Department of Chemical Engineering, Imperial College London , South Kensington Campus, London SW7 2AZ, United Kingdom.
Langmuir. 2017 Oct 24;33(42):11345-11365. doi: 10.1021/acs.langmuir.7b01849. Epub 2017 Sep 5.
The phase behavior and structure of a simple square-well bulk fluid with anisotropic interactions is described in detail. The orientation dependence of the intermolecular interactions allows for the formation of a nematic liquid-crystalline phase in addition to the more conventional isotropic gas and liquid phases. A version of classical density functional theory (DFT) is employed to determine the properties of the model, and comparisons are made with the corresponding data from Monte Carlo (MC) computer simulations in both the grand canonical and canonical ensembles, providing a benchmark to assess the adequacy of the DFT results. A novel element of the DFT approach is the assumption that the structure of the fluid is dominated by intermolecular interactions in the isotropic fluid. A so-called augmented modified mean-field (AMMF) approximation is employed accounting for the influence of anisotropic interactions. The AMMF approximation becomes exact in the limit of vanishing density. We discuss advantages and disadvantages of the AMMF approximation with respect to an accurate description of isotropic and nematic branches of the phase diagram, the degree of orientational order, and orientation-dependent pair correlations. The performance of the AMMF approximations is found to be good in comparison with the MC data; the AMMF approximation has clear advantages with respect to an accurate and more detailed description of the fluid structure. Possible strategies to improve the DFT are discussed.
详细描述了具有各向异性相互作用的简单方阱本体流体的相行为和结构。分子间相互作用的方向依赖性允许形成向列液晶相，除了更常规的各向同性气体和液体相之外。采用了一种经典密度泛函理论（DFT）版本来确定模型的性质，并与在巨正则和正则系综中的蒙特卡罗（MC）计算机模拟的相应数据进行了比较，为评估 DFT 结果的充分性提供了基准。DFT 方法的一个新颖元素是假设流体的结构由各向同性流体中的分子间相互作用主导。采用所谓的增强修正平均场（AMMF）近似来考虑各向异性相互作用的影响。当密度趋于零时，AMMF 近似变得精确。我们讨论了 AMMF 近似在各向同性和向列相图分支、取向有序度和各向异性相关对的准确描述方面的优缺点。与 MC 数据相比，发现 AMMF 近似的性能良好；与更准确和更详细的流体结构描述相比，AMMF 近似具有明显的优势。讨论了改进 DFT 的可能策略。
