Protein−Poly(silicic) Acid Interactions at The Air/Solution Interface

• Published in: The journal of physical chemistry. B 2005-11, Vol.109 (44), p.20878-20886
• Main Authors: Henderson, Mark J, Perriman, Adam W, Robson-Marsden, Hana, White, John W
• Format: Article
• Language: English
• Subjects: Air; Caseins; Colloids; Hydrogen-Ion Concentration; Polymers; Proteins - chemistry; Silicic Acid - chemistry; Solutions; Surface Properties
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp051908k

The structure of the interface generated by a spread layer of β-casein on an aqueous colloidal poly(silicic) acid subphase is described. The results are compared with data for the protein alone spread at the air/water interface and the silicate solution. Films develop at the air−solution interface and a strong pH dependence of the interaction causing this is demonstrated. Reflectometry with X-rays and neutrons was used to probe the interaction as a function of subphase pH and film compression. Film thickness, τ/Å, scattering length density, ρ/Å-2, water volume fraction, φw, and surface coverage, Γ/mg m-2, were used to quantify the interfacial structure. Where possible, the X-ray and neutron data sets were co-refined enabling φw to be evaluated without assumption regarding the protein density. At pH 5−7, strong protein−silicate interaction occurred, the interface comprising three regions:  a discrete protein upper layer on top of a 15 ± 2 Å layer of silicated material followed by a diffuse layer that extended into the subphase.