浓度在决定NaCl成核机制中的作用：路径共存的故事
The Role of Concentration in Determining NaCl Nucleation Mechanism: A Story of Pathways Coexistence
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中文总结 AI 辅助
本研究通过无偏路径采样揭示NaCl成核路径随浓度变化，从一步机制过渡到无定形聚集与结晶解耦的共存路径，调和了先前矛盾观点。
中文摘要 AI 辅助
揭示晶体成核的微观细节仍然是化学物理学中的一个关键开放问题。对于含水氯化钠（NaCl），成核过程中无定形离子聚集的程度，以及两步类行为出现的过饱和区间，仍存在激烈争论。在本工作中，我们使用无偏路径采样和∞RETIS方法，研究了在12、13和14 mol/kg（$S = 3.2, 3.5, 3.8$）浓度下的NaCl成核。通过结合反应坐标分析、联合和条件概率分布、扩散张量分析以及条件自由能景观，我们在没有先验机制假设的情况下识别了成核路径。尽管所有浓度都由相同的反应坐标控制，表明核生长和结构有序化是跨越能垒的核心，但反应轨迹的性质随过饱和度变化。在较低浓度下，成核主要通过簇尺寸和有序化的紧密耦合增加发生，与一步类机制兼容。随着浓度增加，无定形聚集与结晶逐渐解耦，涉及在结晶前大量无定形生长的路径变得越来越可能。重要的是，这些路径在单个宽反应通道内共存，而不是通过不同的亚稳中间态进行。总之，这些结果调和了先前关于NaCl成核机制的矛盾报道，并支持将成核视为竞争性反应路径的集合，其相对概率随过饱和度连续变化。更广泛地说，我们的发现表明，在经典成核框架内可以出现显著的机制多样性，而无需不同的非经典描述。
英文摘要
Deciphering the microscopic details of crystal nucleation remains a key open problem in chemical physics. For aqueous sodium chloride (NaCl), the extent to which nucleation involves amorphous ion aggregation, and the supersaturation regime in which two-step-like behavior emerges, remains actively debated. In this work, we examine NaCl nucleation at 12, 13, and 14 mol/kg ($S = 3.2, 3.5, 3.8$) using unbiased path sampling with $\infty$RETIS. Through a combination of reaction coordinate analysis, joint and conditional probability distributions, diffusion tensor analysis, and conditional free energy landscapes, we identify nucleation pathways without a priori mechanistic assumptions. While all concentrations are governed by the same reaction coordinate, indicating that nucleus growth and structural ordering are central to crossing the barrier, the nature of reactive trajectories changes with supersaturation. At lower concentrations, nucleation mainly occurs via tightly coupled increases in cluster size and ordering, compatible with a one-step-like mechanism. As concentration increases, amorphous aggregation becomes increasingly decoupled from crystallization, and pathways involving substantial amorphous growth prior to crystallization become progressively more probable. Importantly, these pathways coexist within a single broad reaction channel rather than proceeding through distinct metastable intermediate states. Together, these results reconcile earlier, contradictory reports on NaCl nucleation mechanisms and support a view of nucleation as an ensemble of competing reactive pathways whose relative probabilities vary continuously with supersaturation. More broadly, our findings illustrate that substantial mechanistic diversity can emerge within a classical nucleation framework, without requiring distinct non-classical descriptions.
发表机构
- University of Minnesota（明尼苏达大学）
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