Departamento de Física Aplicada, Universidad de Huelva, 21071 Huelva, Spain.
Centro de Investigación de Física Teórica y Matemática FIMAT, Universidad de Huelva, 21071 Huelva, Spain.
J Chem Phys. 2014 Mar 21;140(11):114705. doi: 10.1063/1.4868100.
We have determined the interfacial properties of short fully flexible chains formed from tangentially bonded Lennard-Jones monomeric units from direct simulation of the vapor-liquid interface. The results obtained are compared with those corresponding to rigid-linear chains formed from the same chain length, previously determined in the literature [F. J. Blas, A. I. M.-V. Bravo, J. M. Míguez, M. M. Piñeiro, and L. G. MacDowell, J. Chem. Phys. 137, 084706 (2012)]. The full long-range tails of the potential are accounted for by means of an improved version of the inhomogeneous long-range corrections of Janeček [J. Phys. Chem. B 129, 6264 (2006)] proposed recently by MacDowell and Blas [J. Chem. Phys. 131, 074705 (2008)] valid for spherical as well as for rigid and flexible molecular systems. Three different model systems comprising of 3, 5, and 6 monomers per molecule are considered. The simulations are performed in the canonical ensemble, and the vapor-liquid interfacial tension is evaluated using the test-area method. In addition to the surface tension, we also obtained density profiles, coexistence densities, critical temperature and density, and interfacial thickness as functions of temperature, paying particular attention to the effect of the chain length and rigidity on these properties. According to our results, the main effect of increasing the chain length (at fixed temperature) is to sharpen the vapor-liquid interface and to increase the width of the biphasic coexistence region. As a result, the interfacial thickness decreases and the surface tension increases as the molecular chains get longer. Comparison between predictions for fully flexible and rigid-linear chains, formed by the same number of monomeric units, indicates that the main effects of increasing the flexibility, i.e., passing from a rigid-linear to a fully flexible chain, are: (a) to decrease the difference between the liquid and vapor densities; (b) to decrease the critical temperature and to increase the critical density; (c) to smooth the density profiles along the interfacial region; (d) to increase the interfacial thickness; and (e) to decrease the vapor-liquid surface tension.
我们通过对蒸气-液体界面的直接模拟，确定了由切向键合 Lennard-Jones 单体单元形成的短全柔性链的界面性质。将得到的结果与文献中先前确定的具有相同链长的刚性线性链的结果进行了比较[F. J. Blas、A. I. M.-V. Bravo、J. M. Míguez、M. M. Piñeiro 和 L. G. MacDowell，J. Chem. Phys. 137，084706（2012）]。通过 MacDowell 和 Blas 最近提出的改进型 Janček 不均匀长程修正[J. Phys. Chem. B 129，6264（2006）]，充分考虑了势的全远程长尾，该修正对于球形以及刚性和柔性分子系统均有效。考虑了三种不同的模型系统，每个分子包含 3、5 和 6 个单体。模拟在正则系综中进行，使用测试面积法评估蒸气-液体界面张力。除表面张力外，我们还获得了密度分布、共存密度、临界温度和密度以及界面厚度随温度的函数，特别注意了链长和刚性对这些性质的影响。根据我们的结果，增加链长（在固定温度下）的主要影响是使蒸气-液体界面变锐，并增加两相共存区域的宽度。因此，随着分子链的增长，界面厚度减小，表面张力增加。全柔性链和由相同数目的单体单元形成的刚性线性链的预测结果的比较表明，增加柔性的主要影响是：（a）减小液体和蒸气密度之间的差异；（b）降低临界温度并增加临界密度；（c）沿界面区域平滑密度分布；（d）增加界面厚度；（e）降低蒸气-液体表面张力。
