Shear Viscosity of Langmuir Monolayers in the Low-Density Limit

We have measured the shear viscosity in the liquid phase of several Langmuir monolayers with an accuracy better than $30%$. The method is based on the optical monitoring of the Brownian diffusion of submicron latex spheres floating at the air-water interface. The values are between 1 and 11×10^-10 N s m^-1, which is 10 to 100 times lower than previous data on similar systems. For N-palmitoyl-6-n-penicillanic acid and L-α-dipalmitoylphosphatidylcholine, the variation of the shear viscosity with surface density agrees with a classical free area model, whereas for pentadecanoic acid we observe a compensation effect.