Octadecylurea monolayers

Abstract

Monolayers of octadecylurea have been studied over the temperature range 14—30°C by measuring surface pressure-area isotherms and evaporation resistances. The monolayer can exist in two distinct condensed phases: the α phase, with a limiting area of about 0.195 nm 2 molecule −1, and the β phase, with a limiting area of about 0.250 nm 2 molecule −1. The β phase is stable at low temperatures and the α phase tends to be stable at higher temperatures. The surface pressure of the α-β transition decreases with temperature. Significant evaporation resistances were observed in both the α and β phases and in the α-β transition region. These evaporation data are not consistent with either the energy barrier theory or the density fluctuation theory, but can be explained by the accessible area theory.