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UBC Theses and Dissertations

Are diffusion coefficients calculated using the Stokes-Einstein equation combined with viscosities consistent with measured diffusion coefficients of tracer organics within organics-water mediums? Chenyakin, Yuri

Abstract

Recently, rates of molecular diffusion of organic species within organic-water particles of atmospheric relevance have become an area of intense research. This is because molecular diffusion rates are required for predicting rates of growth and reactivity of organic-water particles in the atmosphere. Due to the shortage of information on the topic, fluorescence recovery after photobleaching (FRAP) was used to measure the diffusion coefficients of three organic tracer dyes in sucrose-water aqueous solutions that serve as proxies for organic-water particles in the atmosphere. Organic tracer dyes used were fluorescein isothiocyanate (FITC)–dextran (molecular weight (MW) of 1.50x10⁵ g/mol, hydrodynamic radius (RH) of 83.1 Å), calcein (MW of 622 g/mol, RH = 7.4 Å) and fluorescein sodium salt (fluorescein) (MW of 376 g/mol, RH = 5.02 Å). For FITC-dextran, diffusion coefficients ranging from 12.6-1.53x10-² µm²/s were measured for water activities (aw) ranging from 0.99 to 0.75. For calcein, diffusion coefficients ranging from 4.10-1.65x10-³ µm²/s were measured for aw from 0.88 to 0.65. For fluorescein, diffusion coefficients ranging from 7.09-2.51x10-⁴ µm²/s for aw ranging from 0.88 to 0.50. The results in this dissertation showed that Stokes-Einstein equation is still valid for molecules at the size scale of fluorescein in sucrose-water mixtures when the aw ≥ 0.50. This corresponds to viscosities ≤ 10⁴ Pa·s and Tg/T ≤ 0.87. This is consistent with the previous studies by Champion et al. (1997) who also observed consistency between the Stokes-Einstein equation and measurements when Tg/T ≤ 0.86 when studying diffusion of fluorescein in sucrose- water mixtures. However, the results are inconsistent with the studies by Corti et al. (2008a) who showed decoupling between the Stokes-Einstein equation and viscosity measurements when Tg/T > 0.65.

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