Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada.
J Am Chem Soc. 2011 Jun 22;133(24):9354-63. doi: 10.1021/ja111492s. Epub 2011 May 31.
The ejection of solvated small ions from nanometer-sized droplets plays a central role during electrospray ionization (ESI). Molecular dynamics (MD) simulations can provide insights into the nanodroplet behavior. Earlier MD studies have largely focused on aqueous systems, whereas most practical ESI applications involve the use of organic cosolvents. We conduct simulations on mixed water/methanol droplets that carry excess NH(4)(+) ions. Methanol is found to compromise the H-bonding network, resulting in greatly increased rates of ion ejection and solvent evaporation. Considerable differences in the water and methanol escape rates cause time-dependent changes in droplet composition. Segregation occurs at low methanol concentration, such that layered droplets with a methanol-enriched periphery are formed. This phenomenon will enhance the partitioning of analyte molecules, with possible implications for their ESI efficiencies. Solvated ions are ejected from the tip of surface protrusions. Solvent bridging prior to ion secession is more extensive for methanol/water droplets than for purely aqueous systems. The ejection of solvated NH(4)(+) is visualized as diffusion-mediated escape from a metastable basin. The process involves thermally activated crossing of a ~30 kJ mol(-1) free energy barrier, in close agreement with the predictions of the classical ion evaporation model.
溶剂化小离子从纳米尺寸液滴中的喷射在电喷雾电离（ESI）过程中起着核心作用。分子动力学（MD）模拟可以深入了解纳米液滴的行为。早期的 MD 研究主要集中在水相体系，而大多数实际的 ESI 应用涉及使用有机溶剂共溶剂。我们对携带过量 NH4+离子的水/甲醇混合液滴进行了模拟。甲醇被发现破坏了氢键网络，导致离子喷射和溶剂蒸发的速率大大增加。水和甲醇逸出率的显著差异导致液滴组成随时间发生变化。在低甲醇浓度下发生分离，从而形成甲醇富集的外围分层液滴。这种现象将增强分析物分子的分配，可能对它们的 ESI 效率产生影响。溶剂化离子从表面突起的尖端被喷射出来。与纯水溶液相比，甲醇/水液滴中离子分离前的溶剂桥接更为广泛。溶剂化 NH4+的喷射被可视化为由亚稳势阱中扩散介导的逃逸。这个过程涉及到约 30 kJ mol-1自由能势垒的热激活跨越，与经典离子蒸发模型的预测非常吻合。
