Surface analysis of magnetite nanoparticles in cyclohexane solutions of oleic acid and oleylamine

Abstract

The surface of magnetite (Fe 3 O 4 , d ¼ 6:3 AE 0:7 nm) nanoparticles dispersed in cyclohexane was studied in the presence of oleic acid and oleylamine using in situ FTIR spectroscopy. Equimolar mixtures of these surfactants are widely used in the chemical synthesis of nanoparticles with a low polydispersity. Here, the IR spectra indicate that oleic acid molecules adsorb to the magnetite surface as a carboxylate. Measurements as a function of surfactant concentration yield an adsorption isotherm, with about two surfactant molecules adsorbed per nm 2 of magnetite at 1 mM surfactant concentration and about 3.5 molecules per nm 2 at 310 mM, of the order expected for full monolayer coverage. No spectral indication is found of oleylamine molecules at the surface of magnetite. In solution, however, almost every oleylamine molecule combines with an oleic acid molecule to form an acid-base complex, with an association constant of 3:5 Â 10 4 dm 3 mol À1 .

Spatial Distribution of Nanocrystals Imaged at the Liquid-Air Interface

“…One candidate would be oleic acid molecules desorbed from the surface of nanoparticles, molecules that are about 2 nm in length. However, oleic acid molecules in aprotic solvents are present as linear apolar dimers [34], which do not adsorb to the liquid-air interface; we verified that the surface tension of decalin remains unchanged (within 0:2 mN=m) after dissolution of pure oleic acid, even up to a concentration of 0:040 mol=l, twice as much as present on the particles (assuming an adsorption density of $3 nm À2 [35]) in the sample with the highest nanoparticle concentration ( ¼ 0:01). A more plausible adsorbing species is lead oleate, consisting of a Pb 2þ cation coordinated by two oleate ions and being one of the chemical precursors present during nanocrystal growth.…”

Complex magnetic susceptibility setup for spectroscopy in the extremely low-frequency range

“…The ferrofluids consisted of magnetite ͑Fe 3 O 4 ͒ nanoparticles of about 7 nm in diameter 51,52 dispersed in a solution of 0.3 vol % oleic acid and 0.3 vol % oleylamine in Decalin. 53 As expected, frequency-independent magnitudes of the magnetic susceptibility were found, and in the 0.1-100 Hz range, an absolute phase smaller than 1°was measured, in line with the frequency independence of the magnetic susceptibility of the samples.…”

Magnetization behavior of ferrofluids with cryogenically imaged dipolar chains

“…Dilutions were done with a 1 mM solution of oleic acid in Decalin to prevent surfactant desorption [14]. Three systems with different average particle diameters are examined, corresponding to three different dipole-dipole coupling parameters (see table 1).…”

“…The values for λ in table 1 assume that the outer diameter σ of the particles is equal to the core diameter d TEM plus a 2 nm oleic acid layer [14] [16] of 1.2 nm for systems A and B, and 0.3 nm for system C. In the light of the 1 nm resolution of TEM, the conclusion is that the particles can be modeled as bulk magnetite spheres and that the particles are so large that surface effects can be neglected in our analysis of the data.…”