Department of Chemistry , The University of Utah , 315 South 1400 East , Salt Lake City , Utah 84112-0850 , United States.
J Phys Chem B. 2018 May 3;122(17):4758-4770. doi: 10.1021/acs.jpcb.8b02381. Epub 2018 Apr 23.
Block copolymers, surfactants, and biomolecules form lamellar, hexagonal, and gyroid mesophases. Across these systems, the nucleation of lamellar from the disordered liquid is the easiest and the nucleation of gyroid the most challenging. This poses the question of what are the factors that determine the rates of nucleation of the mesophases and whether they are controlled by the complexity of the structures or the thermodynamics of nucleation. Here, we use molecular simulations to investigate the nucleation and thermodynamics of lamellar, hexagonal, and gyroid in a binary mixture of particles that produces the same mesophases as those of surfactants and block copolymers. We demonstrate that a combination of averaged bond-order parameters q̅ and q̅ identifies and distinguishes the three mesophases. We use these parameters to track the microscopic process of nucleation of each mesophase and investigate the existence of heterogeneous nucleation (cross-nucleation) between mesophases. We estimate the surface tensions of the liquid/mesophase interfaces from nucleation rates using classical nucleation theory and find that they are comparable for the three mesophases with values that are about a third of those expected for liquid-crystal interfaces. The driving forces for nucleation, on the other hand, are quite different and increase in the order gyroid < hexagonal < lamellar at any temperature. We find that the nucleation rates of the mesophases follow the order of their driving forces. We conclude that the difficulty to nucleate the gyroid originates in its lower temperature of melting and extremely low entropy of melting compared to those of the hexagonal and lamellar mesophases.
嵌段共聚物、表面活性剂和生物分子形成层状、六方和柱状中间相。在这些体系中，从无序液体中形成层状相的成核是最容易的，而柱状相的成核是最具挑战性的。这就提出了一个问题，即是什么因素决定了中间相的成核速率，以及它们是由结构的复杂性还是成核的热力学控制的。在这里，我们使用分子模拟来研究在产生与表面活性剂和嵌段共聚物相同中间相的粒子的二元混合物中，层状、六方和柱状相的成核和热力学。我们证明，平均键序参数 q̅ 和 q̅ 的组合可以识别和区分这三种中间相。我们使用这些参数来跟踪每种中间相的成核微观过程，并研究中间相之间是否存在异质成核（交叉成核）。我们使用经典成核理论从成核速率估计了液体/中间相界面的表面张力，并发现它们对于三种中间相是可比的，其值约为液晶界面预期值的三分之一。另一方面，成核驱动力则大不相同，在任何温度下，驱动力的顺序为柱状相<六方相<层状相。我们发现中间相的成核速率遵循其驱动力的顺序。我们得出结论，柱状相成核困难的原因在于其熔化温度较低，熔化熵极低，与六方相和层状相相比。
