NMR Study of the Water Dynamics in Aqueous Colloidal Magnetic Fluids at Different pH's and Grains Concentrations (original) (raw)
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The Journal of Chemical Physics, 1998
In a previous NMR study [González, C.E.; et al. J. Chem. Phys. 1998, 109, 4670] we observed that the 1 H and 2 H spectra of both surfacted and ionic ferrofluids are broad and asymmetric. In ionic ferrofluids, this effect could be due to (i) electric interactions between the electrically charged magnetic grains and the electric dipole moments of water molecules and/or (ii) the interaction between water molecules and the distribution of magnetic field gradients in the intergrain volume. In this work we study a series of ionic ferrofluids prepared at different magnetic grains concentrations and with different surface charge densities. Our experiments clearly show that the sign and the density of the electric charge of the magnetic grains have no influence on the NMR spectra. On the other hand, spectral widths increase with the magnetic grains concentration, all the samples being far from the motional narrowing regime.
Water dynamics in ionic magnetic colloids studied by 1H nuclear magnetic resonance
Physica B: Condensed Matter, 2002
In a previous nuclear magnetic resonance (NMR) study we observed that the NMR spectra of water in both surfacted and ionic ferrofluids are asymmetric and several orders of magnitude wider than the one of pure water. It has been proposed that this effect is produced by extremely strong magnetic field gradients in the intergrain volume and/or by surface interactions between the carrier liquid molecules and the grains surface. In the case of aqueous ionic ferrofluids the latter possibility should be interpreted as electric interactions between water (polar) molecules and the charges in the grain surface.
Magnetic interactions in water based ferrofluids studied by Mössbauer spectroscopy
Journal of Physics-condensed Matter, 2007
Various samples of ferrofluids consisting of colloidal suspensions of surfacted cobalt ferrite or magnetite nanoparticles in water were studied by x-ray diffraction and temperature dependent Mössbauer spectroscopy. Information about the particle mean size, the size dispersion and the effective magnetic anisotropy energy was obtained for each sample. The results are consistent with the formation of a magnetic dead layer at
Variable temperature electron paramagnetic resonance investigations of an ionic ferrofluid
Indian Journal of Engineering and Materials Sciences, 2004
An aqueous based ionic ferrofluid has been studied by EPR spectroscopy technique from room temperature to liquid nitrogen temperature to detect the magnetic phase transitions and their variation at low temperatures. The line-width of EPR signal has been found to increase with decreasing temperature and this has been assigned to magnetic phase transition from ferromagnetic to a spin glass or cluster glass state mainly due to freezing of the carrier fluid. Line-width variation with temperature also suggests that bulk rotation mechanism is the dominant mode of relaxation. Any evidence of the presence of super-paramagnetism has not been observed.
Nanoparticle Composition of a Ferrofluid and Its Effects on the Magnetic Properties
Langmuir, 2004
Experiments were carried out on a water-based ferrofluid (γ-Fe2O3 with carboxydextran shell) using photon correlation spectroscopy (PCS), atomic force microscopy, and magnetic nanoparticle relaxation measurements. The experiments were designed with the aim to relate the Néel signals that are in theory generated by large single core particles with nanoscopic properties, that is, particle size, particle size distribution, shell properties, and aggregation. For this purpose, the ferrofluid was fractionated by magnetic fractionation and size exclusion chromatography. Nanoparticles adsorbed onto positively charged substrates form a two-dimensional monolayer. Their mean core diameters are in the range of 6 to about 20 nm, and particles above 10 nm are mostly aggregates. The hydrodynamic particle diameters are between 13 and 80 nm. The core diameter of the smallest fraction is confirmed by X-ray reflectometry; the surface coverage is controlled by bulk diffusion. Comparison with the hydrodynamic radius yields a shell thickness of 3.8 nm. Considering the shell thickness to be constant for all particles, it was possible to calculate the apparent particle diameter in the original ferrofluid from the PCS signals of all fractions. As expected, the small cores yielded no Néel relaxation signals in freeze-dried samples; however, the fractions containing mostly aggregates yielded Néel relaxation signals.
Magnetic resonance in nanoscopic particles of a ferrofluid
Journal of Magnetism and Magnetic Materials, 2000
V Fe \V Fe O particles are studied using electron spin resonance (ESR) technique. Measurements at 298 K show that (i) peak-to-peak linewidth decreases with increasing Mn concentration, (ii) inter-particle interaction decreases with Mn-content. Anisotropy values calculated from the spectra agree well with those determined earlier. The e!ect of cooling the sample to 77 K with and without external magnetic "eld on the ESR spectra are examined. Results are analysed in the light of existing theories.
Study of some magnetic properties of ferrofluids filtered in magnetic field gradient
Journal of Magnetism and Magnetic Materials, 2000
The magnetic properties of some ferrofluids obtained by a new technique, in which the stabilization of particles was done by hydrofobization in the absence of the dispersion medium, and the filtration was done in the presence of a magnetic field gradient, are studied. Measurements were performed on three samples originated from the same ferrofluid with magnetite particles dispersed in kerosene. They are distinguished by the type of filtration: in the absence of the field gradient (sample A), in the presence of the field gradient (sample B), and in the presence of the field gradient with a filamentary matrix introduced in the filtration vessel (sample C). The measurements performed in static magnetic field show an increase of approximately 12% and 40% in the saturation magnetization of samples B and C, respectively, in comparison to sample A, due to the increase of the concentration of the magnetic phase, as a result of the filtration in the presence of the field gradient. A decrease of the mean magnetic diameter of the particles in the ferrofluid can be noticed, from the dimensional distribution of the particles, determined by the Chantrell method (from 9.15nm (sample A) to 7.97nm (sample C)).
A theory of correlations in a magnetically aligned ferrofluid
Journal of Magnetism and Magnetic Materials, 1983
We have obtained an analytic form for the structure factor of a colloidal ferrofluid in which the particle moments are completely aligned by an applied magnetic field. The results obtained show the particle chaining first proposed by de Gennes and Pincus.
Journal of Physics D-applied Physics, 2001
From an initial ferrofluid with magnetite particles dispersed in kerosene and stabilized with oleic acid, having particle concentration n = 7.7×1016 cm-3 (determined from magnetic measurements), 11 samples were obtained by successive dilution with kerosene (with a dilution ratio 2:3). Using magnetic resonance measurements for each ferrofluid sample, we have studied the dependence on particle concentration of the effective anisotropy constant of magnetite colloidal particles. The obtained results show an increase of the effective anisotropy constant by increasing the particle concentration. This behaviour is assigned to interparticle magnetic interactions.
The effects of external magnetic field upon the stability of ionic magnetic fluids
Journal of Magnetism and Magnetic Materials, 2004
Effects of an external magnetic field on the energy spectrum and surface charge density of semiconductor nanoparticles (SNPs) in ionic colloids were investigated in the frame of finite element method. The maximum probability for carriers increases with the increasing of the applied magnetic field for both spherical and nonspherical nanoparticles. The effect of an applied external magnetic field on the surface charge density of SNPs is dramatically modified by the nanoparticle shape. Our observations can be well explained through the competition of the quantum confinement effects introduced by both particle geometry and magnetic field. r