Adsorption and Desorption of Bile Salts at Air-Water and Oil-Water Interfaces

Bile Salts (BS) adsorb onto emulsified oil droplets to promote lipolysis and then desorb, solubilizing lipolytic products, a process which plays a crucial role in lipid digestion. Hence, investigating the mechanism of adsorption and desorption of BS onto the oil-water interface is of major importance to understand and control BS functionality. This can have implications in the rational design of products with tailored digestibility. This study shows the adsorption and desorption curves of BS at air-water and oil-water interfaces obtained by pendant drop tensiometry. Three BS have been chosen with different conjugation and hydroxyl groups: Sodium Taurocholate (NaTC), Glycodeoxycholate (NaGDC) and Sodium Glycochenodeoxycholate (NaGCDC). Experimental results show important differences between the type of BS and the nature of the interface (air/oil-water). At the air-water interface, Glycine conjugates (NaGDC and NaGCDC) are more surface active than Taurine (NaTC), and they also display lower surface tension of saturated films. The position of hydroxyl groups in Glycine conjugates, possibly favors a more vertical orientation of BS at the surface and an improved lateral packing. These differences diminish at the oil-water interface owing to hydrophobic interactions of BS with the oil, preventing intermolecular associations. Desorption studies reveal the presence of irreversibly adsorbed layers at the oil-water interface in all cases, while at the air-water interface, the reversibility of adsorption depends strongly on the type of BS. Finally, dilatational rheology shows that the dilatational response of BS is again influenced by hydrophobic interactions of BS with the oil; thus, adsorbed films of different BS at the oil-water interface are very similar, while larger differences arise between BS adsorbed at the air-water interface. Results presented here highlight new features of the characteristics of adsorption layers of BS on the oil-water interface, which are more relevant to lipid digestion than characteristics of BS adsorbed at air-water interfaces.