Real-Time Measurements of pH Changes in Single, Acoustically Levitated Droplets Due to Atmospheric Multiphase Chemistry

Abstract

We demonstrate the application of direct absorption spectroscopy to measure multiphase chemical reactions taking place in single, acoustically levitated droplets. Acidification reactions are monitored using a colored pH indicator dissolved in the droplets. The pH decrease during the uptake of dilute gas-phase carbon dioxide into an aqueous droplet is followed in real time. We show that uptake coefficients may be extracted from such measurements. The acidification is significantly suppressed when the droplet has a near-monolayer coating of 1-octanol. Such inhibition of interfacial transport by near monolayers of slightly soluble amphiphiles, which do not reside entirely at the water surface, has been previously reported for I2 and NO2 but not yet for CO2.