Interfacial properties of mixed films of long-chain organics at the air–water interface

Organic molecules residing at the air–water interface of atmospheric aerosols will have a critical and direct effect on the aerosols’ chemical, physical, and optical properties. It is important to study the interfacial properties of such compounds in order to accurately assess these effects. In this study, the compositions of two organic binary films at the air–water interface were monitored as a function of exposure time to the ambient atmosphere. One film was composed of tetracosanoic acid (lignoceric acid, CH3(CH2)22COOH) and nonacosane (C29H60), and the second film was composed of octadecanoic acid (stearic acid, CH3(CH2)16COOH) and octadecane (C18H38). These films were used as simplified proxies for the organic coating on atmospheric aerosols. The effect of lengthening the hydrocarbon chain on the interfacial longevity of the compounds in the mixed organic film at the air–aqueous interface was determined. The results show that octadecane in a mixed film desorbs from the interface after 72 h while octadecanoic acid remains. For nonacosane, further lengthening of the carbon chain greatly increased its interfacial longevity so that it was comparable with the fatty acids, which remained stable at the interface for at least 144 h. These results are used to explain the preponderance of long-chain fatty acids on the surfaces of collected aerosols and give insight into the degree to which the presence of other long-chain organics may affect the aerosol's chemical and physical properties.