Biosurfactants as CCN : comparison between on-line and off-line measurements

• We are presenting the CCN properties for the bacterial compounds rhamnolipid and surfactin, which are extremely strong surfactants. Three organic:sodium chloride mixtures with mass percentages of 80:20, 50:50 and 20:80 were measured for each biosurfactant. Both on-line Cloud Condensation Nuclei Counter (CCNC) and off-line osmolality combined with surface tension measurements were performed to obtain two sets of critical supersaturations for various dry particle diameters. The critical supersaturations measured by the CCNC were systematically higher than the corresponding supersaturations derived from osmolality/surface tension measurements. A simple surface partitioning-adaption was applied to the off-line data and resulted in a correlation with the results from CCNC measurements for both mixtures with 20 wt% biosurfactant and the 50 wt% rhamnolipid mixture but not for the mixtures with 80 wt% biosurfactant and the 50 wt% surfactin mixture. An explanation can be unreliable CCNC results from the surfactin mixtures as we suspect poor dissolvement of the organic crystals. The choice of the assumed biosurfactant density also has an effect which should not be ignored. However, this indicate that the experimental method using osmolality and surface tension measurements together with a simple surface partitioning model can be used for strongly surfactant compounds as long as they do not dominate the particle mass. We also conclude that biosurfactants in mixed potential CCN particles can activate at relatively low supersaturation compared to other organic mixtures. Still, the critical supersaturation increases with increasing surfactant fraction.

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Applied Environmental Science

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