All isotherms were recorded upon symmetric compression of the monolayer at a constant barrier speed 10 mm.min -1 . In the case of binary mixtures, the surface pressure was plotted against the mean molecular area, obtained by dividing the total surface area by the number of molecules spread on the surface. For each monolayer composition, all the measurements were repeated at least 3 times and the isotherms were recorded at 25 ± 0.1 °C. The standard deviation obtained from compression isotherms was ± 0.5 Å² for the molecular area and ± 0.2 mN.m -1 for the surface pressure. The compression isotherms allowed the determination of the compressibility modulus, C S -1 , as: