Experimental determination of the temperature dependence of water activities for a selection of aqueous organic solutions
This work presents experimental data of the temperature dependence of water activity in aqueous organic solutions relevant for tropospheric conditions (200–273 K). Water activity (<i>a</i><sub>w</sub>) at low temperatures (<i>T</i>) is a crucial parameter for pred...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2014-09-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/14/9993/2014/acp-14-9993-2014.pdf |
| Summary: | This work presents experimental data of the temperature
dependence of water activity in aqueous organic solutions relevant for
tropospheric conditions (200–273 K). Water activity (<i>a</i><sub>w</sub>) at low
temperatures (<i>T</i>) is a crucial parameter for predicting homogeneous ice
nucleation. We investigated temperature-dependent water activities, ice
freezing and melting temperatures of solutions, and vapour pressures of a
selection of atmospherically relevant aqueous organic systems. To measure
<i>a</i><sub>w</sub> over a wide composition range and with a focus on low
temperatures, we use various <i>a</i><sub>w</sub> measurement techniques and
instruments: a dew point water activity meter, an electrodynamic balance
(EDB), differential scanning calorimetry (DSC), and a setup to measure the
total gas phase pressure at equilibrium over aqueous solutions. Water
activity measurements were performed for aqueous multicomponent and
multifunctional organic mixtures containing the functional groups typically
found in atmospheric organic aerosols, such as hydroxyl, carboxyl, ketone,
ether, ester, and aromatic groups. The aqueous organic systems studied at
several fixed compositions over a considerable temperature range differ
significantly in their temperature dependence. Aqueous organic systems of
1,4-butanediol and methoxyacetic acid show a moderate decrease in
<i>a</i><sub>w</sub> with decreasing temperature. The aqueous M5 system (a
multicomponent system containing five different dicarboxylic acids) and
aqueous 2-(2-ethoxyethoxy)ethanol solutions both show a strong increase of
water activity with decreasing temperature at high solute concentrations for
<i>T</i> < 270 K and <i>T</i> < 260 K, respectively. These measurements show
that the temperature trend of <i>a</i><sub>w</sub> can be reversed at low
temperatures and that linear extrapolations of high-temperature data may lead
to erroneous predictions. To avoid this, experimentally determined
<i>a</i><sub>w</sub> at low temperature are needed to improve thermodynamic models
towards lower temperatures and for improved predictions of the ice nucleation
ability of organic–water systems. |
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| ISSN: | 1680-7316 1680-7324 |