The line tension and relative surface potentials of Langmuir monolayer phases are determined in the three-phase coexisting region gas/liquid expanded /liquid condensed of methyl octadecanoate using the pendant droplet technique. The Young-Laplace equation for two-dimensional systems with long-range dipolar interactions is applied to fluorescence images of liquid expanded droplets wetting liquid condensed surfaces in thermodynamic equilibrium. The method yields the line tension of the liquid expanded/gas borderline and the ratio of the surface potentials.