Physical origin underlying the prenucleation-cluster-mediated nonclassical nucleation pathways for calcium phosphate
Physical origin underlying the prenucleation-cluster-mediated nonclassical nucleation pathways for calcium phosphate
复制标题
磷酸钙预成核团簇介导的非经典成核途径的物理起源
DOI：
10.1039/c9cp00919a
复制
发表时间：
2019
影响因子：
3.3
通讯作者：
Yang Xiaoning
中科院分区：
文献类型：
--
作者：
Yang Xiao;Wang Mingzhu;Yang Yang;Cui Beiliang;Xu Zhijun;Yang Xiaoning
在非经典成核理论的框架内，最近已承认预成核团簇（PNC）参与过饱和溶液的结晶过程;然而，在定量水平上，对PNC的形成机制和稳定性，由它们形成的新相，以及它们对成核势垒的影响知之甚少。在本文中，使用累积模拟时间超过5 μs的复杂自由能计算，我们确定了一个优先考虑的PNC介导的磷酸钙成核途径，从溶液中的离子对缔合开始。我们证明，这样的离子缔合不仅发生在阳离子和阴离子之间，而且对于具有相同符号的电荷的多原子物种，这实际上导致通过连续的磷酸根离子与钙的配位形成PNC。自由能分解计算表明，水相能够阻碍或促进上述过程的离子缔合，其具体作用与钙离子周围水合壳层的特性错综复杂地相关。高电荷物种之间的有利相互作用在稳定PNC复合物和PNC形成的聚集体中起着至关重要的作用。此外，我们目前的工作表明，额外的钙离子的摄取是第一个和强制性的步骤，触发PNC聚集成无定形磷酸钙（ACP）通过消除相关的自由能障碍。我们的理论研究成功地提供了大量的实验数据在该领域，这是目前正在激烈的讨论与非经典成核机制的定量解释。在我们目前的工作中开发的计算方法的组合提供了一个可行的和通用的解决方案，定量和系统地研究离子协会和晶体成核/生长在水溶液中的原子水平，这通常是无法访问的大多数现有的实验收购。
The involvement of prenucleation clusters (PNCs) in crystallization from a supersaturated solution has been recently admitted within the framework of nonclassical nucleation theory; however, little is known about PNCs, at the quantitative level, for their formation mechanism and stability, the new phase formed by them, as well as their impact on nucleation barriers. Herein, using the sophisticated free energy calculations with a cumulative simulation time of over 5 μs, we identify a thermodynamically favored pathway of the PNC-mediated nucleation for calcium phosphate, starting with the ion pair association in solution. We demonstrate that such an ion association occurs not only between cations and anions, but also for the polyatomic species with charges of the same sign, which, in fact, leads to PNC formation via the consecutive coordination of the phosphate ions to calcium. The free energy decomposition calculations illustrate that the water phase is capable to either hinder or promote ion association for the abovementioned processes, and its specific role is intricately related to the characteristics of the hydration shell around calcium ions. The favorable interactions between the highly charged species play a crucial role in stabilizing the PNC complexes and the aggregates formed by PNCs. Furthermore, our present work reveals that the uptake of an extra calcium ion is the first and mandatory step to trigger PNC aggregation into amorphous calcium phosphate (ACP) by eliminating the related free energy barriers. Our theoretical study successfully provides quantitative explanations to a large set of experimental data in the field, which is currently under intense discussions associated with the nonclassical nucleation mechanism. The combination of computational methods developed in our present work offers a feasible and general solution to quantitatively and systematically study ion associations and crystal nucleation/growth in an aqueous solution at the atomic level, which are normally inaccessible to most of the existing experimental acquisitions.