CTAB-modified chitosan nanocoatings: Increasing the cationic surface character by self-assembly

Cetyltrimethylammonium bromide (CTAB)-containing chitosan nanolayers were deposited onto glass substrates via dip-coating from aqueous precursor solutions with 1% chitosan and 0-3.0 mM CTAB content. The thickness of the coatings was ca. 200-300 nm, according to UV-visible spectroscopy. While lower surfactant concentrations (0.25-1.0 mM) resulted in homogeneous, semicrystalline coatings, AFM, X-ray diffraction and solid-state NMR evidenced phase separation and the formation of CTAB crystallites in the case of the samples made from precursor solutions with over 1.0 mM CTAB concentration. X-ray photoelectron spectroscopy confirmed a significant interfacial accumulation of CTAB on the coating-air interface (up to eightfold increase in quaternary N), while contact angle measurements indicated an increase in hydrophilicity, with advancing water contact angles decreasing from 70° to ca. 30°, with increasing Lewis-basic surface free energy component. Crucially, surface zeta potential measurements demonstrated an increase in positive surface charge from +22 mV to over +30 mV, and turbidimetric and colorimetric assays confirmed the antibacterial effect (over 80% inhibition) of the coatings against B. subtilis. These findings suggest the formation of a unique molecular structure, where the CTAB was accumulated on the surface of the nanolayer with outward-oriented cationic head groups, providing a robust strategy for developing enhanced antimicrobial coatings.