A simple method for estimating surfactant impurities in solvents and subphases used for monolayer studies.

It is important to assess levels of surface active impurities in solutions used for characterization of monomolecular films and for deposition of Langmuir-Blodgett multilayers. Traditional surface pressure-area measurements lack sufficient sensitivity because of the extremely low surface pressures surfactants exhibit below the formation of a coherent film. In contrast, surface potential measurements at the gas-liquid interface increase in a surfactant-dependent manner in the gaseous-liquid expanded transition region. Using this property of such films together with area reduction, levels of impurities representing less than or equal to 0.1% of a typical coherent monolayer can be quantitated. The measurement does not require ultrapure reference materials and can be performed on a solution immediately before spreading and compression of an experimental monolayer film.