Małolepsza Edyta, Keyes Tom
Department of Chemistry, Boston University , Boston, Massachusetts 02215-2521, United States.
J Phys Chem B. 2015 Dec 31;119(52):15857-65. doi: 10.1021/acs.jpcb.5b06832. Epub 2015 Dec 17.
Under ambient conditions, water freezes to either hexagonal ice or a hexagonal/cubic composite ice. The presence of hydrophobic guest molecules introduces a competing pathway: gas hydrate formation, with the guests in clathrate cages. Here, the pathways of the phase transitions are sought as sequences of states with coexisting phases, using a generalized replica exchange algorithm designed to sample them in equilibrium, avoiding nonequilibrium processes. For a dilute solution of methane in water under 200 atm, initializing the simulation with the full set of replicas leads to methane trapped in hexagonal/cubic ice, while gradually adding replicas with decreasing enthalpy produces the initial steps of hydrate growth. Once a small amount of hydrate is formed, water rearranges to form empty cages, eventually transforming the remainder of the system to metastable β ice, a scaffolding for hydrates. It is suggested that configurations with empty cages are reaction intermediates in hydrate formation when more guest molecules are available. Free energy profiles show that methane acts as a catalyst reducing the barrier for β ice versus hexagonal/cubic ice formation.
在环境条件下，水会冻结成六方冰或六方/立方复合冰。疏水性客体分子的存在引入了一条竞争途径：气体水合物的形成，客体分子位于笼形包合物笼中。在此，利用一种广义副本交换算法，将相变途径作为具有共存相的状态序列来寻找，该算法旨在对它们进行平衡采样，避免非平衡过程。对于200个大气压下甲烷在水中的稀溶液，用全套副本初始化模拟会导致甲烷被困在六方/立方冰中，而随着焓的降低逐渐添加副本则会产生水合物生长的初始步骤。一旦形成少量水合物，水会重新排列形成空笼，最终将系统的其余部分转变为亚稳的β冰，这是水合物的一种骨架。有人认为，当有更多客体分子时，有空笼的构型是水合物形成过程中的反应中间体。自由能剖面图表明，甲烷起到了催化剂的作用，降低了β冰相对于六方/立方冰形成的势垒。
