Journal of Magnetism and Magnetic Materials 305 (2006) 221–227 Preparation and effect of Ca 2+ on water solubility, particle release and swelling properties of magnetic alginate films S. Roger, D. Talbot, A. Bee à Laboratoire des Liquides Ioniques et Interfaces Charge´es (LI2C), CNRS—UMR 7612, Universite´Pierre et Marie Curie, case 63, 4 place Jussieu, 75005 Paris, France Received 22 July 2005; received in revised form 9 December 2005 Available online 7 February 2006 Abstract Magnetic natural films composed of alginate and maghemite nanoparticles are studied. A surface treatment by citrate ions of the magnetic nanoparticles is first required to limit interactions with carboxylate functions of alginate and to stabilize them in neutral pH. Sodium alginate films, with or without nanoparticles, are immersed in a calcium chloride bath to convert them into mixed sodium/ calcium alginate films. The ion exchange process is quantified by the degree of substitution (DS) deduced from sodium and calcium content obtained by atomic absorption spectroscopy. The magnetic nanoparticles content is also analysed to correlate the release of the particles to the amount of calcium present in the film. Nanoparticles do not significantly change the ion exchange process. Water insoluble films, which do not release magnetic nanoparticles, are obtained for a complete conversion of sodium alginate into calcium alginate (DS value is thus close to the stoechiometric ratio equal to 0.5). Such increase in water resistance of alginate films is caused by the formation of a dense network by crosslinking of the alginate polymer with Ca ions which prevents the alginate from going out of the film. Swelling properties of the mixed films are then investigated as a function of calcium concentration and nanoparticles content. It is found that the swelling ability in water of the films decreases by increasing the immersion time in CaCl 2 bath, the swelling ratio is thus a measure of the extent of crosslinking r 2006 Elsevier B.V. All rights reserved. Keywords: Magnetic nanoparticles; Film; Alginate; Magnetic film; Ferrofluid 1. Introduction In the framework of the development of natural packaging films, alginate could be considered as a promising candidate. Alginate is a natural polysaccharide extracted from various species of brown seaweed. It consists of linear copolymers composed of b-D-mannur- onate (M) and a-L-guluronate (G), linked by b 1,4- and a 1,4-glycosidic bonds with varying proportions of M and G units. These two units are organized in homopolymeric M–M and G–G blocks and heteropolymeric M–G blocks [1–6]. The carboxyl groups of the polymer give it the ability to undergo a sol–gel transition in the presence of multi- valent cations and more specifically with calcium ions [7,8]. This crosslinking agent provides the polymer network by connecting the chains. Gelation of alginate is convention- ally described in the terms of the ‘‘egg box model’’ where calcium ions are coordinated to guluronate cavities made by paired up of guluronate sequences in the alginate chains [9]. Ca-alginate gels are extensively used in various domains such as in the food processing industry for producing restructured foods, in the biotechnology indus- try for immobilization of cells or enzymes [10] and in environmental applications for removal of heavy metals [11,12]. Sodium-alginate form strong films, nevertheless they exhibit a poor water resistance because of the hydrophilic nature of Na-alginate polymer which is considered as the water soluble species of alginate. Crosslinking agent such as calcium ions allows to overcome this problem [13–15]. In order to improve the mechanical properties of the films, synthetic and natural polymers [16–18] or inorganic fillers are introduced to strengthen the polymer matrix [19–23]. ARTICLE IN PRESS www.elsevier.com/locate/jmmm 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.005 à Corresponding author. Tel.: +33 1 44 27 30 98; fax: +33 1 44 27 36 75 E-mail address: bee@ccr.jussieu.fr (A. Bee).