Optical detection of surfactants by means of reflective phantom interface method

Surfactants are among the most relevant organic pollutants of water with threatening potential for the aquatic environment. The most common and widespread techniques to detect surfactants in water samples are typically expensive and time consuming. In this context, we propose a new kind of optical sensor, based on an amorphous fluorinated plastic iso-refractive to water and, therefore, barely visible in aqueous solutions. When a thin molecular layer with a different refractive index adsorbs at the interface, the intensity of reflected or scattered light markedly increases, hence enabling a simple and real-time detection. We investigated the interaction between the plastic iso-refractive to water and a non-ionic surfactant (Tween 20) by measuring the intensity of light reflected by a planar interface as a function of time after the addition of different concentrations of surfactant. This detection scheme has been also tested with natural water samples taken from the Lambro river across the city of Milan.