Surface free energy tuning of supported mixed lipid layers†
Supported lipid layers are commonly used as model systems for biological membranes with high potential for diverse (bio)technological applications including the development of novel sensors. The aim of this study was to investigate the influence of mixing ratio and subphase used in the monolayer assembly on the surface free energy (SFE) of supported palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and oleic acid triglyceride (triolein) (TO) mixed lipid monolayers on a mica substrate. The supported layers were formed by Langmuir–Blodgett transfer of mixed monolayers assembled on a subphase of ultra-pure water and electrolyte (0.55 M NaCl, pH 8.1). AFM and fractal/lacunarity analysis were used to elucidate their relevant topographical features while the SFE and its polar and dispersive component were determined using contact angle measurements. The results showed that the monolayers formed on an electrolyte subphase are more stable than those on a water subphase, while their homogeneity and topographical features depend on the mixing ratio. The SFE of a mixed layer depends on the mixing ratio and the subphase type used in the self-assembly of the Langmuir monolayer, but is also influenced by the saturation/unsaturation of the hydrophobic tail chain. Thus, changing the subphase and the POPC/TO mixing ratio allows for change of the supported layer's wetting properties from hydrophilic to strongly hydrophobic. The results of this study should contribute to better understanding of the SFE of supported mixed lipid films and allow the tailoring of surfaces with targeted properties.