SURFACE CONCENTRATION MEASUREMENT OF A SPREADING MONOLAYER FILM USINGA DISPLACEMENT CURRENT TECHNIQUE

There are numerous techniques for measuring surface concentration of m onolayer 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 fi lm. Measurement of the concentration of an octadecanol monolayer while spreading over a water surface is demonstrated with a displacement cu rrent measurement technique in conjunction with Wilhelmy plate measure ments of the surface pressure. It was found that the displacement curr ent technique can easily measure changes in the film concentration wel l below those detectable by the Wilhelmy plate. A ''gaseous'' film reg ion was observed ahead of the leading edge detectable by the Wilhelmy plate. This film region has not been previously reported in the litera ture. Furthermore, the film front appears to diffuse during the spread ing process. Surfactant concentration behavior during spreading is use ful for examining and developing models of surface tension driven flow s.