Electrostatic interactions stress between charged surfaces

The surface viscosity or resistance to shear stress of the surface film is an index of its mechanical strength and is an important parameter related to the stability of films and foams ( ), Surface yield stress of BSA films were determined using the above parameters ( ). Maximum values were obtained in the pH range of 5-6, near the isoelectric point of BSA, and decreased rapidly above pH 6,0 [Table 1]. These observations again reflect the enhanced intermolecular interactions between protein components in the film as the isoelectric point is reached and the enhanced electrostatic repulsion between neighboring molecules as the pH is raised above the isoelectric point, i.e. net charge on the protein surface was increased. 

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