Laboratoire des Fluides Complexes, Faculté des Sciences, Université de Pau, UMR CNRS 5150, BP 1155, Pau Cedex F-64013, France.
J Chem Phys. 2010 Aug 21;133(7):074507. doi: 10.1063/1.3472283.
Short-range correlations of the molecular orientations in liquid n-alkanes have been extensively studied from depolarized Rayleigh scattering and thermodynamic measurements. These correlations between segments induce structural anisotropy in the fluid bulk. This phenomenon, which is characteristic of linear chain molecules when the constituting segments are nor freely jointed, but interact through a given angular potential, is then present in the linear n-Cn series, increasing its magnitude with chain length, and it is therefore less relevant or even completely absent in branched alkanes. This intermolecular effect is clearly revealed in second-order excess magnitudes such as heat capacities when the linear molecule is mixed with one whose structure approaches sphericity. The mixing process of different aspect ratio chain molecules is thought to modify the original pure fluid structure, by producing a diminution of the orientational order previously existing between pure n-alkane chains. However, second-order thermodynamics quantities of pure liquids C(P), ( partial differentialv/ partial differentialT)(P), and ( partial differentialv/ partial differentialP)(P) are known to be very sensitive to the specific interactions occurring at the microscopic level. In other words, the behavior of these derived properties versus temperature and pressure can be regarded as response functions of the complexity of the microscopic interactions. Thus, the purpose of the present work is to rationalize the orientational order evolution with both temperature and molecular chain length from the analysis of pure fluid properties. To this aim, we focused on two linear alkanes, n-octane (n-C(8)) and n-hexadecane (n-C(16)), and two of their branched isomers, i.e., 2,2,4-trimethylpentane (br-C(8)) and 2,2,4,4,6,8,8-heptamethylnonane (br-C(16)). For each compound, we propose a combined study from direct experimental determination of second-order derivative properties and Monte Carlo simulations. We performed density rho, speed of sound c, and isobaric heat capacity C(P) measurements in broad ranges of pressure and temperature allowing a complete thermodynamic characterization of these compounds. Monte Carlo simulations provide a link between the molecular scale model and the experimental thermodynamic properties. Additional information about the microscopic structure of the simulated fluid model was derived, through the calculation of the radius of gyration and average end-to-end distances. Orientational order is clearly revealed by the experimental residual heat capacity trend of pure linear alkanes. The close agreement observed between this experimental macroscopic property and the calculated theoretical structural parameters support the conclusion that the orientational order between segments of linear molecules should be regarded as a conformational effect due to the flexibility of the chain.
已从去偏振瑞利散射和热力学测量中广泛研究了液体正构烷烃中分子取向的短程相关性。这些片段之间的相关性在流体本体中引起结构各向异性。当组成片段不是自由连接而是通过给定的角势相互作用时，这种现象存在于线性 n-Cn 系列中，随着链长的增加而增加，因此在支链烷烃中相关性较小甚至完全不存在。当线性分子与结构接近球形的分子混合时，这种分子间效应在二阶过剩量如热容中明显表现出来。不同纵横比链分子的混合过程被认为通过产生先前存在于纯正构烷烃链之间的取向序的减小来改变原始纯流体结构。然而，纯液体 C(P)、(偏微分 v/偏微分 T)(P)和(偏微分 v/偏微分 P)(P)的二阶热力学量已知对微观水平发生的特定相互作用非常敏感。换句话说，这些导出性质随温度和压力的变化可以看作是微观相互作用复杂性的响应函数。因此，本工作的目的是从纯流体性质的分析中，结合温度和分子链长来合理化取向序的演化。为此，我们专注于两种线性烷烃，正辛烷（n-C(8)）和正十六烷（n-C(16)），以及它们的两种支链异构体，即 2,2,4-三甲基戊烷（br-C(8)）和 2,2,4,4,6,8,8-庚基壬烷（br-C(16)）。对于每种化合物，我们都提出了从二阶导数性质的直接实验测定和蒙特卡罗模拟的综合研究。我们在宽压力和温度范围内进行了密度 ρ、声速 c 和定压热容 C(P)测量，从而全面表征了这些化合物的热力学性质。蒙特卡罗模拟提供了分子尺度模型与实验热力学性质之间的联系。通过计算旋转半径和平均末端到末端距离，从模拟流体模型中获得了有关微观结构的其他信息。纯线性烷烃的实验剩余热容趋势清楚地揭示了取向序。观察到的这种实验宏观性质与计算理论结构参数之间的紧密一致支持了这样的结论，即线性分子片段之间的取向序应被视为由于链的柔性而导致的构象效应。
