COMPARISON BETWEEN MACROSCOPIC AND MICROSCOPIC MEASUREMENTS OF THE SHEAR ELASTIC-CONSTANT OF ALCOHOL MONOLAYERS AT THE AIR-WATER-INTERFACE

We have developed a new version of an apparatus which allows macroscop ic measurements of the shear elastic constant of monolayers, thus prob ing their crystalline state. We have investigated 1-alcohol monolayers at the air-water interface: our aim was to complement and compare the results obtained previously by high resolution X-ray diffraction, at ESRF (ID10 beamline). In two dimensions, the Bragg peak is a power law singularity, whose line shape is related to the shear elastic constan t mu. The X-ray measurement revealed mu = 210 mN/m for a decanol (C10- OH) monolayer and mu = 325 mN/m for a dodecanol (C12-OH) one, far belo w melting transition. With the mechanical technique, we found small va lues of mu, of a few mN/m, increasing with chain length. We measured m u similar to 1 mN/m for chain length of 11 carbons, and mu similar to 21 mN/m for 13 carbons. We thus bring an evidence of large differences between the microscopic and macroscopic shear elastic constants. (C) 1998 Elsevier Science B.V. All rights reserved.