Dilational viscosity of Langmuir monolayers

The dilational viscosity ′ of the Langmuir monolayer is considered in a theoretical model taking into account an orientational effect of the dilational wave on the surface molecules. The surface tension treated in the framework of this model is supposed to be dependent on the degree of molecular ordering in the monolayer plane. This orientational order is described by the surface order parameter Q and the orientational part of the free energy which is given by Landauʹs expansion in powers of Q. The magnitude of surface viscosity determined by the surface tension derivative ∂γ/∂Q is found to be in good accord with the experimentally observed ′. The sign of ′ is positive which indicates that increased ordering in the monolayer plane increases the surface tension.