Diffusion coefficients of organic molecules in sucrose–water solutions and comparison with Stokes–Einstein predictions

摘要

The diffusion coefficients of organic species in secondary organicaerosol (SOA) particles are needed to predict the growth and reactivity ofthese particles in the atmosphere. Previously, viscosity measurements, alongwith the Stokes–Einstein relation, have been used to estimate the diffusion ratesof organics within SOA particles or proxies of SOA particles. To test theStokes–Einstein relation, we have measured the diffusion coefficients ofthree fluorescent organic dyes (fluorescein, rhodamine 6G and calcein) withinsucrose–water solutions with varying water activity. Sucrose–water solutionswere used as a proxy for SOA material found in the atmosphere. Diffusioncoefficients were measured using fluorescence recovery after photobleaching.For the three dyes studied, the diffusion coefficients vary by 4–5 orders ofmagnitude as the water activity varied from 0.38 to 0.80, illustrating thesensitivity of the diffusion coefficients to the water content in the matrix.At the lowest water activity studied (0.38), the average diffusioncoefficients were 1.9  ×  10, 1.5  ×  10 and7.7  ×  10 cm s for fluorescein, rhodamine 6Gand calcein, respectively. The measured diffusion coefficients were comparedwith predictions made using literature viscosities and the Stokes–Einsteinrelation. We found that at water activity  ≥  0.6 (which corresponds toa viscosity of  ≤  360 Pa s and ∕  ≤  0.81), predicteddiffusion rates agreed with measured diffusion rates within the experimentaluncertainty ( represents the glass transition temperature andis the temperature of the measurements). When the water activity was 0.38(which corresponds to a viscosity of 3.3  ×  10 Pa s and a∕ of 0.94), the Stokes–Einstein relation underpredicted thediffusion coefficients of fluorescein, rhodamine 6G and calcein by a factorof 118 (minimum of 10 and maximum of 977), a factor of 17 (minimum of 3 and maximumof 104) and a factor of 70 (minimum of 8 and maximum of 494), respectively. Thisdisagreement is significantly smaller than the disagreement observed whencomparing measured and predicted diffusion coefficients of water insucrose–water mixtures.