free energy is combined with the SAFT free energy to describe the excess thermodynamic properties of binary mixtures of n-alkanes. Predictions from the theory are compared with experimental data taken from the literature. The agreement between the experiments and the theoretical predictions is excellent in all cases. This work shows that although minor microscopic effects, such as the conformational changes in the molecules that form the mixtures, have only a very small effect on the usual thermodynamic properties, such as pressure, chemical potential, phase equilibria, and excess volumes, they can contribute significantly to other thermodynamic properties. In fact, one of the main conclusions of this work is that it is essential that conformational effects be taken into account in molecular-based theories if an accurate description of certain excess properties (excess enthalpy for instance) is desired.
提出了统计缔合流体理论的一种修正形式，即所谓的软统计缔合流体理论（Soft-SAFT）状态方程，用于预测正构烷烃二元混合物的过量热力学性质。正构烷烃分子被建模为完全柔性的 Lennard-Jones 链。这种分子模型考虑了真实链状分子最重要的微观特征：不同化学基团之间的吸引和排斥相互作用以及构成分子的链段的连通性。在这项工作中，我们考虑了一种可能深刻影响某些热力学性质的额外微观效应，即两种不同正构烷烃分子混合时的构象变化。我们遵循 Vega 及其同事的工作[《化学物理杂志》，1999 年，111 卷，3192 页]，提出了一个简单模型来解释分子中的构象变化。由此得到的自由能与 SAFT 自由能相结合，以描述正构烷烃二元混合物的过量热力学性质。将该理论的预测结果与文献中的实验数据进行了比较。在所有情况下，实验与理论预测之间的吻合度都非常好。这项工作表明，尽管诸如构成混合物的分子的构象变化等微小微观效应，对诸如压力、化学势、相平衡和过量体积等常见热力学性质的影响非常小，但它们对其他热力学性质可能有显著贡献。事实上，这项工作的主要结论之一是，如果希望准确描述某些过量性质（例如过量焓），在基于分子的理论中考虑构象效应是至关重要的。
