In Situ Observation of Domain Structure in Monolayers of Arachidic Acid/Fe₂O₃ Nanoparticle Complexes at the Air/Water Interface

γThe Langmuir layer behavior of arachidic acid/γ-Fe₂O₃ nanoparticle complexes was studied at the air/water interface. The subphase was an aqueous colloidal solution (hydrosol) of γ-Fe₂O₃ nanoparticles with an average diameter of 8.3 nm and with a standard deviation of (1.4 nm. Formation of the complex between arachidic acid and γ-Fe₂O₃ nanoparticles was studied with surface pressure-area isotherms, surface potential-area isotherms and Brewster angle microscopy. Increasing surface pressure resulted in a transition from wellseparated domains of the complex to well-compressed, nanoparticulate layers and, ultimately, to multiparticulate layers. The magnetic nanoparticles and layers of nanoparticles on solid substrates were studied with FTIR, Mössbauer spectroscopy and vibrating sample magnetometry (VSM). The γ-Fe₂O₃ nanoparticles and Langmuir-Blodgett films with the nanoparticles showed superparamagnetic properties. The stability of the γ-Fe₂O₃ nanoparticle hydrosol solution was studied by ζ potential measurements. Positively charged γ-Fe₂O₃ nanoparticles in aqueous hydrosol solution at pH 3.5-5 showed excellent long-term colloidal stability.