Computational Modelling of Interfacial Properties of Droplets
Abstract
Aqueous nanodroplets containing reactive species play an important role in atmospheric chemistry and technology. The presence of atmospheric aerosol particles and the chemical reactions that they host plays a critical role in climate, visibility in the atmosphere, quality of air, and health. Man-made aerosols find applications in ink-jet printing, electrospinning, electrospraying, and ionization methods used in mass spectrometry.
Despite their small size, these systems show complex chemical and physical behaviour because a significant portion of the system is occupied by a liquid-vapour interface. Interfaces are distinct regions that are characterized by large mass density gradients, shape fluctuations, the particular orientation of the molecules and curvature effects, especially in the lower-sized nanodroplets. Low temperature and charge are two additional factors that further enhance the complexity of physical chemistry in droplets. For example, supercooled droplets may still be liquid at 200~K or even lower temperatures.
In this dissertation, atomistic modelling of aqueous nanodroplets containing ions and charged proteins are employed to study (a) the structure of the liquid-vapour interface; (b) interconversion reactions between solvent-separated and contact ion pairs, and (c) the rupture of the surface of highly charged droplets by emission of solvated ions. The results are scalable and are extended to the microscale.
There are several important findings in the study. Firstly, the colder the aqueous droplets the more likely is for the ion-pairing to take place near the surface where the thermodynamics and kinetics is shown to be significantly different from those in the bulk solution. Secondly, new maxima in the number density of a single ion in aqueous droplets are detected that were not found in previous literature. Thirdly, for the first time the structure of the interface of a highly charged droplet and the size distribution of the progeny clusters emitted from the surface as a function of the ion type is identified.
The study of the ion-pairing as a function of temperature suggests possible ways for optimizing ionization methods used in native mass spectrometry. These studies open-up new directions in research such as modelling of cryo-preservation of macromolecules in droplets and exploration of the chemistry in conical geometries formed on the droplet surface for possible applications in catalysis.