ELASTIC-MODULUS AND ORDER-PARAMETER AT THE BIDIMENSIONAL SMECTIC-ISOTROPIC TRANSITION IN A MIXED LANGMUIR MONOLAYER

A two-component monolayer, made of stearic acid and amphiphilic carboc yanine dye, is spread at the surface of water. We observe dichroic cir cular domains with absorption dipole ordering. From the light intensit y reflected by this two-dimensional mesophase, we determine the dipola r order parameter and its spatial variations. From the statistical flu ctuations, we infer the elastic modulus K. By changing the mixture com position, we get a transition to a disordered phase, where the optical anisotropy disappears. There is a jump of K which is in agreement wit h the Kosterlitz-Thouless prediction.