Adsorption of proteins and surfactants at air /water and solid /water interfaces
The competitive and sequential adsorption of bovine serum albumin (BSA) and sodium myristate (SM) at the air/water and solid/water interfaces are probed by tensiometry, ellipsometry, and infrared (IR) spectroscopy at 25°C. For BSA at the air/water interface, the layer thicknesses increase with concentration. The steady-state surface tensions are similar, indicating that the BSA molecules may orient differently at the higher concentrations. Results for the competitive adsorption of BSA/SM mixtures show that SM is the main component at the interface for all concentrations. For mixtures with BSA concentrations of 1 wt%, thicker layers are observed, because of the presence of myristic acid (MA) resulting from the protonation of SM at lower pHs. SM is shown to be capable of expelling a preadsorbed layer of BSA. The results have implications in controlling lung surfactant inhibition by serum proteins. ^ At the silicon oxide/water interface, in-situ IR results show that adsorbed densities of BSA quickly reach steady values for concentrations between 0.01 to 1 wt%. The layer thicknesses for all concentrations are less than 4 nm, the smallest dimension of the protein, which indicates monolayer adsorption with possible unfolding. Conformational changes are implied from differences in the spectrum of BSA in solution and in the adsorbed layer. Ex-situ ellipsometry results agree with those determined from in-situ IR. The adsorbed densities of BSA are lower on hydrophobic DPPC monolayers on silicon oxide than on silicon oxide alone. SM can remove most of the adsorbed BSA from the surface in sequential adsorption. BSA adsorbs onto layers of adsorbed SM without displacing them. For competitive adsorption of BSA/SM, small amounts of BSA adsorb quickly, with only SM adsorbing at longer times. SM then becomes protonated to MA at the interface. Upon replacement of the bulk solution with water, only BSA and SM remain, with no evidence of MA in the adsorbed layer. We conclude that the surface is composed mainly of SM with sparse patches of BSA present. The results have implications for designing chromatographic systems and for understanding and designing biomaterial surfaces for specific interactions with proteins, surfactants, or protein/surfactant mixtures. ^
Major Professor: Elias I. Franses, Purdue University.