Monolayers of globular proteins on the air/water interface: Applicability of the volmer equation of state

We performed simultaneous measurements of the instantaneous values of the surface pressure versus time, Pi(t) (by the Wilhelmy plate method), and the adsorption versus time, Gamma(t) (by ellipsometry), for aqueous solutions of a globular protein (beta-lactoglobulin, BLG). The resulting dependence Pi(Gamma) was found to be well described by the Volmer equation of state (when Gamma less than or equal to 1.6 mg/m(2), a value corresponding to an almost complete monolayer), for all times and bulk concentrations. The excluded area per molecule, alpha, turned out to be twice as large as the maximum cross-sectional area of the molecule, omega, in accordance with the theoretical considerations. We processed in the same way available literature data for various proteins (BLG, alpha-lactalbumin, bovine serum albumin), both for equilibrium and for nonequilibrium adsorbed layers, as well as for spread layers. In all cases, the experimental dependencies Pi(Gamma) were fitted well by the Volmer equation; the excluded area either was almost exactly twice the maximum cross-sectional area (for spherical molecules) or could be interpreted in a similar way (for nonspherical molecules), by means of qualitatively equivalent reasoning. These results have led us to the following conclusions for the studied globular proteins: (i) The surface state depends only on the instantaneous adsorption, Gamma, regardless of how it was reached. (ii) The Volmer equation is obeyed for surface coverages close to or lower than the monomolecular adsorption. (iii) No denaturation occurs during the adsorption process.