Effects of interfacial hydrogen bonding and electrostatic interactions on the adsorption and foaming properties in...

Understanding how molecular additives modulate the interfacial structure and foaming behaviour of natural surfactants is essential for the design of high-performance, sustainable formulations. Here, we investigate how hydrogen-bond donor and acceptor additives - glycerol, choline chloride, and urea - modify the interfacial architecture of Quillaja saponin extract solutions. A multi-technique approach combining surface tension, interfacial dilational rheology, foam stability measurements, vibrational sum-frequency generation (SFG) spectroscopy, electrokinetic analysis, and atomistic molecular dynamics (MD) simulations reveals distinct additive-specific mechanisms at the molecular level. Using SFG spectroscopy, we resolved the additive-induced reorganisation of interfacial saponins and interfacial water molecules. Glycerol reduces the fraction of more strongly hydrogen-bonded interfacial water without altering surface tension, indicating interfacial insertion and reorientation while the overall macroscopic surface activity remains largely unchanged. In contrast, urea redistributes spectral weight toward strongly hydrogen-bonded interfacial water without significantly altering the overall surface excess of saponin, while decreasing headgroup-water contributions, thereby impairing the formation of lateral hydrogen bonds between neighbouring saponins and reducing the elasticity of the surface layer. Choline chloride induces pronounced suppression of the OH stretching band from interfacial water, consistent with charge screening and a substantial reduction of the interfacial net charge. MD simulations corroborate these findings by revealing that glycerol has more frequent contacts with the saccharide region of saponin, supporting strong interfacial cohesion. The high propensity of urea to interact with the polar residues of saponin explains the redistribution of interfacial water and headgroup hydroxyls observed in SFG spectra.