The surface chemistry of ions, water molecules and proteins and their ability to form stable networks in foams influences and controls macroscopic properties such as the stability of a macroscopic foam. Despite the importance of protein adsorption at liquid interfaces, a molecular level understanding of proteins and their interactions at aqueous interfaces has been elusive.
Therefore, we address the adsorption of β-lactoglobulin at air-water interfaces with vibrational sum-frequency-generation (SFG) and ellipsometry. SFG provides specific information on composition and average orientation of interfacial molecules, while information on adsorbate thickness is obtained with ellipsometry. Adsorption of charged proteins at aqueous interfaces leads to an electrified interface, pH and ion depending charging and electric field-induced polar ordering of interfacial H2O and proteins. Varying bulk pH or ionic strength of protein dilutions changes the intensities of protein vibrational bands substantially, while dramatic changes in OH stretching bands of interfacial H2O are simultaneously observed. Furthermore a charge reversal is monitored directly at the interface which helps to tune interfacial properties.
These observations allow us to determine the isoelectric point of β-lactoglobulin at the air-water interface. At a pH near the isoelectric point proteins form an amorphous network of possibly agglomerated proteins, while highly charged proteins form monolayers with strong repulsions. By the addition of salts, protein net charge can be screened and to some extend also varied whereby ionic strength, the nature of cations and thus specific ion adsorption play an important role.
Finally, we provide a direct correlation of the molecular structure of the investigated protein interfaces with foam stability1,2,3.
[1] Engelhardt, K. et al., Langmuir, 28, 7780–7787, 2012.
[2] Engelhardt K. et al., Langmuir , 29,11646-11655, 2013.
[3] Engelhardt K. et al., Curr. Opinion. Int. Sci. 19, 2007-215, 2014
