Soft Condensed Matter, Debye Institute for NanoMaterials Science, Utrecht University, Princetonplein 5, 3561 RT Utrecht, The Netherlands.
J Chem Phys. 2011 Mar 7;134(9):094501. doi: 10.1063/1.3552951.
We investigate the phase behavior of a model for colloidal hard platelets and rigid discotic molecules: oblate hard spherocylinders (OHSC). We perform free energy calculations using Monte Carlo simulations to map out the phase diagram as a function of the aspect ratio L∕D of the particles. The phase diagram displays a stable isotropic phase, a nematic liquid crystal phase for L∕D≤0.12, a columnar phase for L∕D≲0.3, a tilted crystal phase for L≲0.45, and an aligned crystal phase for L∕D≳0.45. We compare the results to the known phase diagram of hard cut spheres. Thin cut spheres are almost cylinder-shaped, while the interactions between real discotic mesogens and colloidal platelets are more consistent with the toroidal rims of the OHSC. Since the shapes of the OHSC and the cut spheres are otherwise similar, the phase diagrams of the two types of particles are quite akin. However, the tilted crystal phase for OHSC, which is of a crystal type that is frequently found in experiments on disklike molecules, has not been found for hard cut spheres. Furthermore, although we have found a cubatic phase, it was shown to be definitely unstable, whereas the stability of the cubatic phase of cut spheres is still disputed. Finally, we also show that the phase boundaries differ significantly from those for cut spheres. These are remarkable consequences of a subtle change in particle shape, which show that for a detailed comparison with the phase behavior of experimental particles, the OHSC should be used as a model particle.
扁长硬球柱（OHSC）。我们使用蒙特卡罗模拟进行自由能计算，以描绘出作为粒子纵横比 L∕D 的函数的相图。相图显示出稳定的各向同性相、L∕D≤0.12 时的向列液晶相、L∕D≲0.3 时的柱状相、L≲0.45 时的倾斜晶体相和 L∕D≳0.45 时的取向晶体相。我们将结果与已知的硬切球相图进行了比较。薄切球几乎是圆柱形的，而真实碟状介晶分子与胶体盘之间的相互作用更符合 OHSC 的环形边缘。由于 OHSC 和切球的形状相似，因此两种类型粒子的相图非常相似。然而，OHSC 的倾斜晶体相是实验中经常发现的碟状分子的晶体类型，而在硬切球中尚未发现。此外，虽然我们已经发现了立方相，但它被证明是肯定不稳定的，而切球的立方相的稳定性仍然存在争议。最后，我们还表明，相界与切球的相界有很大的不同。这些都是粒子形状细微变化的显著后果，表明为了与实验粒子的相行为进行详细比较，应该将 OHSC 用作模型粒子。
