Monolayers of Globular Proteins on the Air/Water Interface:  Applicability of the Volmer Equation of State

Abstract

We performed simultaneous measurements of the instantaneous values of the surface pressure versus time, Π(t) (by the Wilhelmy plate method), and the adsorption versus time, Γ(t) (by ellipsometry), for aqueous solutions of a globular protein (β-lactoglobulin, BLG). The resulting dependence Π(Γ) was found to be well described by the Volmer equation of state (when Γ ≤ 1.6 mg/m2, a value corresponding to an almost complete monolayer), for all times and bulk concentrations. The excluded area per molecule, α, turned out to be twice as large as the maximum cross-sectional area of the molecule, ω, in accordance with the theoretical considerations. We processed in the same way available literature data for various proteins (BLG, α-lactalbumin, bovine serum albumin), both for equilibrium and for nonequilibrium adsorbed layers, as well as for spread layers. In all cases, the experimental dependencies Π(Γ) 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, Γ, 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.

Coalescence stability of emulsions containing globular milk proteins

“…18 as a function of protein concentration in the aqueous phase after emulsification (in the serum). In the concentration range between 0.001 and 0.03 wt.%, Γ ≈ 1.5 ± 0.1 mg/m 2 , which is close to the values reported in the literature for saturated (dense) adsorption BLG monolayer at an air-water interface (Γ M ≈ 1.65 mg/m 2 [14,100,102]). A simple geometrical estimate shows that a compact monolayer of intact BLG molecules having approximately spherical shape with diameter 3.58 nm and molecular mass 18,400 g/mol [39,96] would correspond to Γ Int ≈ 2.75 mg/m 2 .…”

“…Effects of protein concentration, electrolyte concentration, and pH on β-lactoglobulin adsorption Protein adsorption on a single air-water interface has been widely studied in the literature by means of different techniques -ellipsometry, neutron reflection, radiolabeling, and others [9][10][11][12][13][14][15][16][96][97][98][99][100][101][102]. The effects on protein adsorption of various factors, such as protein concentration, pH, and aging time of the surface were studied.…”

Emulsion Drops with Complex Interfaces: Globular Versus Flexible Proteins

“…[16] Experimental data for interfacial tensions, adsorption kinetics, and surface pressure isotherms of proteins have been measured by numerous authors, mostly using the Langmuir trough and pendant drop methods, [6,[17][18][19][20] resulting in several suggested adsorption models and equations of state. [15,[21][22][23] Proteins are important gelformers in the bulk, depending on protein concentration, ionic strength, pH, and thermal history. [24,25] When a surface-active protein is adsorbed from a dilute solution to a liquid interface, its local concentration in the interfacial region remains drastically increased as compared to the bulk phase concentration.…”

A model of compression isotherms for analyzing particle layers

“…The models proposed in the literature to describe compression isotherms of molecular films were usually based on the Volmer and Mahnert study [40,41] using Gibbs equation [42,43]. Recently, extensions of the Volmer and Mahnert type models were used to describing compression isotherms of particle films [44,45]. However, these models based on the rigorous thermodynamics may be disappointing when applied to very complex systems.…”