Effects of Fe (II) on stability of aqueous foam prepared by hydrolyzed rice protein in the presence of oil

Abstract

Fluorinated foams have outstanding stability in contact with oil, but the use of fluorocarbon surfactants has been restricted due to toxicity and environmental persistence. The hydrolyzed protein may be a potential non-fluoro replacement for fluorocarbon surfactants to stabilize aqueous foams. In this work, the effects of Fe (II) on the stability of aqueous foam prepared by hydrolyzed rice protein (HRP) in the presence of oil were systematically studied. Three model foams made from surfactants were used for comparison in terms of their stability. Multiscale approaches including small-angle X-ray scattering (SAXS) technique, surface tension measurements and microscopic technique were employed to investigate the interaction between Fe2+ and HRP as well as the foam stabilization mechanism. The results show that the foam stabilized by HRP solely is unstable in contact with oil compared to the fluorinated foams but the stability of HRP foam can be remarkably enhanced after adding FeSO4. The addition of FeSO4 allows the formation of HRP-Fe2+ complexes in solution to stabilize foam and SAXS results reveal the interaction between Fe2+ and HRP is well described by the Beaucage model. The improved foam stability is related to the adsorption of HRP-Fe2+ complexes at the air-liquid interface, which protects the bubbles against coalescence and rupture, rather than lowering the surface tension. The proposed formulation can be a potential fluoro-replacement, which provides better foam stability than silicone and glucoside surfactants.