Competitive adsorption of bovine serum albumin and n-dodecyl-β-d-maltoside in foam films

Abstract

The competitive adsorption of Bovine Serum Albumin (BSA) and n-dodecyl-β-d-maltoside (C12G2) on foam films surfaces formed from mixed solutions with a composition (BSA:C12G2=1:7.5; 1:50; 1:100), corresponding to weak and strong binding of C12G2 molecules to BSA molecule was experimentally investigated. In order to answer the question which one of the surfactants or both of them is/are adsorbed on the film surfaces data for: (i) the kinetics of surface tension of the single surfactants and their mixtures; (ii) the AFM images of single BSA surface layer and its mixed layers with C12G2; (iii) the viscosity in the foam film determined from its thinning; (iv) the kinetics behavior and equilibrium properties of foam films from single and mixed solutions at pH=4.9, 3.8; (v) the BSA concentration in the solution obtained by dissolving of a dried foam were obtained.Data for the surface activity of investigated solutions showed that in the mixed adsorption layers from solution with molar ratio BSA:C12G2=1:7.5 BSA molecules predominate, while from BSA:C12G2=1:50 and 1:100-C12G2 molecules. This conclusion is supported by AFM images. The lower viscosity calculated from the thinning of foam films stabilized by these mixtures than this from the thinning of films from individual BSA solution is explained by the less aggregation between albumin molecules due to their hydrophilization by bind C12G2 molecules. The stable foams produced from the mixtures BSA:C12G2=1:50 and 1:100 were dried to such an extent enough for black foam films formation in them. Quantitatively, it was found that the concentration of BSA in the solution obtained after dissolving of the dried foam is an order lower than the concentration of the solution drained from the foam during its drying. The thinning evolutions of the films from pure and mixed solutions, recorded at pH=4.9 showed that C12G2 films thin up to 90nm, while the thinning of films from the mixture (1:100) depends on the time of film interfaces aging: the films with fresh interfaces have a similar drainage as the pure C12G2 films, but they thin to the thickness of 67nm; the films with intermediate times of aging become unstable, or stable ones when black spots (mobile or frozen) are formed in them; the aged films thin fastest and they jump to NBF as the films from C12G2 solution but with pH=3.8 or with 10−1M NaBr. It means that the films electrostatic stabilization is screened which can be due to BSA adsorption. This suggestion was checked by an estimation based on the surface charge density of the following interfaces: air/water; air/C12G2 solution and air/BSA+C12G2 solution. It was found that the number of OH− ions expelled by BSA is one order smaller that those by C12G2. Nevertheless, it is a proof for the presence of BSA in the film interfaces, as well as for its role on the film thinning. The values of films equilibrium thickness, surface potential and drainage in the case when pH of the mixed solution is 3.8 indicate that the films are common ones. This finding demonstrates again that BSA molecules adsorb on the film interfaces. At pH=3.8 BSA molecule is charged positively that is why the film interfaces are recharged.