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The relationship between diffusion coefficients and viscosity in organic-water matrices as proxies for secondary organic aerosol Evoy, Erin

Abstract

The diffusion coefficients of large and small molecules in organic-water mixtures are important to atmospheric chemistry, as organic-water mixtures are useful proxies for atmospheric organic aerosol. Diffusion coefficients of large molecules (molecules with radius Rdiff ≥ the radius of the organic matrix molecule, Rmatrix) and small molecules (Rdiff < Rmatrix) in organic-water mixtures have been presented in the literature. However, these data are limited. Frequently, the Stokes-Einstein relation, which relates diffusion and viscosity (D ∝ 1/η), where D is the diffusion coefficient and η is the viscosity, is used to calculate diffusion coefficients. An alternative relation is the fractional Stokes-Einstein relation (D ∝ 1/η^ξ), where ξ is a fractional exponent. However, the accuracy of neither of these relations has been thoroughly assessed for predicting diffusion coefficients in organic-water mixtures. This thesis combines new diffusion measurements with literature data to test the Stokes-Einstein and fractional Stokes-Einstein relations. When Rdiff/Rmatrix ≥ 1, the Stokes-Einstein relation is able to describe most diffusion coefficients within a factor of 10, up to a viscosity of 10^6 Pa s. However, a fractional Stokes-Einstein relation with a single ξ value does a better job of describing the data. When a data set includes both Rdiff/Rmatrix ≥ 1 and Rdiff/Rmatrix < 1, the Stokes-Einstein relation describes only 75% of the data. A fractional Stokes-Einstein relation, where ξ is a function of Rdiff/Rmatrix, is able to describe 98% of the data. These equations are tested in more realistic and chemically complex aerosol samples. The Stokes-Einstein relation is able to accurately describe diffusion coefficients of organic molecules in one lab-generated organic aerosol sample, while the fractional Stokes-Einstein relation is required to describe the diffusion coefficients in the second sample. Finally, diffusion measurements of a very large organic molecule (Rdiff >> Rmatrix) are combined with the Stokes-Einstein relation to calculate the viscosity of an organic-water mixture, resolving a discrepancy in the literature between two previously published viscosity data sets. The results presented here increase our ability to quantify the relationship between diffusion of large and small molecules and the viscosity of organic-water mixtures, which are useful proxies for atmospheric organic aerosol.

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