Interfacial adsorption of lipopeptidesurfactants at the silica/water interface studied by neutron reflection

Abstract

Lipopeptide surfactants are composed of amino acids and fatty acids and carry the inherent benefits of biocompatibility, degradability, functionality and easiness for structural design. In this work, the interfacial adsorption of a novel lipopeptide surfactant C₁₄K₁ has been examined by combining the measurements of spectroscopic ellipsometry (SE) and neutron reflection (NR). As for other model surfactants, the silicon oxide/water interface has been adopted to undertake the interfacial screening due to its readiness to facilitate the parallel measurements. SE revealed that the dynamic interfacial adsorption was characterized by an initial fast step within the first 2–3 min, followed by a slow relaxation step over the subsequent 30 min or so. Majority of the lipopeptide was found to adsorb with the fast initial step but the subsequent structural relaxation and adjustment led to the adsorption plateau. Interestingly, this pattern of dynamic adsorption is very different from amphiphilic peptides such as V₆K₁ and V₆K₂, but is similar to diblock copolymers bearing a hydrophilic MPC block and a pH responsive DEA block comprised of tertiary amine groups. Subsequent NR, in conjunction with deuterium labeling, revealed the formation of bilayer structure over a wide concentration and pH range, with the hydrophobic tails sandwiched in the middle and hydrophilic charged head groups projected outwards. The fully packed bilayer is some 40 Å thick and changes in pH did not affect layer thickness much but did alter the packing density due to the different electrostatic repulsion associated with the charge dissociation of the head groups. Whilst surface adsorption of C₁₄K₁ behaved in many aspects like nonionic C₁₂E₆ (surface tension, salt effect and bilayer packing), detailed structural determination revealed little intermixing between the heads and tails, an observation different from C₁₂E₆.