Specific effects of Ca(2+) ions and molecular structure of β-lactoglobulin interfacial layers that drive macroscopic foam stability

Abstract

β-Lactoglobulin (BLG) adsorption layers at air-water interfaces were studied in situ with vibrational sum-frequency generation (SFG), tensiometry, surface dilatational rheology and ellipsometry as a function of bulk Ca(2+) concentration. The relation between the interfacial molecular structure of adsorbed BLG and the interactions with the supporting electrolyte is additionally addressed on higher length scales along the foam hierarchy - from the ubiquitous air-water interface through thin foam films to macroscopic foam. For concentrations <1 mM, a strong decrease in SFG intensity from O-H stretching bands and a slight increase in layer thickness and surface pressure are observed. A further increase in Ca(2+) concentrations above 1 mM causes an apparent change in the polarity of aromatic C-H stretching vibrations from interfacial BLG which we associate to a charge reversal at the interface. Foam film measurements show formation of common black films at Ca(2+) concentrations above 1 mM due to considerable decrease of the stabilizing electrostatic disjoining pressure. These observations also correlate with a minimum in macroscopic foam stability. For concentrations >30 mM Ca(2+), micrographs of foam films show clear signatures of aggregates which tend to increase the stability of foam films. Here, the interfacial layers have a higher surface dilatational elasticity. In fact, macroscopic foams formed from BLG dilutions with high Ca(2+) concentrations where aggregates and interfacial layers with higher elasticity are found, showed the highest stability with much smaller bubble sizes.

Fig. 1. (a) Schematic representation of the structure of a BLG foam film (not to scale). Note that this scheme is only relevant for very low Ca²⁺ concentrations before significant charge screening and a possible charge reversal occurs at the interface; see also the discussion of Fig. 2. (b) Photograph of the modified porous plate cell with horizontal part of the capillary for experiments at low capillary pressures; the liquid from the porous plate is transferred into the horizontal part of the capillary and stays at the level of the film thus avoiding the action of a hydrostatic pressure.

Fig. 2. Vibrational SFG spectra of β-lactoglobulin (BLG) modified air–water interfaces as a function of Ca²⁺ concentration. (a) C–H stretching region that is dominated by CH₃ bands originating from interfacial BLG proteins. (b) Shows the frequency region that is dominated by O–H stretching band from hydrogen bonded interfacial water molecules. Ca²⁺ concentrations for spectra were as indicated in (a) and (b).

Fig. 3. Effects of Ca²⁺ concentration on (a) the bulk ζ-potential and (b) layer thickness h ₁ (black circles) of β-lactoglobulin (BLG) modified at air–water interfaces as measured by ellipsometry and surface pressure Π (red triangles). (c) Intensity of O–H stretching band at 3200 cm^–1 from SFG spectra in Fig. 2b. (d) Surface dilatational elasticity E′ (black squares) and viscosity E′′ (red circles). (e) Foam film thickness h at a capillary pressure P _C of (100 ± 4) Pa. (f) Critical pressure of film rupture P _cr and (g) stability of macroscopic foams from aqueous 15 μM solutions of BLG as a function of Ca²⁺ concentration and foam age. Lines are a guide to the eye.

Fig. 4. Images of BLG foam films for different Ca²⁺ concentrations; CTF – common thin film and CBF – common black film. The films were obtained in the tube cell at a capillary pressure P _C of 100 ± 4 Pa.

Fig. 5. Structure and aging of macroscopic foam from 15 μM β-lactoglobulin solutions with 1 M, 30 mM and 1 mM CaCl₂ concentrations. Foam age was as indicated, 30 s indicate that time after the gas flow was stopped. The lateral resolution was for images identical and is as indicated in the top left image. Bubbles are false colour coded for certain size fractions. Additional concentrations can be found in the ESI.