Diffusion coefficients of organic molecules in sucrose–water solutions and comparison with Stokes–Einstein predictions
The diffusion coefficients of organic species in secondary organic aerosol (SOA) particles are needed to predict the growth and reactivity of these particles in the atmosphere. Previously, viscosity measurements, along with the Stokes–Einstein relation, have been used to estimate the diffusion rates...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-02-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/17/2423/2017/acp-17-2423-2017.pdf |
| Summary: | The diffusion coefficients of organic species in secondary organic
aerosol (SOA) particles are needed to predict the growth and reactivity of
these particles in the atmosphere. Previously, viscosity measurements, along
with the Stokes–Einstein relation, have been used to estimate the diffusion rates
of organics within SOA particles or proxies of SOA particles. To test the
Stokes–Einstein relation, we have measured the diffusion coefficients of
three fluorescent organic dyes (fluorescein, rhodamine 6G and calcein) within
sucrose–water solutions with varying water activity. Sucrose–water solutions
were used as a proxy for SOA material found in the atmosphere. Diffusion
coefficients were measured using fluorescence recovery after photobleaching.
For the three dyes studied, the diffusion coefficients vary by 4–5 orders of
magnitude as the water activity varied from 0.38 to 0.80, illustrating the
sensitivity of the diffusion coefficients to the water content in the matrix.
At the lowest water activity studied (0.38), the average diffusion
coefficients were 1.9 × 10<sup>−13</sup>, 1.5 × 10<sup>−14</sup> and
7.7 × 10<sup>−14</sup> cm<sup>2</sup> s<sup>−1</sup> for fluorescein, rhodamine 6G
and calcein, respectively. The measured diffusion coefficients were compared
with predictions made using literature viscosities and the Stokes–Einstein
relation. We found that at water activity ≥ 0.6 (which corresponds to
a viscosity of ≤ 360 Pa s and <i>T</i><sub>g</sub>∕<i>T</i> ≤ 0.81), predicted
diffusion rates agreed with measured diffusion rates within the experimental
uncertainty (<i>T</i><sub>g</sub> represents the glass transition temperature and
<i>T</i>
is the temperature of the measurements). When the water activity was 0.38
(which corresponds to a viscosity of 3.3 × 10<sup>6</sup> Pa s and a
<i>T</i><sub>g</sub>∕<i>T</i> of 0.94), the Stokes–Einstein relation underpredicted the
diffusion coefficients of fluorescein, rhodamine 6G and calcein by a factor
of 118 (minimum of 10 and maximum of 977), a factor of 17 (minimum of 3 and maximum
of 104) and a factor of 70 (minimum of 8 and maximum of 494), respectively. This
disagreement is significantly smaller than the disagreement observed when
comparing measured and predicted diffusion coefficients of water in
sucrose–water mixtures. |
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| ISSN: | 1680-7316 1680-7324 |