Surface and volumetric properties of a saponin-rich aqueous soapwort extract

Saponin-rich plant extracts are increasingly being used in many fields of industry. They are multi-component mixtures of a large number of biomolecules whose composition depends on the type of plant and methods of their preparation. In this research, a saponin-rich extract (SE) was obtained by boiling the Saponaria officinalis L. (soapwort) roots in water. It was examined by measuring the surface tension, density, conductivity and pH in a wide range of concentrations. Based on the surface tension, density and conductivity isotherms, the critical micelle concentration values (CMC) were determined. Additionally, the partial volume of model saponin and its density were calculated. Since the hydrophilic groups of the major surface-active components of SE are composed of sugar units, saccharose was used as a simple headgroup (glycone) model, while hederagenin was chosen as a simple model of the saponin tail (aglycone). To better understand the adsorption and micellar properties of the soapwort extract, the surface tension components of SE, hederagenin and saccharose layers deposited on a polytetrafluoroethylene (PTFE) surface were determined from the contact angle values of the model liquids (water, formamide, glycerol and diiodomethane). Using these data, SE was thermodynamically analyzed at the solution-air interface and in the bulk. Both the excess Gibbs concentration (Γ) at the solution-air interface and CMC for SE described in the present contribution differ from those described in the literature, yet still remain within the same orders of magnitudes. The free energy of adsorption and micellization for the saponin-rich soapwort extract are close to those of Triton TX-165.