Advances in the theoretical and molecular simulation studies of the ion chemistry in droplets

摘要

We review recent theoretical and computational advances in the understanding of ejection mechanisms of solvated ions and macromolecular ions (macroions) from highly charged nanodrops. The excess charge is carried by simple ions or macroions. On the one hand, the principal factors that lead to the instability of a droplet due to its high charge are captured by fundamental theories. These theories consider the competition between the electrostatic interactions between ions of the same sign that tend to fragment the droplet and the surface tension that tends to keep the droplet connected. On the other hand, a detailed description of the molecular mechanism of the fragmentation in these complex systems has still not been developed. Development of a comprehensive model for the droplet fragmentation is further complicated by chemical modifications of the macromolecule by binding to alkali ions, divalent ions or protons in a constantly changing droplet environment. We describe our major findings in understanding the disintegration mechanisms of charged droplets. For droplets containing simple ions, we utilise computational methodology for activated processes to describe the ejection of solvated ions. We found that the transition state involves bottle-neck configurations where the ions leave a droplet by shape fluctuations that are similar to "Taylor" cones or string formations of the solvent. In the presence of macromolecular ions, due to the connectivity of the charge along the macromolecular backbone, droplets take different shapes from those that contain simple ions. By using molecular dynamics simulations we classified these droplet states as contiguous extrusion of the macroion from a droplet, drying-out, star-like formation of solvent surrounding a macroion and pearl formation along the macromolecular chain. Quantitative analysis of each of the droplet states calls for its own theoretical description to generalise the simulation findings. Using a combination of analytical theory and molecular simulations we proposed a theoretical model for the contiguous extrusion of a macromolecule from a droplet. (C) 2014 Elsevier B.V. All rights reserved.