THE DETERMINATION OF THE SURFACE SHEAR PROPERTIES OF ADSORBED PROTEIN LAYERS

The surface shear properties of protein monolayers adsorbed at the air/water interface have been determined at small periodic deformations by means of a modified Bernard's double-ring surface viscometer. The results have been analyzed in terms of the elastic and viscous shear moduli of these layers by adapting an earlier theory of de Feijter [J. Colloid Interface Sci., 69 (1979) 375] to the geometry of this apparatus. The method is shown to characterize the shear properties of adsorbed layers in terms of physically meaningful parameters. The elastic surface shear moduli of ovalbumin and of bovine serum albumin (BSA) increase with decreasing amplitude of deformation; the true values of these moduli are obtained by extrapolation to the limit of zero deformation. Network formation of adsorbed BSA molecules during aging was followed by combining data on the surface dilational modulus and the surface shear modulus as a function of surface age. The effect of this age on the surface shear modulus is quite different from that on the surface dilational modulus. Both the surface shear and the surface dilational moduli of adsorbed protein monolayers increase in the sequence Na caseinate < BSA < ovalbumin, i.e. with the transition from a random to a globular configuration.