A novel approach for structure quantification of fatty acids films on rain water

A complete compositional or structural description of naturally occurring surfactants on rainwater is not currently feasible. A main limitation of previous work has been the lack of means for correlating force-area characteristics with the chemical makeup of the films. Instead of analyzing the chemical composition of rain water film-forming organics, it is postulated here to introduce the novel scaling procedures (2D virial equation of state and 2D polymer film scaling theory) applied to the surface pressure-area (π−A) isotherms and surface pressure-temperature (π−T) isochors, and resulting from generalized physical formalisms modified to a multicomponent surfactant film. A set of the introduced structural film state parameters could become sensitive indicators for surface-active source-specific organic matter pathways tracing, where the measurement of surfactant concentration and chemical analyses are avoided. Performed comprehensive film studies on rain, marine and snow-melted water samples exhibited significant and differentiated film structural parameters variability. The developed procedure allows one to recover the film parameters (π, Γ, Alim, Eisoth) present originally at the raindrop surface from the Langmuir trough data supplemented with the simultaneously taken rain event characteristics (rain rate and rain drop diameter distribution). It requires the partitioning effect of the surfactant molecules between the surface and bulk phases to be estimated where the entering quantities are: the partitioning coefficient Kp=Γ/c and a degree of the rain water interfacial system area development Ar/Vr evaluated here using the fatty acids concentrations as model input data. The latter parameter depends on the rain rate and the form of the drop size distribution function differing significantly from the Marshall–Palmer one at low Ir (<1 mm h−1). The partitioning factor Kp related to the physicochemical composition of the film-composing material exhibited a seasonal and source-specific features and varies in a rather narrow range (5.3–13.7 cm) being of secondary importance in comparison to the interface development term. Original rain drop films exhibited low surface pressures (1.9–5.4 mN m−1) characteristic of a quasi-ideal 2D gas behavior found in a diluted surfactant solution (with c≈10−3a) and are composed of slightly soluble immiscible components of mean molecular masses within 261–270 Da in agreement with chemical analyses. The concept is quantified using the Langmuir trough film and disdrometer data on a set of rainwater events observed at Gdańsk, Poland in May 2002.