VISCOELASTICITY OF MONOLAYERS OF N-PENTADECANOIC ACID - A LIGHT-SCATTERING STUDY

The viscoelastic properties of monolayers of n-pentadecanoic acid at the air-water interface have been studied using surface light scattering. The monolayers displayed first-order liquid-condensed to liquid-expanded transitions: the surface properties reported are those of well defined, reproducible monolayer states. Two viscoelastic moduli were determined: for shear transverse to the surface and for uniaxial dilation in the surface. For experiments both above and below the triple point the surface viscoelasticity was found to depend upon the monolayer phase in a complex fashion. Both moduli displayed viscoelastic relaxation, the strength and timescales being different for the two moduli, as well as depending upon the monolayer state. At one point in the phase diagram it was shown that the high-frequency dilational viscosity measured by light scattering could be identified with the conventional surface shear viscosity. The results confirm the complexity of the viscoelastic behaviour of molecular films which is not adequately represented by a single surface viscosity.