Tailoring structure and properties of long-lived emulsion foams stabilized by a natural saponin glycyrrhizic acid: Role of oil phase.

Novel supramolecular nanofibrils assembled from food-grade saponin glycyrrhizic acid (GA) are effective building blocks to make complex multiphase systems, e.g., emulsion foams. In this work, the effects of different oil phases (castor oil, sunflower oil, dodecane, and limonene) on the formation, stability and structural properties of long-lived emulsion foams prepared by GA nanofibrils (GNs) were investigated. The obtained results showed that soft-solid emulsion foams (4 wt% GNs) can be fabricated, independently of oil phase, and their structural properties, viscoelasticity, and tribological properties can be well tuned by oil phase polarity. Compared to the GNs aqueous foams, the presence of jammed emulsion droplets in the liquid channels and at the surfaces of bubbles can provide a higher bubble stability for emulsion foams. For more polar oil phase (castor oil), GNs showed a higher affinity to the oil-water interface with a lower interfacial tension, thus forming smaller oil droplets and bubbles, which leads to the higher mechanical strength, denser network microstructures, and lower friction coefficients of emulsion foams. However, the limonene foam exhibited weak storage stability and rheological properties, as well as the relatively low lubrication, which may be related to the formation of oil droplet aggregates and clusters induced by the volatility of limonene. GN-based emulsion foams are thermoresponsive, independently of oils, and the temperature-switchable process for the destabilization and regeneration of foams can be controlled and repeated. These emulsion foams based on natural saponin nanofibrils with tunable properties have potential sustainable applications in foods, pharmaceuticals, and personal care products.