Influence of surfactant charge and concentration on the surface and foaming properties of biocompatible silk fibroin

Abstract

As a kind of fibrous amphiphilic protein, the biocompatible silk fibroin (SF) extracted from domesticated silkworms can adsorb at the air/water interface, form thick interfacial layers, and stabilize foams that can be used to fabricate porous scaffolds and regenerate soft tissue. In this study, three surfactants, cationic cetyl trimethyl ammonium bromide (C16TAB), nonionic polyoxyethylene sorbitan monolaurate (Tween 20), and anionic sodium dodecyl sulphate (SDS) were utilized to investigate the effects of surfactant charge and concentration on the adsorption dynamics, interfacial viscoelasticity, and foaming properties of SF. SF is negatively charged at neutral pH in aqueous solution. All these surfactants tend to enhance the surface activity of SF, thus leading to faster interfacial adsorption and establishment of the adsorption equilibrium, thicker interfacial layers, and an increase in the foaming rate. Meanwhile, these surfactants also improve the toughness of interfacial SF networks, and then promote the foam stability. However, at high concentrations, the nonionic Tween 20 and anionic SDS reduce the interfacial elasticity and stability due to the emergence of surfactant domains that interrupt the interfacial SF networks, while the cationic C16TAB thickens the interfacial layers and significantly decreases the interfacial stability and foamability due to its counter-ion screening effect that promotes the aggregation of SF molecules.