Understanding Surface pKa of Molecules in Macroscale Solution and Microdroplets: A partitioning perspective

Rebika Tamang, Jacob Twichell, Madelyn Johnston, MICHAEL JACOBS, Texas State University

     Abstract Number: 197
     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 macroscale solutions. These results have broad implications from understanding chemistry in the atmosphere to suggesting new methods to run reactions more sustainably. Most studies largely ascribe the observed accelerated chemistry in microdroplets to the increased importance of interfacial chemistry and the unique properties of the air-water interface. For example, several studies invoke the “superacidity” of the air-water interface to explain accelerated chemistry in microdroplets. The ability of surface water to either accept or donate protons has been rigorously debated, with some results suggesting the surface is proton-rich, but others suggesting it is hydroxide ion-rich. Studies have presented a ‘surface pKa’ for molecules that can be shifted by several units relative to their bulk pKa values. For example, alkyl acids can have a surface pKa > 7, indicating a proton-rich interface. However, molecular origins of shifted surface pKa values remain unclear. Do they report on the acidity of surface water, or do they report on the different surface affinities of protonated/deprotonated species? Furthermore, it also remains unclear how different surface pKa values can alter the composition and acidity of microdroplets to promote chemical reactions.

We use a quadrupole electrodynamic trap (QET) to study the physicochemical properties of levitated microdroplets and explore how these unique physicochemical properties influence the partitioning of molecules to the air-water interface. In this talk, we will present recent experimental results aimed at understanding how the acidity of surface water influences molecular partitioning to the air-water interface. Using careful macroscale surface tension measurements, we demonstrate the shifts in the apparent ‘surface pKa’ of alkyl acids and amines can largely be explained using a simple competitive adsorption model that separately describes the surface partitioning of protonated and deprotonated molecules. We use our partitioning model to describe how changing surface pKa values can alter the pH of microdroplets. We compare our model results to preliminary surface tension measurements of microdroplets containing alkyl acids. Ultimately, we are working to develop a fundamental understanding of how confinement and differences in partitioning behavior may alter the chemical composition of microdroplets to promote and accelerate a wide array of chemical reactions.