F.E.S.S. · Foams and Emulsions Stabilized by Saponins

Foams and Emulsions Stabilized by Saponins

In the current project we studied the surface properties of saponins and their ability to stabilize foams. Saponins are a class of natural compounds found in more than 500 plant species. They consist of hydrophobic head group, called aglycone, with one or several hydrophilic oligosaccharide (sugar) chains connected to the aglycone via glycoside bonds. The term “saponin” includes a great variety of compounds, differing both in structure and in sugar composition. The saponins are classified on the basis of: (i) The type of aglycone (triterpenoid or steroid), and (ii) The number of sugar chains, attached to it. The most common saponins have two sugar chains (bidesmosidic type), some have one sugar chain (monodesmosidic type) and in rare cases – three. Triterpenoid saponins are more proliferated in nature than steroid saponins. They have a number of positive biological effects (anti-bacterial, anti-microbial, cholesterol lowering, anti-oxidant, anti-inflammatory etc.). Due to the amphiphilic structure of their molecules, saponins have strong surface activity. They adsorb on the oil-water or air-water interface and form adsorption layers, which prevent the oil droplets or bubbles from coalescing. In other words they act as foam and emulsion stabilizers. A number of different products in everyday life represent foams or emulsions: food, pharmaceuticals, cosmetics etc. Saponins are an excellent choice for surfactants to be used in such products because they can perform a dual role: biological and surface-active agents. The chemical structure, isolation and biological effects of saponins have been studied very well in the literature. However their surface activity is still not well understood. It is known that the properties of the surfactant adsorption layer affect the behavior of the respective bulk system (foam or emulsion). The aim of the current project is to characterize in detail the surface properties of a variety of saponins both on the air/water and oil/water interface. We also study the kinetics of Ostwald ripening in concentrated foams, produced with saponin solutions. Based on the results we seek to derive 2 types of correlations: 1. Relation between the chemical structure of the saponin and the properties of the surfactants layer (structure, mechanical properties). 2. Relation between the surface properties of the surfactant layers and the behavior of the bulk system. We studied 11 different saponin extracts (8 triterpenoids and 3 steroids). We characterized in detail the mechanical properties of the adsorption layers on the air-water interface. It was established that several saponins (Tea Saponin, Quillaja saponin, Escin, Berry Saponin, Ginsenosides) form layers with extremely high mechanical strength. The strength of these layers was characterized via the surface modulus, G. The value of G for these systems was considerably higher than the value for the majority of all other known surface-active agents (proteins, polymers, particles, low-molecular surfactants). It was established that triterpenoids with a single sugar chain have especially high G, followed by triterpenoids with 2 sugar chains. In contrast steroids had negligible G. The high mechanical strength of the layers was attributed to strong H-bonds between the molecules in the layer. Molecules with certain structure promote the formation of stronger and/more H-bonds, which results in higher overall mechanical strength of the layer. We also studied the properties of the layers on the oil-water interface. For most of the saponins similar characteristic behavior was observed as on the air-water interface. We observed that in general the presence of the oil phase decreases significantly the surface modulus. This was explained with a different structure of the saponin layers on the oil-water interface, leading to different strength of the interaction between the molecules. The mechanical properties of the saponin layers were characterized with 2 different instruments. In order to obtain a rigorous and self-consistent physical description of the studied systems we compared results obtained with: (1) Different instruments and the same rheological test; (2) The same instrument but different rheological test. Such study is important not only for the characterization of the concrete systems. It proposes a more rigorous approach to the measurement and analysis of mechanical properties of adsorption layers, compared to the protocols which are usually employed. The adsorption layers were studied with neutron scattering techniques (ISIS neutron source, Rutherford Appelton Laboratory, UK). In this experiment the layers are bombarded with a beam of neutrons and the scattering of the neutrons is analyzed. The analysis yields information on the structure if the adsorption layers as: thickness of the layer, number of molecules per unit area of the layer. The obtained results were consistent with data obtained via different experimental approach (surface tension isotherms). We studied the kinetics of Ostwald ripening (OR) in concentrated foams stabilized with saponins. OR is a process of molecular transport of gas from smaller to bigger bubbles in the foam, which leads to overall increase in the mean bubble size. The mean bubble size is an important parameter which determines properties of the foam as mechanical properties, stability to coalescence etc. It is rate important to decrease the OR as much as possible, and one of the ways to do that is by employing an appropriate surface active agent. The rate of Ostwald ripening varied in a wide range depending on the particular saponin which was used. However there was no clear correlation between the rate of OR on one hand and the chemical structure of the saponin or the mechanical properties of the saponin layers on the other. Saponins as escin and Tea Saponin decreased significantly the rate of OR. Based on the results obtained in this project, we prepared 7 articles to be published in peer-reviewed journals. The main achievement of the project is that provides comprehensive information on the surface properties of a variety of saponins. We built a self-consistent description of the studied systems by employing a number of experimental methods which complement each other. We established several important relations between the molecular structure of the saponin molecules and the properties of the adsorption layers. These results are a foundation for the further study of saponins and their ability to stabilize foams and emulsions. A deep and detailed understanding of the properties of saponins as surface-active agents is essential for their effective application in commercial products. The results obtained in the current project are an important first step towards such an understanding.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз