Time dependent NMR spectroscopy on ionic ferrofluids

D. HEINRICH, A.R. GOÑI, L. CERIONI, T. OSÁN, D.J. PUSIOLC, C. THOMSEN

Cambridge, USA

Conferencia; 9th International Bologna Conference Magnetic Resonance in Porous Media; 2008

Magnetic nanoparticles colloidally suspended in a ferrofluid exhibit a tendency to form clusters and chain-like structures under the influence of an external magnetic field. Its behavior resembles a liquid in a porous media. Recently we used Raman spectroscopy to monitor the metastable cluster formation and its dynamics in surfacted and ionic ferrofluids]. In this work we present results of a complementary study of the magnetic-field induced behavior of a water-based ionic ferrofluid (IFF) with a concentration of 1 vol.% using nuclear magnetic resonance (NMR) spectroscopy.  For the measurements we used a low-resolution NMR spectrometer working at room temperature with an homogenous magnetic field of 400 mT. In the experiments, an electrostatically stabilized ferrofluid which has not been exposed previously to any magnetic field is placed at 300 K in the bore of the NMR spectrometer. The NMR spectrum displays an asymmetric feature which is composed by two peaks of different intensity. The main peak is blue-shifted by approximately 17 kHz from the resonance frequency of “pure and free” water molecules. The less intense peak appears to be centered at around 3 kHz above the pure water frequency. Both peaks are attributed to two different dynamic environments of water molecules in the ferrofluid, namely, molecules moving freely within the volume among the magnetic particles but far from them (3 kHz feature) and molecules from the solvation layers close around the magnetic grains (17 kHz structure). Figure 2 shows the time evolution of the peak amplitude of both peaks in a time scale of more than one hour. The amplitude of the NMR signal of the 17 kHz feature exhibits a slight increase in the first 300 s followed by a strong reduction in intensity, reaching its minimum after approximately 15 min. Simultaneously, the amplitude of the 3 kHz peak corresponding to the NMR signal stemming from water molecules far from the magnetic nanoparticles increases monotonically, saturating at times longer than one hour. This contrasting behavior of both NMR signals is readily understood by considering the dynamical processes within the ferrofluid triggered by an external magnetic field, as revealed by Raman spectroscopy.