Alkyl group dependence of the surface-induced assembly of nonionic disaccharide surfactants

N-Alkylmaltonamide nonionic diblock surfactants of varying hydrophobic segment lengths were synthesized, and their surface active properties on highly oriented pyrolytic graphite were characterized by atomic force microscopy (AFM). The N-alkylmaltonamide surfactants are composed of maltose, a hydrophilic disaccharide, with amide-linked alkyl groups of varying length, from octyl to octadecyl. All the surfactants readily adsorb uniformly onto the hydrophobic graphite surface from solution. Surfactants with alkyl lengths of 10 or more carbons exhibit epitaxial adsorption on graphite, forming ordered hemicylinders with a diameter that increases with increasing surfactant length. The solution concentration necessary for inducing surface ordering decreases with increasing alkyl chain length. N-Octadecylmaltonamide, which is insoluble in water, adsorbed from methanol solution without ordering and upon solvent replacement with water, assembled into hemicylinders. Once formed, the surfactant structures appeared stable in pure water and under high scanning forces. The effects of alkyl chain length on surface-induced assembly and molecular packing are discussed from the viewpoint of surfactant composition and adsorbed configurations.