``Surface Elasticity'' of Protein Films. I. Egg Albumin

Abstract

The limiting area is the quantity by which protein films are usually described. This may give some indication of the ``spreading tendency'' of the protein, but it is not necessarily related to other mechanical properties of the film, since an unknown proportion of the protein may remain unspread, or may pass into solution in the substrate. This is demonstrated by an experimental study of the limiting areas and the surface elasticities, M_S = — A · dF/dA, of egg albumin films as a function of pH. It is shown that the M_S — F curves for this protein are of characteristic form, with a single well‐defined maximum, (M_S)_max. It is shown further that the characteristic large variations of limiting area with pH are not reflected in the variation of (M_S)_max and it is concluded that throughout the range of H ion concentrations studied the observed elasticities are due to a monomolecular film of true limiting area ∼1.0 m²/mg. The lower values of the apparent limiting area obtained at certain hydrogen ion concentrations are due principally to unspread material which does not contribute significantly to the mechanical properties of the film. It is suggested that the M_S — F curves should be employed in place of the apparent limiting areas in the description of protein films and that the values of (M_S)_max may provide a convenient means of showing specific differences between films of different proteins.