Interfacial and rheological properties of long-lived foams stabilized by rice proteins complexed to transition metal ions in the presence of alkyl polyglycoside

Hypothesis: Transition-metal coordination complexes are hopeful to make advanced structural materials, since the metal-coordination bonds, unlike typical covalent bonds, can regenerate after rupture, allowing for dynamic, tunable, and reversible mechanical characteristics. Integration of metal-coordinate crosslinking in foam material has rarely been reported.Experiments: We developed the hydrolyzed rice proteins (HRP) as the building block for amphiphilic transition-metal coordination complexes that could be used to make long-lived foams with high yield stress. Surface properties of the foaming solution were determined using equilibrium and dynamic ten-siometers. Structural information of aggregates in the foaming solution was detected by small-angle X-ray scattering (SAXS) and Cryo-Tem. Visualization of liquid flow in the interfacial liquid film was studied by the reflective optical interference technique. Rheological response of liquid foam was characterized by a rheometer with amplitude-sweep and frequency-sweep modes.Findings: In the presence of transition metal ions, HRP formed a mechanically strong rigid film. In the absence of transition metal ions or the addition of alkyl polyglycoside (APG), HRP was desorbed to pro-duce a mobile film with a detergent state. The two interfacial states could be actively switched based on facile changes in bulk solution composition (metal ions or alkyl glycoside or chelating agent), and the switching between the two states led to the formation of extremely stable foam with high yield stress or the collapse of foam with a significant decrease in yield limit. The transition-metal coordination com-plexes adsorbed on the surface of the liquid film could increase the elastic modulus of liquid foam by more than an order of magnitude without increasing the viscosity of the foaming solution. We further revealed the origin of foam stability/instability and used other well-characterized proteins to prepare transition-metal coordination complexes to make long-lived foams. The cases described in this work illustrate the universal nature of the strategy, which in principle can be extended to many types of protein.(c) 2022 Elsevier Inc. All rights reserved.

Automated summary

Transition-metal coordination complexes have the potential to be used in advanced structural materials due to their ability to regenerate after rupture. The integration of these complexes in foam materials has been rarely reported. In this study, hydrolyzed rice proteins (HRP) were used to create long-lived foams with high yield stress, and the presence of transition metal ions resulted in a mechanically strong film.