Department of Chemical Engineering, Imperial College London, UK.
Faraday Discuss. 2010;144:187-202; discussion 203-22, 467-81. doi: 10.1039/b901601e.
We present coarse-grained molecular dynamics simulations of mixtures of a model of T-shaped polyphilic bolaamphiphile liquid crystal molecules with a solvent. Based on the premise that the most important features of the liquid structure stem from the balance between the close range repulsions and the strong directional forces typical of hydrogen bonding and association, we have employed a coarse-graining approach that simplifies and minimises the attractions present in the system. The model consists of six fused rigid spheres, where the two end spheres have a significant attraction amongst themselves while the rest are repulsive in nature. A weakly self-attracting lateral chain consisting of fully flexible tangently bonded spheres is attached to one of the central spheres. Thus, the T-shaped molecule is composed of three mutually repulsive segments which allow the pure system to self-assemble into a liquid crystalline honeycomb columnar phase. The stability of the columnar phase is probed by the sequential addition of a solvent that has affinity with only one of the segments of the molecule. Our coarse-graining technique allows us to observe dynamically not only the 1st level nanoscale segregation but also the 2nd level reorganization which leads to the formation of replicated periodic structures. It is seen how this latter structuring takes place at times which are an order of magnitude longer than the former, and by itself explains the practical limitations of studying self-assembly with more detailed atomistic models. Mobility coefficients (related to diffusion constants), order parameters and direct visualization of the configurations are used to present a phase diagram for the solvated system in the whole concentration range. At low solvent density, the solvent solvates the honeycomb structure, but does not alter the order significantly. At modest volume fractions of solvent, the solvent mostly segregates into a distinct phase, while the T-shaped molecules retain a phase with columnar structure. At very large solvent concentrations, the T-shaped molecules form structureless aggregates, while at the infinite dilution limit present themselves as dimers and monomers.
我们呈现了一种粗粒化分子动力学模拟，模拟了 T 形多亲性双偶极液晶分子与溶剂的混合物。基于这样的前提，即液体结构的最重要特征源自于近距离排斥和氢键和缔合的强方向性力之间的平衡，我们采用了一种简化和最小化系统中吸引力的粗粒化方法。该模型由六个融合的刚性球体组成，其中两个末端球体之间具有显著的吸引力，而其余球体则具有本质上的排斥性。一个由完全柔性相切键合球体组成的弱自吸引侧链连接到一个中心球体上。因此，T 形分子由三个相互排斥的片段组成，这些片段允许纯体系自组装成液晶蜂窝柱状相。通过顺序添加仅与分子的一个片段有亲和力的溶剂来探测柱状相的稳定性。我们的粗粒化技术不仅允许我们动态观察到 1 级纳米级分相，还允许我们观察到导致复制周期性结构形成的 2 级重组。可以看到这种后者结构形成的时间比前者长一个数量级，并且本身就解释了使用更详细的原子模型研究自组装的实际限制。迁移率系数（与扩散常数相关）、序参数和配置的直接可视化用于在整个浓度范围内呈现溶剂化系统的相图。在低溶剂密度下，溶剂溶剂化蜂窝结构，但不会显著改变有序性。在中等体积分数的溶剂下，溶剂主要分离成一个明显的相，而 T 形分子保留柱状结构的相。在非常高的溶剂浓度下，T 形分子形成无定形聚集体，而在无限稀释极限下呈现为二聚体和单体。
