pH effects on the molecular structure and charging state of β-Escin biosurfactants at the air-water interface

Saponins like beta-escin exhibit an unusually high surface activity paired with a remarkable surface rheology which makes them as biosurfactants highly interesting for applications in soft matter colloids and at interfaces. We have applied vibrational sum-frequency generation (SFG) to study beta-escin adsorption layers at the air-water interface as a function of electrolyte pH and compare the results from SFG spectroscopy to complementary experiments that have addressed the surface tension and the surface dilational rheology. SFG spectra of beta-escin modified air-water interfaces demonstrate that the SFG intensity of O-H stretching vibrations from interfacial water molecules is a function of pH and dramatically increases when the pH is increased from acidic to basic conditions and reaches a plateau at a solution pH of > 6. These changes are attributable to the interfacial charging state and to the deprotonation of the carboxylic acid group of beta-escin. Thus, the change in O-H intensity provides qualitative information on the degree of protonation of this group at the air-water interface. At pH < 4 the air-water interface is dominated by the charge neutral form of beta-escin, while at pH > 6 its carboxylic acid group is fully deprotonated and, consequently, the interface is highly charged. These observations are corroborated by the change in equilibrium surface tension which is qualitatively similar to the change in O-H intensity as seen in the SFG spectra. Further, once the surface layer is charge neutral, the surface elasticity drastically increases. This can be attributed to a change in prevailing intermolecular interactions that change from dominating repulsive electrostatic interactions at high pH, to dominating attractive interactions, such as hydrophobic , dispersive interactions, as well as, hydrogen bonding at low pH values. In addition to the clear changes in O-H intensity from interfacial H2O, the SFG spectra exhibit drastic changes in the C-H bands from interfacial beta-escin which we relate to differences in the net molecular orientation. This orientation change is driven by tighter packing of beta-escin adsorption layers when the beta-escin moiety is in its charge neutral form (pH < 4). (C) 2021 Elsevier Inc. All rights reserved.

Automated summary

Saponins like beta-escin have high surface activity and unique surface rheology, making them interesting for applications in soft matter colloids and interfaces. Using vibrational sum-frequency generation (SFG), beta-escin adsorption layers at the air-water interface were studied, showing changes in O-H intensity with pH due to interfacial charging state and deprotonation of carboxylic acid. The SFG spectra also revealed drastic changes in C-H bands from interfacial beta-escin, related to differences in molecular orientation.