Properties of Organic Films on Aqueous Subphases

Recently, it has been determined that organics represent a significant percentage of the composition of certain atmospheric aerosols. The air/aqueous interface of such an aerosol has the ability to act as a concentrator and selector of organic surfactants. Amphiphilic organics, such as fatty acids and alcohols, have been found to partition to the interface of aerosols thus creating a hydrophobic organic coating on an aqueous core. The selectivity of the interface was studied by monitoring the composition of various films, via GC-MS, as a function of exposure time. A Langmuir-Blodgett trough was used to contain and collect the self-assembled films that were produced from the addition of binary solutions of surfactants to the surface of an aqueous subphase. Surfactants with differing carbon number and head group functionalities were studied. The stability of the films was examined by varying the thickness of the organic films and the pH of the subphase. For a multilayer film containing equimolar stearic acid and lauric acid on a distilled water subphase, it was found that the acid with the longer hydrocarbon tail (stearic acid) remained at the interface much longer that the shorter acid. Films containing 1-octadecanol and stearic acid, both of which have identical carbon numbers, showed similar lifetimes at the air/water interface. Octadecane was found to have a longer lifetime at the interface when dissolved in equimolar stearic acid than when in a homogeneous film. Multilayer films and films formed on acidic subphases were found to be the most stable for both fatty acids studied. The relevance of these findings as they relate to organic aerosol content and structure as well as atmospheric processing and transport will be discussed.