Dynamics of Competitive Adsorption of αs-Casein and β-Casein at Planar Triolein−Water Interface:  Evidence for Incompatibility of Mixing in the Interfacial Film

Competitive adsorption of α s -casein and β-casein from a bulk solution mixture to the triolein−water interface has been studied. Although the binding affinity of α s -casein to the triolein−water interface was lower than that of β-casein in single-component systems, in a 1:1 mixture of α s -casein and β-casein in the bulk solution the ratio of interfacial concentrations of α s -casein to β-casein at equilibrium was about 2:1, indicating that α s -casein was preferentially adsorbed to the triolein−water interface. Furthermore, the equilibrium composition of α s -casein and β-casein in the interfacial film at various bulk concentration ratios did not follow a simple Langmuir adsorption model. This deviation from ideal behavior was mainly due to thermodynamic incompatibility of mixing of these caseins in the interfacial region. The value of the incompatibility parameter, X 12, for these caseins at the triolein−water interface was much greater than that at the air−water interface. Displacement experiments showed that while α s -casein could dynamically displace β-casein when the latter was in an unsaturated monolayer state at the interface, it could not do so when β-casein was in a saturated monolayer film state. It is hypothesized that, because of thermodynamic incompatibility of mixing, the α s -casein and β-casein mixed film at the oil−water interface may undergo two-dimensional phase separation.