Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada.
J Chem Phys. 2013 Jan 28;138(4):044314. doi: 10.1063/1.4789018.
The relation between the charge state of a macromolecule and its ejection mechanism from droplets is one of the important questions in electrospray ionization methods. In this article, effects of solvent-solute interaction on the manifestation of the charge induced instability in a droplet are examined. We studied the instabilities in a prototype system of a droplet comprised of charged poly(ethylene glycol) and methanol, acetonitrile, and water solvents. We observed instances of three, previously only conjectured, [S. Consta, J. Phys. Chem. B 114, 5263 (2010)] mechanisms of macroion ejection. The mechanism of ejection of charged macroion in methanol is reminiscent of "pearl" model in polymer physics. In acetonitrile droplets, the instability manifests through formation of solvent spines around the solvated macroion. In water, we find that the macroion is ejected from the droplet through contiguous extrusion of a part of the chain. The difference in the morphology of the instabilities is attributed to the interplay between forces arising from the macroion solvation energy and the surface energy of the droplet interface. For the contiguous extrusion of a charged macromolecule from a droplet, we demonstrate that the proposed mechanism leads to ejection of the macromolecule from droplets with sizes well below the Rayleigh limit. The ejected macromolecule may hold charge significantly higher than that suggested by prevailing theories. The simulations reveal new mechanisms of macroion evaporation that differ from conventional charge residue model and ion evaporation mechanisms.
大分子的荷质比与其从液滴中逐出机制之间的关系是电喷雾电离方法中的重要问题之一。本文考察了溶剂-溶质相互作用对液滴中电荷诱导不稳定性表现的影响。我们研究了由带电荷的聚乙二醇和甲醇、乙腈和水溶剂组成的液滴原型系统中的不稳定性。我们观察到了三种以前仅推测存在的[S. Consta, J. Phys. Chem. B 114, 5263 (2010)]大分子逐出机制的实例。在甲醇中带电荷大分子的逐出机制类似于聚合物物理中的“珍珠”模型。在乙腈液滴中，不稳定性通过围绕溶剂化大分子形成溶剂刺来表现。在水中，我们发现大分子通过连续挤出链的一部分从液滴中逐出。不稳定性形态的差异归因于大分子溶剂化能和液滴界面表面能之间相互作用产生的力。对于带电荷大分子从液滴的连续挤出，我们证明所提出的机制导致大分子从远低于瑞利极限的液滴尺寸中逐出。逐出的大分子可能带有比现有理论所建议的显著更高的电荷。模拟揭示了与传统的电荷残基模型和离子蒸发机制不同的新的大分子蒸发机制。
