Effect of Calcium Ions and Environmental Conditions
on the Properties of b -Casein Stabilized
Films and Emulsions
O. D. Velev, B. E. Campbell and R. P. Borwankar
Langmuir, 14, 4122 (1998)
Thin emulsion films stabilized with b-casein were studied by microinterferometry. The data demonstrate that the film thickness and intersurface adhesion are influenced by the electrolyte concentration, the pre-history of the protein solution, pH and the presence of fatty acids in the oil phase. A major shift of the film parameters occurs in the presence of calcium ions. The decrease of the film thickness above 1 mM of Ca2+ is consistent with earlier neutron reflectivity and hydrodynamic thickness measurements. The most interesting effect is the strong cross-binding of the film surfaces above a threshold concentration of 12 mM of calcium. Electrophoretic mobility data suggest that at this concentration one Ca2+ ion per two protein molecules is adsorbed on the outermost segments of the protein layers. The possible origin of the cross-binding is discussed on the basis of the structure of the adsorbed b -casein molecules. Data for the resolution of batch emulsions demonstrate that the effect of calcium observed with emulsion films is directly related to stability of practical systems. Surface force apparatus measurements indicate that the effect, although attenuated, may also be present in suspensions of casein-stabilized solids.
Four distinctive cases of b -casein stabilized emulsion films. a) Bright, thick plane-parallel film at low electrolyte concentration (film diameter 110 m m). b) Film at a high electrolyte concentration during the thinning stage - the bright spot is a remainder from a dimple, which slowly diffuses out. c) Equilibrium plane-parallel thin film. d) Thinning to a black film through a spot-like transition in the system with 20 mM of Ca2+ at pH 6.5.
The thickness of the measured emulsion films presented as a function of the concentration of Ca2+. All samples contain 150 mM of NaCl. The shaded area marks the transition from low to high intersurface adhesion.
Dependence of the zeta-potential and estimated charge adsorbed per protein molecule on the Ca2+ concentration.
Schematics of the first 40 residues at the N-terminus of the b-casein tail. The phosphorylated residues that are able to bind Ca2+ specifically are denoted by "p". The radius of the circle around the first phosphoseryl residue is equal to the Debye screening length at I = 45 mM. Three glutamic acid residues near the outer end of the tail that can eventually participate in calcium adsorption are marked with "-".
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Last updated Sep. 21, 1999. Please note that this is part of a copyrighted material.