Surface activity of saponin from Quillaja bark at the air/water and oil/water interfaces

Surface activity of Sigmaʹs Quillaja bark saponin (QBS) was studied by means of dynamic interfacial tension and surface dilational rheology at three fluid/fluid interfaces with the polarity of the non-aqueous phase increasing in the order: air/water, tetradecane/water and olive oil/water. The equilibrium interfacial tension isotherms were fitted to the generalized Frumkin model with surface compressibility for the air/water and tetradecane/water interfaces, whereas the isotherm for the third interface displays a more complex shape. Upon fast compression of a drop of concentrated “Sigma” QBS solution immersed in olive oil, a clearly visible and durable skin was formed. On the other hand, no skin formation was noticed at the air/water interface, and only a little at the tetradecane/water interface. Addition of a fatty acid, however, improved slightly the skin-formation ability of the QBS at the latter interface. The surface behavior of the QBS from Sigma was compared with that from Desert King, Int. (“Supersap”), employed in a recent study by Stanimirova et al. [22]. The two products exhibit different areas per molecule in the saturated adsorbed layer (0.37 nm2 vs. 1.19 nm2 for “Sigma” and “Supersap”, respectively). Also their surface rheology is different: although both QBSs form predominantly elastic layers, for “Sigma” the surface storage modulus, εr = 103 mN m−1, while for “Supersap” εr = 73 mN m−1 at 10−3 mol l−1 (i.e., around their cmc). The two saponin products exhibit also different ionic character, as proven by the acid-base titration of their aqueous solutions: QBS from Sigma is an ionic surfactant, while the “Supersap” from Desert King is a non-ionic one.