Surface concentration measurement of a spreading monolayer film using a displacement current technique

There are numerous techniques for measuring surface concentration of monolayer films. While most techniques are suitable only for stationary films, a novel technique is examined for its feasibility in measuring the concentration of a polar monolayer during the spreading of the film. Measurement of the concentration of an octadecanol monolayer while spreading over a water surface is demonstrated with a displacement current measurement technique in conjunction with Wilhelmy plate measurements of the surface pressure. It was found that the displacement current technique can easily measure changes in the film concentration well below those detectable by the Wilhelmy plate. A gaseous film region was observed ahead of the leading edge detectable by the Wilhelmy plate. This film region has not been previously reported in the literature. Furthermore, the film front appears to diffuse during the spreading process. Surfactant concentration behavior during spreading is useful for examining and developing models of surface tension driven flows.