The effect of pH on structural, topographical, and rheological characteristics of β-casein–DPPC mixed monolayers spread at the air–water interface

In this work we have analyzed the interactions between dipalmitoyl-phosphatidylcholine (DPPC) and β-casein in mixed monolayers spread at the air–water interface by means of surface pressure (π)–area (A) isotherms (monolayer structure, elasticity (E), and miscibility), surface dilatational and shear properties, and Brewster angle microscopy (BAM), as a function of surface pressure, pH (at pHs 5, 7, and 9), and β-casein/DPPC interfacial ratio. DPPC can be present in aqueous solutions with neutral net charge, as zwitterions and with negative net charge depending on the pH. Thus, the analysis of β-casein–DPPC mixed monolayers at different pH would give insight into the importance of electrostatic interactions with respect to the interfacial characteristics of mixed monolayers at the air–water interface. From the study of the π–A isotherm of β-casein–DPPC mixed monolayers — including the application of the additivity rule on miscibility — and monolayer elasticity, it has been concluded that, at a macroscopic level and at π lower than that for the protein collapse a mixed β-casein–DPPC monolayer may exist at the air–water interface, but with weak interactions between monolayer forming components. At higher π the collapsed protein is displaced from the interface by DPPC. The existence of weak interactions between β-casein and DPPC and/or the repulsion between these components on a basic aqueous subphase facilitates displacement of β-casein by DPPC from the air–water interface. The observation of the interface by BAM confirms at a microscopic level the phenomena observed from the π–A isotherm for spread β-casein–DPPC mixed monolayers. The interactions between monolayer forming components have an effect on the β-casein displacement by DPPC and surface dilatational and shear characteristics of mixed monolayers.