Short- and mid-term adsorption behaviour of Quillaja Bark Saponin and its mixtures with lysozyme

The short- and mid-term adsorption kinetics of Quillaja Bark Saponin (QBS) biosurfactant at water–air interface was studied using a maximum bubble pressure and drop shape analysis techniques, respectively. The low values of diffusion coefficients at initial and close-to equilibrium stages of adsorption suggest that adsorption of QBS is mixed-diffusion barrier controlled. Increasing the ionic strength or decreasing pH helps to screen the negative charge of the glucuronic acid moiety of QBS and results in enhanced surface activity of QBS. The surface tension isotherms at pH 2.6 (HCl solution, unbuffered), 6.2 (pure water, unbuffered) and 7.4 (phosphate buffer, I = 0.1 M) were fitted using the Frumkin model. The influence of a model protein (hen egg lysozyme, LYS) on adsorption of QBS on short- and mid-term timescales was studied at fixed LYS concentration of 10−5 M and varying QBS concentration. The synergistic effects in QBS adsorption were observed due to formation of LYS–QBS complex, whose surface activity increases at low QBS concentrations. The complex may be formed either in bulk solution, prior to adsorption, or directly at the surface, through co-adsorption of the slower diffusing component on the pre-adsorbed layer of the other component. At high concentrations of QBS, the latter dominates the adsorption process, depleting the surface from LYS and LYS–QBS complex. The synergistic effects found in adsorption kinetics of LYS–QBS mixtures were assessed in foamability tests using a modified Bikerman method.

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