Co-adsorption of peptide amphiphile V₆K and conventional surfactants SDS and C₁₂TAB at the solid/water interface

Recent research has reported many attractive benefits from short peptide amphiphiles. A practical route for them to enter the real world of applications is through formulation with conventional surfactants. This study reports the co-adsorption of the surfactant-like peptide, V₆K, with conventional anionic and cationic surfactants at the solid/water interface. The time-dependant adsorption behaviour was examined using spectroscopic ellipsometry whilst adsorbed layer composition and structural distribution of the components were investigated by neutron reflection with the use of hydrogen/deuterium labelling of the surfactant molecules. Both binary (surfactant/peptide mixtures) and sequential (peptide followed by surfactant) adsorption have been studied. It was found that at the hydrophilic SiO₂/water interface, the peptide was able to form a stable, flat, defected bilayer structure however both the structure and adsorbed amount were highly dependent on the initial peptide concentration. This consequently affected surfactant adsorption. In the presence of a pre-adsorbed peptide layer anionic sodium dodecyl sulfate (SDS) could readily co-adsorb at the interface; however, cationic dodecyl trimethyl ammonium bromide (C₁₂TAB) could not co-adsorb due to the same charge character. However on a trimethoxy octyl silane (C₈) coated hydrophobic surface, V₆K formed a monolayer, and subsequent exposure to cationic and anionic surfactants both led to some co-adsorption at the interface. In binary surfactant/peptide mixtures, it was found that adsorption was dependent on the molar ratio of the surfactant and peptide. For SDS mixtures below molar unity and concentrations below CMC for C₁₂TAB, V₆K was able to dominate adsorption at the interface. Above molar unity, no adsorption was detected for SDS/V₆K mixtures. In contrast, C₁₂TAB gradually replaced the peptide and became dominant at the interface. These results thus elucidate the adsorption behaviour of V₆K, which was found to dominate interfacial adsorption but its exact adsorbed amount and distribution were affected by interfacial hydrophobicity and interactions with conventional surfactants.