Electrostatic stabilization of foam films from β-lactoglobulin solutions

The equivalent thickness hw of microscopic foam films obtained from buffered solutions of β-lactoglobulin is measured for various buffer concentrations and pH. The films obtained from solutions at a pH close to the electrophoretic isoelectric point pI ≈ 5.1 show formations of a bilayer black film. At protein concentration of 10 μM the films rupture during the irregular expansion of the black spots whereas at 100 μM stable black films are obtained. The films at the lower protein concentration can be stabilized by changing the solution pH ≠ pI. These films reach larger equilibrium hw and are stabilized by repulsive surface forces originating from the increased diffuse electrical double-layer potential of the film surfaces due to the adsorption of charged protein molecules. The electrostatic origin of such stabilization mechanism is confirmed by the dependence of hw on the buffer concentration. Around pI the protein net charge and respectively the double-layer potential are negligible; therefore the electrostatic barrier in the disjoining pressure isotherm Π(hw) for such films is readily overcome which leads to formation of a black film with constant thickness independent of Π. The action of such pressure barrier in the films is confirmed by measuring the Π(hw) isotherm for foam films from solutions at pH 3.5 and 7. The pressure of black film formation decreases with increasing ionic strength and disappears at pH → pI. It is demonstrated that the electrostatic stabilization of β-lactoglobulin foam films is governed by a cooperative effect caused by individual/simultaneous variations in pH or/and ionic strength of the solutions.