Negative effective surface viscosities in insoluble fatty acid monolayers: effect of phase transitions on dilational viscoelasticity.

The viscoelastic properties of insoluble monolayers have been investigated by the excited electrocapillary waves method. Effective negative values of dilational viscosities have been obtained in the liquid expanded and liquid condensed phases of insoluble monolayers of myristic, pentadecanoic, and stearic acids. However, the surface viscosity remains positive for the more expanded monolayers of ethyl palmitate ester. Possible origins of such a behavior are discussed in terms of transitional effects between the two-dimensional coexisting phases.