Influence of salt addition on the surface and foaming properties of silk fibroin

Adsorption, Surface Viscoelasticity, and Foaming Properties of Silk Fibroin at the Air/Water Interface

“…The addition of salt effectively reduces the electrostatic repulsion between SF molecules and, thus, the energy barrier to SF adsorption. As a consequence, both σ and t ind decrease with increasing C salt and ion valence [36]. Obviously, the surface activity of SF is remarkably promoted in the presence of salt, and salt addition not only accelerates the diffusion but also enhances the interfacial adsorption of SF molecules.…”

“…Aqueous SF solutions are typically prepared from fresh domestic Bombyx mori cocoon shells by sericin extraction, cocoon fiber dissolution, and solution dialysis. They are then diluted with the solutions at different solution conditions to the desired concentration (C SF ) [34][35][36][37]. The pI of SF, at which the zeta potential vanishes, was determined to be 4.2 [7].…”

“…At pH 7, the smaller aggregate size and higher molecular flexibility make SF reorganize and pack at the interface more efficiently than at pH 3. When salt is added, δ e becomes thicker and increases with the salt concentration (C salt ) and the ion valence from NaCl to NdCl 3 [36]. SF in aqueous solution is negatively charged at neutral pH, and the counter-ion screening effect of salt significantly reduces the electrostatic repulsion between SF molecules, thus causing a gradual molecular aggregation in the bulk and at the interface.…”

“…At sufficiently low C SF and short t ad , changes in the adsorbed amounts usually do not directly lead to changes in σ, and this time range of negligible σ changes is called induction period (t ind ) [44]. When C SF exceeds 0.002 mg/mL, σ begins to decrease immediately until the equilibrium surface tension (σ eq ) is reached [34][35][36]. The initial rapid decrease in σ is related to the adsorption of SF, and the following slow change of σ is more related to the structure formation of adsorption layers.…”

“…The time for σ to reach equilibrium is around 6 h for SF [34], much longer than that for β-LG [5], gliadin nanoparticles (GNPs) [8], or amaranth protein isolates [4] at comparable bulk concentrations, indicating that SF needs more time to complete the structural reorganization into gelled β-sheets during the formation of water-insoluble films [45]. SF shows a lower σ eq and a much shorter t ind [35,36] than ovalbumin (OVA) with its more compact and rigid structure [10,46], but a higher σ eq than the highly hydrophobic bovine serum albumin (BSA) and the more deformable wheat protein fractions [46]. Obviously molecular size, structure, and hydrophobicity of proteins significantly influence their adsorption characteristics at interfaces.…”