Keratin–rhamnolipids and keratin–sodium dodecyl sulfate interactions at the air/water interface

Keratin–surfactant interactions at the air/solution interface may be indicative for skin irritation by surfactants. In this study pure keratin was extracted from chicken feathers as described by Schrooyen at al. [1] and [2] and the adsorption properties of biologically produced rhamnolipids (RL) and sodium dodecyl sulfate (SDS) mixed with keratin molecules were investigated by surface tension measurements. Keratin was added in three specific amounts to RL and SDS aqueous solutions of varying concentrations, respectively, and keratin–RL and keratin–SDS interactions were investigated at pH 6.2 and 5.0.The surface tension measurements showed that surfactant–keratin complexes were formed in the solutions at pH 6.2. At low RL concentrations the added amount of keratin decreased the surface tension as a result of the hydrophobic keratin–RL complexes formed, which have covered the air/solution interface. Beginning from moderate RL concentrations, less surface active keratin–RL complexes were formed due to hydrophilization of the former complexes and they were removed from the interface to the bulk phase as a result of the competitive adsorption mechanism and pure RL molecules covered the interface gradually. At low SDS concentrations surface tension decrease was also observed by the covering of the air/solution interface with keratin–SDS complexes. At moderate SDS concentrations the keratin–SDS complexes exhibited a more surface active feature. Unlike the keratin–RL complexes, they have remained at the interface until the higher SDS concentrations and keratin ratios. However, at the minimum surface tension value of the pure SDS at its CMC value, the surface tension of the keratin–SDS mixture reached the value of pure SDS solution and the air/solution interface has been covered again only with pure SDS monomers. The keratin–surfactant solutions prepared at pH 5 showed lower surface tension values compared to that measured at pH 6.2 and the surface tension behavior of RL and SDS resembled each other. Above a certain surfactant concentration the addition of keratin to surfactant solutions rendered the surface less surface active compared to their pure surfactant solutions at pH 5.0. The consideration of the surface tension behavior of two anionic surfactants has shown that the RL–keratin interactions are milder than SDS–keratin interactions at pH 6.2 and 5.0.