Producción CyT
Extended Abstracs Book 9 Ferrofluid Workshop - Time dependent NMR spectroscopy on ionic and citrate ferrofluids
Congreso
Autoría:
D. Heinrich ; A.R. Goñi ; L. Cerioni ; T. M. Osán ; D.J. Pusiol ; C. ThomsenFecha:
2009Editorial y Lugar de Edición:
Technische Universität Dresden, Fakultät MascResumen *
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; an effect which in the past years has been extensively studied theoretically [1-3] as well as experimentally [4-7]. Recently, we used Raman spectroscopy to monitor the metastable cluster formation and its dynamics in surfacted and ionic ferrofluids [5-7]. Here we present results of a complementary study of the magnetic-field induced behavior of a water-based ionic (IFF) and citrate (CFF) ferrofluid with a concentration of 1 vol.% using nuclear magnetic resonance (NMR) spectroscopy. For the measurements we used a lowresolution NMR spectrometer working at room temperature with a homogeneous magnetic field of 225 mT. In the experiments, ionic and citrate electrostatically stabilized ferrofluids which had not been exposed previously to any magnetic field are placed at 300 K in the bore of the NMR spectrometer. Figure 1 shows the NMR spectra of the two samples, in comparison to that of pure water. The main peak of the IFF and the CFF is blue-shifted with respect to the resonance frequency of ?pure and free? water molecules [8] by approximately 17 kHz and 75 kHz, respectively. Both peaks are attributed to the dynamic environments of water molecules from the solvation layers close around the magnetic grains in the corresponding ferrofluid [8]. Figure 2 shows the time evolution of the amplitude and frequency of the NMR peak of the IFF on a time scale of more than one hour. The amplitude of the NMR signal exhibits a slight increase in the first 300 s followed by a strong reduction in intensity, reaching its minimum after approximately 15 minutes. In contrast, the frequency shift changes abruptly from 17 kHz to 3 kHz at the point when minimum amplitude is reached. We attribute the peak at 3 kHz to the NMR signal stemming from water molecules far from the magnetic particles (low field regions). Its amplitude increases monotonically, saturating at times longer than one hour. This contrasting behavior of NMR signal is readily understood by considering the dynamical processes within the ferrofluid triggered by an external magnetic field, as revealed by Raman spectroscopy [7]. The decay of the amplitude of the peak at 17 kHz is, thus, attributed to the fieldinduced clustering of the magnetic nanograins to form chain-like structures. As a result, the amount of water molecules in the high-field regions around the magnetic particles continuously decreases due to the building up of the chains, leading to the observed reduction in peak intensity. In fact, a characteristic time constant of (130 ± 15) s is obtained for this decay, which is in very good agreement with the clustering times measured with Raman on the same IFF sample [7]. On the other hand, the much slower increase in amplitude of the NMR signal, now at 3 kHz, corresponding to water molecules far from the grains, gives evidence of a sluggish long-ranged ordering of the chains in a sort of ?layered crystal structure? [9], forming in the homogeneous magnetic field of the NMR spectrometer. The characteristic time for this ordering obtained from a fit to the data points is (420 ± 50) s. Hence, the sudden change in frequency of the NMR signal is taken as evidence of a first-order phase transition from a ferrofluid containing chains of magnetic grains to a phase where the chains are closely packed into layers forming ordered stacks in the magnetic field direction. This transition, in contrast, occurs in the citrate FF only in the presence of magnetophoresis induced by an inhomogeneous magnetic field. Figure 3 displays the first and the last spectrum of a time-dependent NMR series for the CFF in two different situations. The fluid in Fig. 3a was not placed in any external field, whereas the sample of Fig. 3b was subjected to the strongly inhomogeneous field of a permanent magnet for 120 s. Without magnetophoresis (Fig. 3a) the amplitude of the NMR signal changes slightly but the frequency remains nearly constant at a high value of 75 kHz. For the fluid which was placed in the field gradient (Fig. 3b) the NMR peak behaves similarly but having a much lower frequency from the start. This clearly indicates that the CFF needs the net force of the gradient field for the building of the chain-like structures and the formation of the layered phase. We believe that this is a consequence of the much smaller size (10 nm compared to about 100 nm) of the nanograins of the citrate compared to the ionic ferrofluid, respectively. References [1] A.O. Ivanov, Z. Wang, and C. Holm, Phys. Rev. E 69, 031206 (2004). [2] H. Morimoto, T. Maekawa, and Y. Matsumoto, Phys. Rev. E 68, 061505 (2003). [3] Z. Wang, C. Holm, and H.W. Müller, Phys. Rev. E 66, 021405 (2002). [4] A. Wiedenmann, J. Magn. Magn. Mat. 272-276, 1487 (2004). [5] J.E. Weber, A.R. Goñi, D.J. Pusiol, and C. Thomsen, Phys. Rev. E 66, 021407 (2002). [6] J.E. Weber, A.R. Goñi, and C. Thomsen, J. Magn. Magn. Mat. 277, 96 (2004). [7] D. Heinrich, A.R. Goñi, and C. Thomsen, J. Chem. Phys. 126, 124701 (2007). [8] C.E. González, D.J. Pusiol, A.M.F. Neto, M. Ramia, and A. Bee, J. Chem. Phys. 106, 4670 (1998). [9] J. Jordanovic, S.H.L. Klapp, Phys. Rev. Lett. 101, 038302 (2008). Información suministrada por el agente en SIGEVAPalabras Clave
FERROFLUIDSLONG-TIME DYNAMICSMAGNETIC COLOIDSTD-NMR