Dynamics of competitive adsorption of alphas-casein and beta-casein at planar triolein-water interface: evidence for incompatibility of mixing in the interfacial film

Competitive adsorption of alpha(s)-casein and beta-casein from a bulk solution mixture to the triolein-water interface has been studied. Although the binding affinity of alpha(s)-casein to the triolein-water interface was lower than that of beta-casein in single-component systems, in a 1:1 mixture of alpha(s)-casein and beta-casein in the bulk solution the ratio of interfacial concentrations of alpha(s)-casein to beta-casein at equilibrium was about 2:1, indicating that alpha(s)-casein was preferentially adsorbed to the triolein-water interface. Furthermore, the equilibrium composition of alpha(s)-casein and beta-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 alpha(s)-casein could dynamically displace beta-casein when the latter was in an unsaturated monolayer state at the interface, it could not do so when beta-casein was in a saturated monolayer film state. It is hypothesized that, because of thermodynamic incompatibility of mixing, the alpha(s)-casein and beta-casein mixed film at the oil-water interface may undergo two-dimensional phase separation.