Ayappa K G, Mishra Ratan K
Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India.
J Phys Chem B. 2007 Dec 27;111(51):14299-310. doi: 10.1021/jp0728860.
Using grand ensemble simulations, we show that octamethyl-cyclo-tetra-siloxane (OMCTS) confined between two mica surfaces can form a variety of frozen phases which undergo solid-solid transitions as a function of the separation between the surfaces. For atomically smooth mica surfaces, the following sequence of transitions 1[triangle up] --> 1[triangle up]b --> 2B --> 2 square --> 2[triangle up] are observed in the one- and two-layered regimes, where n[triangle up], n[square], and nB denote triangular, square, and buckled phases, respectively, with the prefix n denoting the number of confined layers. The presence of potassium on mica is seen to have a strong influence on the degree of order induced in the fluid. The sequence of solid-solid transitions that occurs with the smooth mica surface is no longer observed. When equilibrated with a state point near the liquid-solid transition, a counterintuitive freezing scenario is observed in the presence of potassium. Potassium disrupts in-plane ordering in the fluid in contact with the mica surface, and freezing is observed only in the inner confined layers. The largest mica separations at which frozen phases were observed ranged from separations that could accommodate six to seven fluid layers. The extent of freezing and the square-to-triangular lattice transition was found to be sensitive to the presence of potassium as well as the thermodynamic conditions of the bulk fluid. The implications of our results on interpretation of surface force experiments as well as the generic phase behavior of confined soft spheres is discussed.
通过巨正则系综模拟，我们发现八甲基环四硅氧烷（OMCTS）限制在两个云母表面之间时可形成多种冻结相，这些冻结相随着表面间距的变化会发生固 - 固转变。对于原子级光滑的云母表面，在单层和双层体系中观察到以下转变序列：1[三角] --> 1[三角]b --> 2B --> 2方形 --> 2[三角]，其中n[三角]、n[方形]和nB分别表示三角形、方形和屈曲相，前缀n表示受限层数。云母上钾的存在对流体中诱导的有序程度有很大影响。光滑云母表面发生的固 - 固转变序列不再出现。当与接近液 - 固转变的状态点平衡时，在有钾存在的情况下观察到一种违反直觉的冻结情况。钾破坏了与云母表面接触的流体中的面内有序性，仅在内部受限层中观察到冻结。观察到冻结相的最大云母间距范围为可容纳六到七个流体层的间距。发现冻结程度和方形到三角形晶格转变对钾的存在以及体相流体的热力学条件都很敏感。讨论了我们的结果对表面力实验解释以及受限软球的一般相行为的影响。
