**4.3. Magnetic properties**

58 Ion Exchange Technologies

of about 50Å of thickness.

with PdNPs.[32]

In this sense and among other techniques, SEM has been established as a referent in the field of the surface characterization. Because polymeric matrices usually are not conductive, in some cases (in matrices without MNPs or with a low content of MNPs) it is imperative to prepare the sample for the study, by a sputter coating with gold, carbon or palladium layers

For membranes and films, cross-section images can be obtained by cutting the samples under liquid N2. For resin nanocomposites, it is necessary to embed the material in an epoxy

resin to cut it transversally with a microtome, as shown in Figure 17.

**Figure 17.** Sample preparation of a resin bead for SEM characterization.

**Figure 18.** SEM images of (a) SPES-C membrane surface, (b) bare SPES-C membrane, (c) SPES-C membrane with PdNPs, (d) Blend membrane surface, (e) bare Blend membrane and (f) Blend membrane The metallic nanoparticles have larger magnetization compared to metal oxides, which is interesting for many applications. But metallic magnetic nanoparticles are not air stable, and are easily oxidized, resulting in changes or loss in their magnetization properties.

Thus, IMS of magnetic NPs open a new range of research. Lack of stability of this kind of nanoparticles finds a counterpart by their stabilization on a polymeric matrix.

Magnetic properties of metallic nanoparticles are dependent on the oxidative state of the NPs components. Therefore, the true knowledge of the degree of nanoparticle oxidation is necessary for the forecasting of magnetic characteristics of the obtained samples. This is not easy, but techniques such as XANES (X-Ray Absorption Near Edge Structure) may do it achievable by interaction of the atom core with the source of energy. By comparison with previously placed and analyzed patterns, information about chemical bonding and oxidative states is obtained. Figure 19 shows XANES analysis of nanocomposites containing either Ag or Ag@Co MNPs (with a superparamagnetic Co0-core) on sulfonic resin. Standard elements spectra were linearly combined and fitted with the sample in order to determine the oxidative state of each element in the sample. The linear combination results are also included (normalized) inset. Ag@Co NPs in sulfonated matrices showed an average Co spectrum similar to that recorded by the Co0 standard. In fact, linear combination fitting results confirmed that all the Co present in that sample was Co0.

Furthermore, to characterize magnetic behavior SQUID (Superconducting Quantum Interference Device) magnetization curves are obtained. In order to do that, sample is placed in a changing magnetic field over at room temperature. Magnetization loops are registered through the overall process.

In general, ferromagnetic species have normally evident hysteresis curves. However, superparamagnetic materials (frequently, ferromagnetic materials at nanometer scale) shows a lack of hysteresis but high magnetic saturation. Figure 20 presents SQUID magnetization curves for Pd@Co MNPs supported on a sulfonic (C100E) and carboxylic(C104E) cation exchange resin.

**Figure 19.** XANES spectra of Ag in comparison with Ag standards for Ag@Co-C100E sample (blue line in the graphic) and the linear combination fitting among all of the compounds analyzed is also shown in a fitting range from -20 to 30 eV. Ag0 (red) and AgNO3 (green) are standards.

**Figure 20.** SQUID Magnetic curves obtained of Pd@Co-NPs stabilized in C100E and C104E supports
