Arjun A, Bolhuis P G
van 't Hoff Institute for Molecular Sciences, University of Amsterdam, PO Box 94157, 1090 GD Amsterdam, The Netherlands.
J Phys Chem B. 2021 Jan 14;125(1):338-349. doi: 10.1021/acs.jpcb.0c09915. Epub 2020 Dec 30.
Carbon dioxide hydrate is a solid built from hydrogen-bond stabilized water cages that encapsulate individual CO molecules. As potential candidates for reducing greenhouse gases, hydrates have attracted attention from both the industry and scientific community. Under high pressure and low temperature, hydrates are formed spontaneously from a mixture of CO and water via nucleation and growth. Yet, for moderate undercooling, i.e., moderate supersaturation, studying hydrate formation with molecular simulations is very challenging due to the high nucleation barriers involved. We investigate the homogeneous nucleation mechanism of CO hydrate as a function of temperature using transition path sampling (TPS), which generates ensembles of unbiased dynamical trajectories across the high barrier between the liquid and solid states. The resulting path ensembles reveal that at high driving force (low temperature), amorphous structures are predominantly formed, with 456 cages being the most abundant. With increasing temperature, the nucleation mechanism changes, and 56 becomes the most abundant cage type, giving rise to the crystalline sI structure. Reaction coordinate analysis can reveal the most important collective variable involved in the mechanism. With increasing temperature, we observe a shift from a single feature (size of the nucleus) to a 2-dimensional (size and cage type) variable as the salient ingredient of the reaction coordinate, and then back to only the nucleus size. This finding is in line with the underlying shift from an amorphous to a crystalline nucleation channel. Modeling such complex phase transformations using transition path sampling gives unbiased insight into the molecular mechanisms toward different polymorphs, and how these are determined by thermodynamics and kinetics. This study will be beneficial for researchers aiming to produce such hydrates with different polymorphic forms.
二氧化碳水合物是一种由氢键稳定的水笼构成的固体，这些水笼包裹着单个的CO₂分子。作为减少温室气体的潜在候选物，水合物已引起了工业界和科学界的关注。在高压和低温下，水合物通过成核和生长过程由CO₂和水的混合物自发形成。然而，对于适度过冷，即适度过饱和情况，由于涉及高成核势垒，使用分子模拟研究水合物形成极具挑战性。我们使用过渡路径采样（TPS）研究了CO₂水合物的均相成核机制随温度的变化，TPS可生成跨越液态和固态之间高势垒的无偏动态轨迹系综。所得的路径系综表明，在高驱动力（低温）下，主要形成无定形结构，其中456笼最为丰富。随着温度升高，成核机制发生变化，56笼成为最丰富的笼型，从而形成晶体sI结构。反应坐标分析可以揭示该机制中最重要的集体变量。随着温度升高，我们观察到作为反应坐标显著成分的变量从单一特征（核的大小）转变为二维（大小和笼型）变量，然后又变回仅核的大小。这一发现与从无定形到晶体成核通道的潜在转变一致。使用过渡路径采样对这种复杂的相变进行建模，能够对形成不同多晶型物的分子机制以及这些机制如何由热力学和动力学决定提供无偏的见解。这项研究将对旨在制备具有不同多晶型形式的此类水合物的研究人员有益。
