Role of interfacial elasticity for the rheological properties of saponin-stabilized emulsions

TL;DR

This study investigates how the elastic properties of saponin-stabilized interfaces influence the rheological behavior of emulsions, revealing that interfacial elasticity significantly affects shear elasticity but not viscous stress.

Contribution

It provides new insights into the relationship between interfacial dilatational elasticity and bulk emulsion rheology using natural saponins as model surfactants.

Findings

Interfacial elasticity strongly impacts emulsion shear elasticity.

Saponins form solid shells on certain oil-water interfaces.

Interfacial elasticity has limited effect on viscous stress.

Abstract

Hypothesis Saponins are natural surfactants which can provide highly viscoelastic interfaces. This property can be used to quantify precisely the effect of interfacial dilatational elasticity on the various rheological properties of bulk emulsions. Experiments We measured the interfacial dilatational elasticity of adsorption layers from four saponins (Quillaja, Escin, Berry, Tea) adsorbed on hexadecane-water and sunflower oil-water interfaces. In parallel, the rheological properties under steady and oscillatory shear deformations were measured for bulk emulsions, stabilized by the same saponins (oil volume fraction between 75 and 85 %). Findings Quillaja saponin and Berry saponin formed solid adsorption layers (shells) on the SFO-water interface. As a consequence, the respective emulsions contained non-spherical drops. For the other systems, the interfacial elasticities varied between 2 mN/m and 500 mN/m. We found that this interfacial elasticity has very significant impact on the emulsion shear elasticity, moderate effect on the dynamic yield stress, and no effect on the viscous stress of the respective steadily sheared emulsions. The last conclusion is not trivial, because the dilatational surface elasticity is known to have strong impact on the viscous stress of steadily sheared foams. Mechanistic explanations of all observed effects are described.