Adsorption Behavior of Hydrophobin and Hydrophobin/Surfactant Mixtures at the Air-Water Interface

The adsorption of the surface-active protein hydrophobin, HFBII, and the competitive adsorption of HFBII with the cationic, anionic, and nonionic surfactants hexadecyltrimethylammonium bromide, CTAB, sodium dodecyl sulfate, SDS, and hexaethylene monododecyl ether, C12E6, has been studied using neutron reflectivity, NR. HFBII adsorbs strongly at the air-water interface to form a dense monolayer similar to 30 angstrom thick, with a mean area per molecule of similar to 400 angstrom(2) and a volume fraction of similar to 0.7, for concentrations greater than 0.01 g/L, and the adsorption is independent of the solution pH. In competition with the conventional surfactants CTAB, SDS, and C12E6 at pH 7, the HFBII adsorption totally dominates the surface for surfactant concentrations less than the critical micellar concentration, cmc. Above the cmc of the conventional surfactants, HFBII is displaced by the surfactant (CTAB, SDS, or C12E6). For C12E6 this displacement is only partial, and some HFBII remains at the surface for concentrations greater than the C12E6 cmc. At low pH (pH 3) the patterns of adsorption for HFBII/SDS and HFBII/C12E6 are different. At concentrations just below the surfactant cmc there is now mixed HFBII/surfactant adsorption for both SDS and C12E6. For the HFBII/SDS mixture the structure of the adsorbed layer is more complex in the region immediately below the SDS cmc, resulting from the HFBII/SDS complex formation at the interface.