Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada.
J Am Chem Soc. 2010 Aug 18;132(32):11270-7. doi: 10.1021/ja1041989.
Aqueous nanodroplets that contain excess charge carriers play a central role during the electrospray ionization (ESI) process. An interesting question concerns the charge carrier location in these systems. In analogy to the behavior of metallic conductors, it is often assumed that excess ions are confined to a thin layer on the droplet surface. However, it is unclear whether simple electrostatic arguments adequately reflect the nanodroplet behavior. In particular, most ions tend to be heavily solvated, such that placing them at the liquid/vapor interface would be enthalpically unfavorable. In this work, molecular dynamics simulations are used to study the properties of Na(+)-containing water nanodroplets close to the Rayleigh limit. In apparent violation of the surface charge paradigm, it is found that the ions reside inside the droplet. Electrostatic mapping reveals that all of the excess charge is nonetheless located on the surface. This conundrum is resolved by considering the effects of orientational water polarization. Buried Na(+) ions cause large-scale dipole ordering that extends all the way to the droplet periphery. Here, the positive ends of water dipoles preferentially point into the vapor phase. These half-dipoles in the outermost droplet layers assume the role of surface charge, while solvation effectively neutralizes Na(+) ions in the interior. Overall, our data reaffirm the validity of the surface charge paradigm for ESI nanodroplets, albeit with the caveat that this paradigm does NOT require charge carriers (ions) to be located at the water/vapor interface.
在电喷雾电离（ESI）过程中，含有过量载流子的水纳米液滴起着核心作用。一个有趣的问题涉及这些体系中载流子的位置。类似于金属导体的行为，通常假定过量离子被限制在液滴表面的薄层内。然而，简单的静电论点是否能充分反映纳米液滴的行为尚不清楚。特别是，大多数离子往往被严重溶剂化，因此将它们放置在液体/蒸气界面上在焓方面是不利的。在这项工作中，使用分子动力学模拟来研究接近瑞利极限的含 Na(+) 的水纳米液滴的性质。明显违反表面电荷范例的是，发现离子位于液滴内部。静电映射表明，所有的过剩电荷都位于表面。通过考虑取向水极化的影响解决了这个难题。埋置的 Na(+)离子引起了大规模的偶极有序，一直延伸到液滴的外围。在这里，水分子偶极的正端优先指向气相。这些最外层液滴层中的半偶极子承担表面电荷的作用，而溶剂化有效地中和了内部的 Na(+)离子。总的来说，我们的数据再次证实了 ESI 纳米液滴的表面电荷范例的有效性，尽管有一个警告，即该范例并不要求载流子（离子）位于水/蒸气界面。
