Chemical functionality at the liquid surface of pure unsaturated fatty acids

The C=C double bonds in unsaturated fatty acids found within atmospheric aerosols play an important role in tropospheric pollution cycles. An understanding of the affinity of such double bonds for the surface of these aerosols is thus vital to the accurate modelling of reactions occurring at these interfaces. To simulate the structures of the liquid surfaces of a series of C18 fatty acids with increasing degrees of unsaturation molecular dynamics calculations have been carried out using the GROMACS suite of programs, employing the general Amber force field. The relative surface coverage of HC=CH, carboxylic acid, methyl and CH₂ groups has been probed and found to be significantly different from that predicted based on a purely statistical distribution of groups or assumed based on X-ray diffraction data. HC=CH, methyl, and CH₂ groups dominate the surface, with the methyl groups being found disproportionally at the surface relative to their bulk concentration. The HC=CH surface exposure does not increase linearly as the acids become more unsaturated. The distinct structure found at the interface extends only a few nanometers into the sample, suggesting that surface order only affects the first layer of molecules at the interface, but is key to the exposure of highly reactive chemical functional groups to atmospheric oxidants.