Solvent property of air-water interface was evaluated based on the fluorescence spectra of 1,2'-dinaphthylamine in water containing ultrafine bubbles (average diameter: 103 nm, standard deviation: 38 nm). Among naphthylamine derivatives whose fluorescence spectra were responsive to microscopic hydrophobicity, 1,2'-dinaphthylamine (DN) was selected because its wavelength of the maximum emission (λmax) was significantly dependent on the concentration and microenvironment of the ultrafine bubble. The λmax value of DN in water was 486 nm, while it shifted to shorter wavelength (408 nm) in the presence of 1.09 × 109 mL-1 of ultrafine bubbles. The shift of λmax value indicates that DN adsorbs on the surfaces of ultrafine bubbles and exists in hydrophobic region rather than in bulk water. By comparing with the λmax values in different solvents, the surface of ultrafine bubble was found to have similar solvent property to ethyl ether or ethyl acetate that are widely used as extracting solvents for hydrophobic organic compounds.
Keywords: 1,2’-Dinaphthylamine; Air–water interface; Fluorescent spectra; Solvent property; Ultrafine bubbles.
© 2023. The Author(s), under exclusive licence to The Japan Society for Analytical Chemistry.