The Importance of Surfactants in Atmospheric Aerosols

By Prof Mohd  Talib Latif and Prof Peter Brimblecombe.

Surface-active agent or “surfactants” is a substance that, when present at low concentration in a system, has the property of adsorbing onto surfaces or interfaces of the system. Surfactants are the most widely encountered and useful products of modern chemical technology. Their areas of application are as diverse as motor oils and other lubricants, laundry detergents, personal care products, food additives and specialised pharmaceutical formulations.

Surfactants are seen as potentially important in environmental chemistry because of their ability to alter the movement of materials across aqueous interfaces and to affect the solubility of compounds in aqueous systems. In atmospheric chemistry, the composition of the organic surfactant which typically covers the surface of atmospheric particles is expected to affect all of the surface related aerosol properties. It is reasonable to expect that the surfactant in aerosol plays a significant role in light scattering, interaction with human tissue, aggregation of particles, and the nucleation of cloud droplets. The occurrence of surface-active organic materials can cause substantial enrichment of hydrophobic toxic substances, such as pesticides in fog waters.

Surfactants within a water droplet can reduce surface tension, alter evaporation and condensation rates of H2O, modify drop coalescence efficiency, and promote droplet instability. Early reports of surfactants in rain water and atmospheric aerosols showed that their concentrations were too low to have any effect on cloud physical processes at high dilution. However, at high concentrations, fine sized, or when they are in the growing droplets, the situation may be different. In such situations, a decrease in the surface tension of cloud droplets could lead to an increase in the smaller sized droplet population. This can cause the lifetime of clouds to be prolonged, resulting in overall enhancement of cloud albedo and an associated cooling effect in the atmosphere.  Surfactants also can increase the solubility of aerosol particles.

Surfactants influence the formation of cloud albedo.

The presence of surfactant coatings on aerosol particles may influence how a coated particle grows in a humid environment, and understanding this effect is important in understanding the particles’ role in atmospheric processes. The presence of an organic layer on a hydrophobic core particle, such as elemental carbon, may increase water sorption and cause a coated, originally hydrophobic particle to grow as the particle is exposed to high relative humidities in the atmosphere. Since surfactant molecules have both a hydrophilic and hydrophobic part, it is possible that under certain orientations a layer of surfactant molecules may increase the hygroscopicity of an aerosol particle.

Surfactants are able to interact with heavy metals and many organic substances which are considered pollutants. The surface properties of particles seem to be of special importance in the interaction of chemical substances with biological membranes such as human tissue, and affect human health. For example, surfactants may reduce or alter the surface tension of the tear film of the eye. It has also been reported that respirable particles contaminated with surfactants can destabilise the mucus membrane of the respiratory system, leading to allergies and asthma. Surfactants play a crucial role in lung physiology, acting as a primary immune barrier. When particles are deposited on the airway wall, that is, on the surfactant film, they are wetted by surface forces and displaced into the liquid phases. Thus, the surfaces of the particles may be changed by the surfactant or by surfactant components.

This article is based on the published paper:
MT Latif and P Brimblecombe. Surfactants in Atmospheric Aerosols. Environ. Sci. Technol. 2004, 38(24), pp 6501-6506.

About the Authors:

Prof Mohd Talib Latif is based at the School of Environmental and Natural Resource Sciences (Faculty of Science and Technology) and the Institute for Environment and Development (LESTARI) in Universiti Kebangsaan Malaysia (UKM).

Prof Peter Brimblecombe is based at the School of Energy and Environment, City University of Hong Kong, Kowloon, Hong Kong, China.