In Situ Observation of Domain Structure in Monolayers of Arachidic Acid/γ-Fe 2 O 3 Nanoparticle Complexes at the Air/Water Interface Young Soo Kang,* ,² Don Keun Lee, ² and Choong Sub Lee ‡ Department of Chemistry, Pukyong National UniVersity, Pusan 608-737, Korea Pieter Stroeve Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, California 95616 ReceiVed: December 10, 2001; In Final Form: June 24, 2002 The Langmuir layer behavior of arachidic acid/γ-Fe 2 O 3 nanoparticle complexes was studied at the air/water interface. The subphase was an aqueous colloidal solution (hydrosol) of γ-Fe 2 O 3 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-Fe 2 O 3 nanoparticles was studied with surface pressure-area isotherms, surface potential-area isotherms and Brewster angle microscopy. Increasing surface pressure resulted in a transition from well- separated 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, Mo ¨ssbauer spectroscopy and vibrating sample magnetometry (VSM). The γ-Fe 2 O 3 nanoparticles and Langmuir-Blodgett films with the nanoparticles showed superparamagnetic properties. The stability of the γ-Fe 2 O 3 nanoparticle hydrosol solution was studied by  potential measurements. Positively charged γ-Fe 2 O 3 nanoparticles in aqueous hydrosol solution at pH 3.5-5 showed excellent long-term colloidal stability. Introduction There has been growing interest in the synthesis of nanoscale inorganic materials, due to the novel properties exhibited by particles of very small dimensions. Materials such as CdS, TiO 2 , γ-Fe 2 O 3 , and Fe 3 O 4 have been widely studied. Cadmium sulfide and TiO 2 nanoparticles have excellent photocatalytic properties. Magnetic nanoparticles are of great interest for applications in information storage systems, catalysts, ferrofluids, and medical diagnostics. 1-6 Among magnetic particles, many studies have been devoted to nanoparticles of magnetite (Fe 3 O 4 ) and maghemite (γ-Fe 2 O 3 ). These iron oxides with sizes less than 10 nm have been synthesized in various matrix materials such as polymers, 7 micelles, 8 vesicles, 9 and lipid bilayer membranes. 10 Nanocomposites of organic materials and inorganic nanopar- ticles are of great promise as composites for utilization in high speed and high capacity optical and magnetic information storage media. 3,11 The focus of this study is to investigate the Langmuir behavior of nanocomposite monolayers, composed of arachidic acid and γ-Fe 2 O 3 nanoparticles at the air/water interface, with pressure-area isotherms and surface potential- area isotherms. The in situ domain structure and molecular orientation of the complex were measured with Brewster angle microscopy (BAM). 12 The characterization of Langmuir- Blodgett (L-B) nanocomposite films was carried out with FTIR and UV-vis spectroscopy. The magnetic properties of the magnetic nanoparticles and the Langmuir-Blodgett films were characterized with vibrating sample magnetometry (VSM) and Mo ¨ssbauer spectroscopy. Experimental Section Materials. The chemicals FeCl 2 ‚4H 2 O (99+%), FeCl 3 •6H 2 O (99+%), and arachidic acid (99+%) were obtained from Aldrich Chemical Co. and used without further purification. Distilled water was passed through a six-cartridge Barnstead Nanopure II purification train with a Macropure pretreatment. Organic solvents, such as chloroform and methanol, were either spec- troanalyzed or HPLC grade. Synthesis and Characterization of Iron Oxide Nanopar- ticles. Volumes of 0.28 mL of 11.5 N HCl and 8.3 mL of purified, deoxygenated water (resistance of 18 MΩ, by nitrogen gas bubbling for 30 min) were combined, and 2.86 g of FeCl 3 ‚ 6H 2 O and 1.00 g of FeCl 2 ‚4H 2 O were dissolved in the prepared solution with stirring for 20 min. The resulting solution was added dropwise into 100 mL of 1.5 M NaOH solution under vigorous stirring for 30 min. The precipitate was isolated in a magnetic field, and the supernatant was removed from the precipitate by decantation. A volume of 250 mL of purified water was added to the precipitate, and the solution was decanted after centrifugation at 5000 rpm for 10 min. After repeating the last procedure three times, 150 mL of 0.01 M HCl solution was added to the precipitate with stirring to neutralize the anionic charges on the nanoparticles. The clear hydrosol of γ-Fe 2 O 3 at pH 3.5 was used as a subphase for Langmuir monolayers in a Langmuir trough. The concentration of hydrosol of γ-Fe 2 O 3 was determined to be 9.29 × 10 -5 M with an atomic absorption spectrophotometer (AAS Vario 6). The  potential of γ-Fe 2 O 3 nanoparticles in an aqueous colloidal solution was measured with a Brookhaven Instrument Co. Model ZetaPlus. Transmis- sion electron microscopy (TEM) experiments were carried out on a JEOL 200 CX transmission electron microscope. The TEM samples were prepared on 400 mesh copper grids coated with carbon that were purchased from Electron Microscope Co. A ² Department of Chemistry. ‡ Department of Physics. 9341 J. Phys. Chem. B 2002, 106, 9341-9346 10.1021/jp014484c CCC: $22.00 © 2002 American Chemical Society Published on Web 08/17/2002