Measuring the Surface Tension and Composition of Model "Electrospray" Microdroplets

MICHAEL JACOBS, Texas State University

     Abstract Number: 181
     Working Group: Aerosol Chemistry

Abstract
Numerous classes of reactions (from condensation to redox to biomolecular) have been reported to occur several orders of magnitude faster (up to 10¹¹!) in microdroplets than equivalent reactions in beaker-scale solutions. While many studies attribute these large rate enhancements to the increased importance of and accelerated interfacial chemistry, most techniques study droplets from electrospray sources and are unable to decouple surface-driven chemistry from other acceleration mechanisms (e.g., solvent evaporation or potential gas phase chemistry). This leaves many lingering questions, including: How do interfacial reactions accelerate chemistry in microdroplets when the kinetic timescale for molecules to sample the air-water interface (>10s ms) is much longer than the lifetime of electrospray droplets (<1 ms)? As a result, complementary techniques with more control over microdroplet size and composition are needed to develop a fundamental understanding of which physicochemical properties are responsible for the accelerated chemistry in microdroplets.

We use a quadrupole electrodynamic trap to measure the physicochemical properties of and chemical reactions in levitated microdroplets with well characterized composition and size. In this talk, we will present our recent work to explore how the large surface charge densities found in electrospray microdroplets affects molecular transport to and concentration at the air-water interface. We use quasi-elastic light scattering to measure the surface tension of microdroplets of varying size, charge, and composition. Kinetic modeling is used to relate measured surface tension to interfacial concentrations. Preliminary results indicate surface tension decreases as microdroplets charge approaches the Rayleigh limit. These results could be indicative of charge-facilitated mass transport to the air-water interface and suggest reactions rates at electrified air-water interfaces are distinct from their uncharged counterparts. Ultimately, we are working to develop a fundamental understanding of molecular transport to the electrified air-water interface that can (partially) explain the unique chemistry of microdroplets.