Surface Rheology of Saponin Adsorption Layers

Abstract

Extracts of the Quillaja saponaria tree contain natural surfactant molecules called saponins that very efficiently stabilize foams and emulsions. Therefore, such extracts are widely used in several technologies. In addition, saponins have demonstrated nontrivial bioactivity and are currently used as essential ingredients in vaccines, food supplements, and other health products. Previous preliminary studies showed that saponins have some peculiar surface properties, such as a very high surface modulus, that may have an important impact on the mechanisms of foam and emulsion stabilization. Here we present a detailed characterization of the main surface properties of highly purified aqueous extracts of Quillaja saponins. Surface tension isotherms showed that the purified Quillaja saponins behave as nonionic surfactants with a relatively high cmc (0.025 wt %). The saponin adsorption isotherm is described well by the Volmer equation, with an area per molecule of close to 1 nm(2). By comparing this area to the molecular dimensions, we deduce that the hydrophobic triterpenoid rings of the saponin molecules lie parallel to the air-water interface, with the hydrophilic glucoside tails protruding into the aqueous phase. Upon small deformation, the saponin adsorption layers exhibit a very high surface dilatational elasticity (280 ± 30 mN/m), a much lower shear elasticity (26 ± 15 mN/m), and a negligible true dilatational surface viscosity. The measured dilatational elasticity is in very good agreement with the theoretical predictions of the Volmer adsorption model (260 mN/m). The measured characteristic adsorption time of the saponin molecules is 4 to 5 orders of magnitude longer than that predicted theoretically for diffusion-controlled adsorption, which means that the saponin adsorption is barrier-controlled around and above the cmc. The perturbed saponin layers relax toward equilibrium in a complex manner, with several relaxation times, the longest of them being around 3 min. Molecular interpretations of the observed trends are proposed when possible. Surprisingly, in the course of our study we found experimentally that the drop shape analysis method (DSA method) shows a systematically lower surface elasticity, in comparison with the other two methods used: Langmuir trough and capillary pressure tensiometry with spherical drops. The possible reasons for the observed discrepancy are discussed, and the final conclusion is that the DSA method has specific problems and may give incorrect results when applied to study the dynamic properties of systems with high surface elasticity, such as adsorption layers of saponins, lipids, fatty acids, solid particles, and some proteins. The last conclusion is particularly important because the DSA method recently became the preferred method for the characterization of fluid interfaces because of its convenience.

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“…They also decrees blood cholesterol level and may be used as adjuvant in vaccines. [8][9][10][11][12][13][14] In addition, saponins are used in preparation of soaps, detergents, fire extinguishers, shampoos, beer and cosmetic. 15 Many saponins exhibit haemolytic activity, have a bitter taste and are toxic to fish.…”

“…Terpenoid and steroid saponins are usually found in dicotyledonous and monocotyledonous plants, respectively. 4,9,17 Saponins with a glucuronic acid moiety at C-3 of oleanolic acid are found in the flowers, while saponins with a glucose moiety at the same position are found in the roots 18 . Due to their amphiphilic nature, saponin molecules form micelles in aqueous solutions.…”

“…The size, shape, and structure of the saponin micelles depend on their plant origin, pH, temperature and the presence of electrolyte in the solution. 9 Saponins are a large family of structurally-related compounds of steroid or triterpenoid aglycone (sapogenin) linked to one or more oligosaccharide moieties by glycosidic linkage. The aglycone, or sapogenin, may contain one or more unsaturated C-C bonds.…”

Green synthesis and characterization of cuprous oxide nanoparticles in presence of a bio-surfactant

“…Among the various stabilizing agents, natural surfactant molecules, such as saponins, which are found in plant extracts of both dicotyledonous and monocotyledonous plants are used as efficient stabilizing agents not only due to their amphiphilic nature but also due to rich biological activity and physiochemical properties [10]. In addition, these natural surface active agents are found to be vital components in vaccines, food supplements, soft drinks, anti-aging creams, and cosmetics [10].…”

“…In addition, these natural surface active agents are found to be vital components in vaccines, food supplements, soft drinks, anti-aging creams, and cosmetics [10]. Saponin in food decreases the cholesterol level in blood and lowers the risk of heart diseases.…”