Kwan Victor, Consta Styliani
Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada.
J Phys Chem A. 2022 May 26;126(20):3229-3238. doi: 10.1021/acs.jpca.2c02056. Epub 2022 May 12.
The ion evaporation mechanism (IEM) is perceived to be a major pathway for disintegration of multi-ion charged droplets found in atmospheric and sprayed aerosols. However, the precise mechanism of IEM and the effect of the nature of the ions in the emitted cluster size distribution have not yet been established despite its broad use in mass spectrometry and atmospheric chemistry over the past half century. Here, we present a systematic study of the emitted ion cluster distribution in relation to their spatial distribution in the parent droplet using atomistic modeling. It is found that in the parent droplet, multiple kosmotropic and weakly polarizable chaotropic ions (Cs) are buried deeper within the droplet than polarizable chaotropic ions (Cl, I). This differentiation in the ion location is only captured by a polarizable model. It is demonstrated that the emitted cluster size distribution is determined by dynamic conical deformations and not by the equilibrium ion depth within the parent droplet as the IEM models assume. Critical factors that determine the cluster size distribution such as the charge sign asymmetry that have not been considered in models and in experiments are presented. We argue that the existing IEM analytical models do not establish a clear difference between IEM and Rayleigh fission. We propose a shift in the existing view for IEM from the equilibrium properties of the parent droplet to the chemistry in the conical shape fluctuations that serve as the centers for ion emission. Consequently, chemistry in the conical fluctuations may also be a key element to explain charge states of macromolecules in mass spectrometry and may have potential applications in catalysis due to the electric field in the conical region.
离子蒸发机制（IEM）被认为是大气和喷雾气溶胶中多离子带电液滴解体的主要途径。然而，尽管在过去半个世纪中IEM在质谱分析和大气化学中得到了广泛应用，但其精确机制以及发射簇尺寸分布中离子性质的影响尚未确定。在这里，我们使用原子模型对发射的离子簇分布与其在母液滴中的空间分布之间的关系进行了系统研究。研究发现，在母液滴中，多个具有盐析作用和弱极化性的离液序列高的离子（Cs）比可极化的离液序列高的离子（Cl、I）更深地埋在液滴内部。离子位置的这种差异仅由可极化模型捕获。结果表明，发射的簇尺寸分布是由动态锥形变形决定的，而不是像IEM模型所假设的那样由母液滴内的平衡离子深度决定。本文提出了一些决定簇尺寸分布的关键因素，如模型和实验中未考虑的电荷符号不对称性。我们认为，现有的IEM分析模型没有明确区分IEM和瑞利裂变。我们建议将现有的IEM观点从母液滴的平衡性质转变为作为离子发射中心的锥形形状波动中的化学过程。因此，锥形波动中的化学过程也可能是解释质谱中大分子电荷状态的关键因素，并且由于锥形区域中的电场，可能在催化方面具有潜在应用。
