Surface potential of a two-dimensional film of pentadecanoic acid in the coexistence region

We report measurements of the change in the work function of a $pH-2\mathrm{}$ water surface upon spreading a monolayer of pentadecanoic acid. The measurements were made as a function of density at a temperature of 20.1°C, which is below the critical temperature for this monolayer. Large variations in the work function were observed in the density range corresponding to the region in which the liquid and vapor phases of the monolayer coexist, offering further evidence for the presence of two distinct phases.