Effect of Hydration and Base Contaminants on Sulfuric Acid Diffusion Measurement: A Computational Study

We used quantum chemical formation free energies of hydrated sulfuric acid-containing molecular clusters and a dynamic model to simulate a flow tube measurement, and determined the effective diffusion coefficient of sulfuric acid as a function of relative humidity. This type of measurement was performed by Hanson and Eisele (2000, J. Phys. Chem. A, 104:1715–1719), who presented and applied a fitting method to obtain equilibrium constants <i>K</i>₁ and <i>K</i>₂ for the formation of sulfuric acid mono- and dihydrates, respectively, from the experimentally determined diffusion coefficients. The fit is derived assuming that only H₂SO₄ molecules hydrated by up to two water molecules are present. To study the sensitivity of the results to this assumption, we implemented the same fit to the modeled diffusion coefficient data, computed including also larger H₂SO₄ hydrates with more than two waters. We show that according to quantum chemical equilibrium constants, the larger hydrates are likely to be present in non-negligible amounts, which affects the effective diffusion coefficient. This results in the fitted value obtained for <i>K</i>₁ being lower and for <i>K</i>₂ being higher than the actual values. The results are further altered if contaminant base molecules, such as amines, capable of binding to H₂SO₄ molecules, are able to enter the system for example with the water vapor. The magnitude and direction of the effect of the contaminants depends not only on the contaminant concentration, but also on the H₂SO₄ concentration and on the hygroscopicity of the H₂SO₄–base clusters.