First measurement of the liquid-solid line energy in a Langmuir monolayer

We observe the homogeneous 2D nucleation of solid anisotropic domains from a liquid phase in an amphiphilic nitrobenzoxadiazole stearic acid monolayer. The nucleation rate decreases exponentially with the inverse of the excess surface pressure. We measure the line energy {λ} between the two phases. Its value, averaged over the contour of a seed, is (5±{}0.5)×{}10^-7 erg/cm at 20 ^∘{}C. It decreases with temperature and when stearic acid impurities are added to the monolayer. We also discuss the anisotropy of {λ} in the frame of a model based on dipolar electric interactions between molecules.