On the structure of organic-coated water droplets: From "net water attractors" to "oily" drops

Organic-coated aerosols are believed to play an important role in atmospheric processes thereby influencing global climate. In our earlier works involving water droplets coated with hydrocarbon chains (C-12, dodecanoic acid) we found that these particles prefer an inverted micelle structure with an aqueous core and a hydrophobic surface. Such particles maintained their overall spherical shapes (with a spherical water-fatty acid interface) leading to a negative surface tension. Consequently, such particles were seen to be net "water attractors" despite a hydrophobic surface. In this paper, we report the results of molecular dynamics studies, in which we investigated the effect of fatty acid chain length, chain branching, and terminal group on the morphology and properties of water droplets coated with organics. It has been seen that for particles coated with longer/branched surfactants, the fatty acid chains tend to align parallel to each other, forcing local flattening and significant distortion of the underlying water substrate. Sticking coefficient calculations of water vapor on such particles showed that these newly formed particles behave in a manner that is consistent with an "oily" surface as opposed to particles with shorter chains that can process water and are "net water attractors."