Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, India.
Department of Chemistry, Stanford University, Stanford, California 94305, USA.
J Chem Phys. 2018 Jun 28;148(24):244704. doi: 10.1063/1.5030114.
Several recent mass spectrometry experiments reveal a marked enhancement of the reaction rate of organic reactions in microdroplets. This enhancement has been tentatively attributed to the accumulation of excess charge on a surface, which in turn can give rise to a lowering of activation energy of the reaction. Here we model the reactions in droplets as a three-step process: (i) diffusion of a reactant from the core of the droplet to the surface, (ii) search by diffusion of the reactant on the surface to find a reactive partner, and finally (iii) the intrinsic reaction leading to bond breaking and product formation. We obtain analytic expressions for the mean search time (MST) to find a target located on the surface by a reactant in both two- and three-dimensional droplets. Analytical results show quantitative agreement with Brownian dynamics simulations. We find, as also reported earlier, that the MST varies as R/D, where R is the radius of the droplet and D is the diffusion constant of the molecules in the droplet medium. We also find that a hydronium ion in the vicinity can substantially weaken the bond and hence lowers the activation barrier. We observe a similar facilitation of bond breaking in the presence of a static dipolar electric field along any of the three Cartesian axes. If the intrinsic reaction is faster compared to the mean search time involved, it becomes primarily a diffusion-controlled process; otherwise the reaction cannot be accelerated in the droplet medium. The air-droplet interface provides a different environment compared to the interior of the droplet. Hence, we might also expect a completely different mechanism and products in the case of droplet reactions.
最近的一些质谱实验表明，在微滴中有机反应的反应速率显著提高。这种增强作用被初步归因于表面上多余电荷的积累，这反过来又可以降低反应的活化能。在这里，我们将液滴中的反应模拟为三个步骤：（i）反应物从液滴核心扩散到表面，（ii）反应物在表面上扩散以找到反应伙伴，最后（iii）导致键断裂和产物形成的固有反应。我们获得了反应物在二维和三维液滴中通过表面搜索目标的平均搜索时间（MST）的解析表达式。分析结果与布朗动力学模拟定量一致。我们发现，正如之前报道的那样，MST 随 R/D 变化，其中 R 是液滴的半径，D 是液滴介质中分子的扩散常数。我们还发现，附近的氢离子可以大大削弱键，从而降低活化能垒。我们观察到在沿三个笛卡尔轴中的任何一个存在静态偶极电场的情况下，键断裂也会得到类似的促进。如果固有反应比所涉及的平均搜索时间快，则它主要成为扩散控制过程；否则，反应不能在液滴介质中加速。与液滴内部相比，气液滴界面提供了不同的环境。因此，我们也可能期望在液滴反应的情况下出现完全不同的机制和产物。
