YOUNGS MODULUS OF A SOLID 2-DIMENSIONAL LANGMUIR MONOLAYER

LANGMUIR monolayers-films of amphiphilic molecules at the surface of water-exhibit many phases 1,2. Some of these behave like two-dimensional solids on experimental timescales, but previous measurements of the shear modulus of these 'solid' monolayers 3-5 have yielded a value too small to be compatible with a two-dimensional crystal. The interpretation of these is complicated, however, by the likelihood of inhomogeneities in the films, which are probably assemblies of microscopic crystalline domains. Here we describe measurements of the Young's modulus of an isolated 'solid' domain of NBD-stearic acid monolayers. We obtain a value large enough to be compatible with the modulus of a two-dimensional crystal 6-8. This suggests that Langmuir monolayers should provide model systems for studies of melting in two dimensions 6-8.