Energy and Angle-Resolved Uptake of Organic Molecules in Concentrated Sulfuric Acid

Abstract

The surface chemistry of sulfuric acid is of interest to heterogeneous atmospheric chemistry. We have measured the uptake of reactive gases on the surface of concentrated (98.8 wt%) sulfuric acid at 298 K. Our goal is to determine the fraction of gas molecules that dissolve in and react with concentrated sulfuric acid as a function of impact angle ({theta}{sub i}), collision energy (E{sub i}), and basicity (pK{sub BH+}). These gases include industrially important molecules such as alcohols, ethers, aldehydes, and carboxylic acids. In particular, we have investigated how scattering and salvation compete at high and low impact energies and at grazing and head-on approach directions. We find that the sticking probability (S) decreases slowly with increasing impact energy. Preliminary measurements indicate that S of formic acid, for example, decreases from {approx}73% at E{sub i} = 15 kJ/mole to {approx}67% at E{sub i} = 85 kJ/mole. The sticking probability of low incident energy gas molecules varies more gradually with increasing impact angle than high energy gases. The sticking probability of low energy formic acid changes insignificantly from {theta}{sub i} = 0{degrees} to 50{degrees}, whereas high incident energy formic acid decreases by approximately 10% over the same range. Sticking probabilities do changemore » dramatically with gas functionality and scale monotonically with the molecule`s pK{sub BH+}, which is a measure of the molecule`s solution phase basicity. Thus, S decreases in the order ethanol, dimethyl ether, formic acid, acetaldehyde, and propene.« less