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 $11\times {10}^{-10}{\mathrm{N}\mathrm{s}\mathrm{m}}^{-1}$, which is 10 to 100 times lower than previous data on similar systems. For $N$-palmitoyl-6-$n$-penicillanic acid and $L\mathrm{\text{−}}\alpha$-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.

• Received 11 January 2001

DOI:https://doi.org/10.1103/PhysRevLett.90.126104