Doctor of Philosophy
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.
Summary for Lay Audience
Aqueous nanodroplets containing ions, mainly Na+ and Cl-, are omnipresent in the environment and in many technologies. Because of the presence of ions these droplets can be found at different charge states, such as neutral, lightly charged with one or a few ions, or highly charged with many ions. Moreover, these liquids droplets can be found at different temperatures. They can be found at room temperature in the surroundings of human action but they also participate in thundercloud processes in the upper troposphere, where the temperature may be as low as 220K.
Nanodroplets at a low or neutral charge state are often encountered in the atmospheric aerosols in the lower troposphere where most of the human action takes place. The chemical and physical processes that occur in aerosols as well as the presence of aerosol particles themselves play a critical role in climate, visibility in the atmosphere, quality of air, and health.
Highly charged droplets are encountered in thunderclouds, and in many technological applications such as ink-jet printing, electrospinning, electrospraying, and ionization methods used in mass spectrometry. Modern experiments use small aerosol droplets as nano and micro-reactors, where orders of magnitude acceleration of reactions may be achieved relative to that in the traditional ``beaker'' chemistry. The physical chemistry and reactivity of these small systems are much less understood relative to reactivity in large amounts of solution. The reason is that these small systems have a large surface-to-volume ratio. A large surface implies that a large portion of the system is the vapour-liquid interface. Interfaces are complex regions because the system properties change as the system transitions from the neat liquid to the neat vapour phase.
In this thesis, I use molecular modelling to examine (a) the manner in which Na+ and Cl- ions approach one another and dissociate in aqueous droplets in the temperature range from supercooling (at approximately -70 degrees Celsius) to room temperature (at approximately 25 degrees Celsius) and (b) the properties and the rapture of the droplet-vapour interface in highly charged droplets. The computational results transform the analytical models that have been widely used in the fields of mass spectrometry so far.
The results of the studies allow us to propose methods to optimize experimental processes such as ionization methods used in mass spectrometry. These experimental methods are widely used in the medicine and pharmaceutical sector for the analysis of biological specimens.
Kwan, Victor, "Computational Modelling of Interfacial Properties of Droplets" (2022). Electronic Thesis and Dissertation Repository. 8596.
Available for download on Thursday, December 01, 2022