Salt Enrichment and Dynamics in the Interface of Supercooled Aqueous Droplets,Journal of the American Chemical Society

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Salt Enrichment and Dynamics in the Interface of Supercooled Aqueous Droplets
Journal of the American Chemical Society ( IF 14.4 ) Pub Date : 2022-06-17 , DOI: 10.1021/jacs.2c01159
Victor Kwan ₁ , Shoubhik R Maiti _{1,

2} , Ivan Saika-Voivod ₃ , Styliani Consta ₁

Affiliation

The interconversion reaction of NaCl between the contact-ion pair (CIP) and the solvent-separated ion pair (SSIP) as well as the free-ion state in cold droplets has not yet been investigated. We report direct computational evidence that the lower is the temperature, the closer to the surface the ion interconversion reaction takes place. In supercooled droplets the enrichment of the subsurface in salt becomes more evident. The stability of the SSIP relative to the CIP increases as the ion-pairing is transferred toward the droplet’s outer layers. In the free-ion state, where the ions diffuse independently in the solution, the number density of Cl^– shows a broad maximum in the interior in addition to the well-known maximum in the surface. In the study of the reaction dynamics, we find a weak coupling between the interionic NaCl distance reaction coordinate and the solvent degrees of freedom, which contrasts with the diffusive crossing of the free energy barrier found in bulk solution modeling. The H₂O self-diffusion coefficient is found to be at least an order of magnitude larger than that in the bulk solution. We propose to exploit the enhanced surface ion concentration at low temperature to eliminate salts from droplets in native mass spectrometry ionization methods.

中文翻译：

过冷水滴界面中的盐富集和动力学

NaCl 在接触离子对 (CIP) 和溶剂分离离子对 (SSIP) 之间的相互转化反应以及冷液滴中的自由离子状态尚未研究。我们报告了直接的计算证据，即温度越低，离子相互转化反应就越接近表面。在过冷液滴中，盐分的地下富集变得更加明显。SSIP 相对于 CIP 的稳定性随着离子对转移到液滴的外层而增加。在自由离子状态下，离子在溶液中独立扩散，Cl 的数密度^-除了表面上众所周知的最大值外，还显示了内部的宽最大值。在反应动力学研究中，我们发现离子间 NaCl 距离反应坐标与溶剂自由度之间存在弱耦合，这与本体溶液建模中发现的自由能垒的扩散交叉形成鲜明对比。发现H ₂ O 自扩散系数至少比本体溶液中的大一个数量级。我们建议利用低温下增强的表面离子浓度来消除天然质谱电离方法中液滴中的盐分。

更新日期：2022-06-17

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