Effect of surfactants on the interfacial viscoelasticity and stability of silk fibroin at different oil–water interfaces

Abstract BACKGROUND The biocompatible amphiphilic silk fibroin, extracted from domesticated silkworms, can adsorb at the oil–water interface and form elastic interfacial layers. In this study, three surfactants (cationic cetyltrimethylammonium bromide, the nonionic polyoxyethylene sorbitan monolaurate, and the anionic sodium dodecyl sulfate) were selected to investigate, through interfacial shear rheology, the influences of surfactants on the interfacial viscoelasticity and stability of silk fibroin at the interfaces between water and two different oils. RESULTS The presence of surfactant prolongs the equilibration time and enhances the interfacial elastic modulus and toughness of the interfacial silk fibroin layers, especially at the nonpolar dodecane–water interface. However, when the surfactant exceeds a critical concentration, the shear modulus and stability of interfacial silk fibroin layers begin to decrease due to the competitive adsorption of surfactant molecules and the weakening of the protein network. Owing to electrostatic interactions, the ionic surfactants cetyltrimethylammonium bromide and sodium dodecyl sulfate can form more hydrophobic complexes with silk fibroin, which results in higher shear moduli than for silk fibroin and silk fibroin–polyoxyethylene sorbitan monolaurate mixture. CONCLUSION Both the surfactant type and oil polarity play important roles in the adsorption, shear viscoelasticity, and stability of silk fibroin at the oil–water interface. Enhanced interactions between a silk fibroin–surfactant mixture and the oil phase delay the equilibration of the adsorption layers but strengthen the stability of interfacial layers. © 2023 Society of Chemical Industry.