The elasticity of adsorption layers of reorientable surfactants

A theoretical model is discussed which describes the relaxation of an adsorption layer under a harmonic surface area perturbation (oscillating bubble method). The model assumes sufficiently high oscillation frequencies that a diffusional exchange of matter can be neglected. As relaxation process the change in the molar area of an adsorbed molecule is considered, caused for example by changing the orientation at the interface. The model calculations show that the use of a reorientation isotherm first leads to a different dilational elasticity isotherm as compared to that obtained for a Langmuir adsorption model. Moreover, it is shown that the surface relaxation due to molecular orientation changes can influence the viscoelasticity significantly. Qualitative comparisons with literature data show good agreement. (C) 2001 Academic Press.