Majumdar Arijit, Li Haoyuan, Muhunthan Priyanka, Späh Alexander, Song Sanghoon, Sun Yanwen, Chollet Matthieu, Sokaras Dimosthenis, Zhu Diling, Ihme Matthias
Mechanical Engineering Department, Stanford University, Stanford, CA, 94305, USA.
SLAC National Accelerator Laboratory, Menlo Park, CA, 94025, USA.
Nat Commun. 2024 Dec 3;15(1):10540. doi: 10.1038/s41467-024-54782-1.
Supercritical fluids exhibit distinct thermodynamic and transport properties, making them of particular interest for a wide range of scientific and engineering applications. These anomalous properties emerge from structural heterogeneities due to the formation of molecular clusters at conditions above the critical point. While the static behavior of these clusters and their effects on the thermodynamic response functions have been recognized, the relation between the ultrafast cluster dynamics and transport properties remains elusive. By measuring the intermediate scattering function in carbon dioxide at conditions near the critical point with X-ray photon correlation spectroscopy, we directly capture the cross-over dynamics between 4 and 13 picoseconds, revealing the transition between ballistic and diffusive motion. Complementary analysis using large-scale molecular dynamics simulations reveals that this behavior arises from collisions between unbound molecules and clusters. This study provides direct evidence of the ultrafast momentum exchange between clusters, which has significant impact on transport properties, solvation processes, and reaction kinetics in supercritical fluids.
超临界流体具有独特的热力学和传输性质，这使得它们在广泛的科学和工程应用中备受关注。这些异常性质源于临界点以上条件下分子簇的形成所导致的结构不均匀性。虽然这些簇的静态行为及其对热力学响应函数的影响已得到认可，但超快簇动力学与传输性质之间的关系仍然难以捉摸。通过用X射线光子相关光谱法在接近临界点的条件下测量二氧化碳中的中间散射函数，我们直接捕捉到了4到13皮秒之间的交叉动力学，揭示了弹道运动和扩散运动之间的转变。使用大规模分子动力学模拟的补充分析表明，这种行为源于未结合分子与簇之间的碰撞。这项研究提供了簇之间超快动量交换的直接证据，这对超临界流体中的传输性质、溶剂化过程和反应动力学具有重大影响。
