Structure and surface characterization of co-adsorbed layer of oleic acid and octadecylamine on detonation nanodiamond

• Dispersibility of nanodiamond (DND) in apolar solvents or matrixes can be optimized by alkyl surfactants. The acid–base interaction of alkyl dispersants can be observed in apolar systems. The synergy effect of surfactants induces crystallization and results in an orderly assembled co-adsorbed layer on DND. Alkyl-functionalization featured with acid-base interaction can improve modification efficiency of DND additives.

Aiming to form an alkyl-functionalized surface and to realize the particle dispersibility in apolar solvents, oleic acid (OA) and octadecylamine (ODA) were introduced to modify detonation nanodiamond (DND). Compared with their single addition, the combined use of OA and ODA resulted in better particle dispersion and suspension stability. The co-adsorbed layer containing both OA and ODA is of a crystal structure, while the single-added OA and ODA form only amorphous structures on DND. The acid–base interaction originated from the electrostatic attraction between the amino groups on ODA and the carboxyl groups on OA may contribute to the assembly of both surfactants. With the significant increase of adsorption concentration and layer thickness, the steric repulsion of DND particles can be strengthened. By taking this approach, the alkyl-functionalization effect and the dispersion of DND particles in organic solvents or in polymer composites can be improved.

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