UNIVERSAL JUMPS IN THE ELASTIC PROPERTIES OF MONOMOLECULAR FILMS

The dilational elastic modulus of monomolecular films at the air-water interface can be determined by surface light scattering. For two different amphiphilic molecules, n-pentadecanoic acid and glycerol monooleate, this modulus increases discontinuously at a particular surface concentration on film compression. The magnitude of the increase, in units of k(B)T/A, where A is the area per molecule in the film, is the same in both cases and is compatible with the value 4pi. This suggests that the effect arises from the transition of the quasi-two-dimensional film from a hexatic to a solid phase.