Water dynamics in ionic magnetic colloids studied by 1H nuclear magnetic resonance

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. In this work we study a series of ionic and surfacted ferrofluids prepared at different magnetic grain concentrations and sizes, and with different surface charge densities. Our experiments clearly show that the sign and the density of the electric charge on the magnetic grains have no influence on NMR spectra. On the other hand, spectral widths increase with the magnetic grain concentration. (C) 2002 Elsevier Science B.V. All rights reserved.