Physica B 320 (2002) 226–229 Water dynamics in ionic magnetic colloids studied by 1 H nuclear magnetic resonance C.E. Gonz ! alez a , D.J. Pusiol a, *, M. Ramia a , A. Bee b , A. Bourdon c , A.M. Figueiredo Neto d a Facultad de Matem ! atica, Astronomia y F! ısica, Universidad Nacional de C ! ordoba, Ciudad Universitaria, 5000 C ! ordoba, Argentina b Laboratoire de Physicochimie Inorganique, Universit ! e Pierre et Marie Curie, B # atiment F Porte 557, 4 P. Jussieu, 75252 Paris, France c Laboratoire des Milieux D! esordonn ! es et H! et ! erog ! e nes, UMR 7603 du CNRS, Universit ! e Pierre et Marie Curie, case 86, 4 place Jussieu, 75252 Paris, France d Instituto de F! ısica, Universidade de S * ao Paulo, caixa postal 66318, 05315–970 S * ao Paulo SP, Brazil Abstract 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. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Magnetic colloids; Nuclear magnetic resonance 1. Introduction Magnetic colloids, usually named ferrofluids [1], are suspensions of small magnetic grains (typical dimension of about 10 nmÞ; coated with a repul- sive agent and dispersed in a liquid carrier. Two types of ferrofluids are actually available: sur- facted (SFF) and ionic (IFF) [2] ferrofluids. In SFF the grains are coated with surfactant agents, with a double or monolayer coating, depending on the type of liquid carrier (non-polar or polar, respectively). IFF grains are electrically charged (positive or negative), being dispersed usually in water. Nowadays, the magnetic grains are chemi- cally synthesized in both types of ferrofluids. Different materials are used to make the grains: maghemite [3], magnetite [4] and ferrites [5]. Liquid carriers can be polar (e.g. water) or non- polar such as light mineral oils. The typical magnetic moment ðmÞ of a grain in a maghemite IFF is [6] mB10 19 Am 2 : One inter- esting question that arises in the case of the water- based ferrofluid is how the water molecules behave *Corresponding author. Fax: +54-351-433-4054. E-mail address: pusiol@fis.uncor.edu (D.J. Pusiol). 0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0921-4526(02)00688-9