Understanding Surface Mediated Redox Chemistry in Charged Microdroplets

Michael Jacobs, FIROZ AHMED, Texas State University

     Abstract Number: 253
     Working Group: Instrumentation and Methods

Abstract
Chemical reactions can occur more than six orders of magnitude faster in microdroplets than in a macroscale solution. A possible reason for the distinct chemistry observed in microdroplets is that the thermodynamics of reactive processes can be altered significantly in microdroplets relative to the bulk phase. Several methods exist for generating microdroplets, such as ultrasonication, condensation, gas nebulizers, and electrospray ionization (ESI). ESI is particularly popular due to its enhanced reaction rates and compatibility with mass spectrometry. Recently, researchers have focused on hydrogen peroxide formation in water microdroplets to explore microdroplet redox chemistry. Various mechanisms have been proposed for this process, including radical-mediated and electron-mediated pathways, however, the exact reason for faster redox chemistry at the microdroplet surface compared to the bulk phase remains unclear. It is believed that the electrical charge in microdroplets plays a crucial role in accelerating these reactions. The Rayleigh limit (RL) is the maximum amount of charge a droplet can carry and represents the point where electric repulsive forces overcome surface tension forces, leading to droplet fission. While the theoretical basis for RL was described in the 1880s, numerous experimental studies show droplets are unstable at sub-RL net charges. Computational studies suggest that achieving 20-30% of RL could significantly lower the reaction barrier, making redox reactions thermodynamically favorable. However, the effect of charge has to promote redox reactions in microdroplets has not been rigorously assessed experimentally.

In this talk, we use single droplet levitation techniques to explore the maximum amount of charge a droplet can carry and how charge influences redox reactivity in microdroplets. We use the quasi-elastic light scattering (QELS) technique to measure the surface tensions of levitated microdroplets of different compositions (high and low ionic strength aqueous droplets and non-aqueous organic droplets) with varying net charges. We present data validating the technique and show preliminary results that indicate surface tension of microdroplets decreases as their charge approaches the Rayleigh limit, leading to sub-RL coulombic fission. Ultimately, a rigorous understanding of how much charge a microdroplet can carry is necessary to understand how charge might influence redox chemistry in microdroplets. Going forward, these surface tension measurements will inform future studies examining how charge influences redox chemistry (and hydrogen peroxide formation) in microdroplets.

Keywords - Microdroplets, Redox Chemistry, Quadrupole Electrodynamic Balance, Rayleigh Limit, Electrospray Ionization