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Concept

Ion-exchange resin

Summary

An ion-exchange resin or ion-exchange polymer is a resin or polymer that acts as a medium for ion exchange. It is an insoluble matrix (or support structure) normally in the form of small (0.25–1.43 mm radius) microbeads, usually white or yellowish, fabricated from an organic polymer substrate. The beads are typically porous (with a specific size distribution that will affect its properties), providing a large surface area on and inside them where the trapping of ions occurs along with the accompanying release of other ions, and thus the process is called ion exchange. There are multiple types of ion-exchange resin. Most commercial resins are made of polystyrene sulfonate, followed up by polyacrylate. Ion-exchange resins are widely used in different separation, purification, and decontamination processes. The most common examples are water softening and water purification. In many cases ion-exchange resins were introduced in such processes as a more flexible alternative to the

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The main objective of this PhD thesis is to link the atomic scale structure to the catalytic properties of self-assembled nanostructures. The growth and characterization of these nanostructures was carried out in an ultra-high vacuum (UHV) chamber initially designed for the study of the kinetics of epitaxial growth of thin films and nanostructures by variable temperature scanning tunneling microscopy. During this PhD thesis, we built a very sensitive and selective gas detector based on a quadrupole mass spectrometer and adapted to the geometry of this UHV system. The proper functioning of the gas detector is confirmed by temperature programmed desorptions of Xe on Ag(100) which demonstrate the very high sensitivity of the device of about

10^{-6}

~monolayer (ML). The zero-order desorption parameters obtained for the first monolayer of Xe adsorbed on Ag(100) are

E_d=0.22-0.23

~eV with a prefactor

\nu_0

between

4\times10^{12}

^{-1}

and

1 \times10^{13}

^{-1}

. An exchange process between Fe adatoms and Ag atoms from the Ag(100) surface is identified by means of temperature programmed desorption of N

_2

. Desorption spectra suggest that the exchange is complete from an annealing temperature of 140~K. Our study of the catalytic activity of metal nanostructures focuses on the ability of passivated Fe clusters to adsorb N

_2

molecules without dissociating them, which is a fundamental step in the bio-inspired heterogeneous catalysis of ammonia. The first system investigated for the growth of Fe clusters is MgO/Ag(100). Diffusion of Fe adatoms deposited on a MgO monolayer is observed from an annealing temperature of 280~K onwards. Fe clusters with an average size of

12.1\pm 0.5

~atoms are obtained on a MgO monolayer by thermal ripening of 0.072~ML of Fe deposited at 40~K. No sign of N

_2

adsorption was observed on these clusters by thermal programmed desorption investigation. The second system chosen for the growth of Fe clusters is graphene/Ir(111) obtained by chemical vapor deposition. The deposition of 0.3 and 0.4 ML of Fe on a graphene/Ir(111) sample at 50~K, followed by annealing at 300~K, generates a superlattice of Fe clusters which matches the periodicity of the moiré formed by graphene/Ir(111). The theoretically expected mean cluster size is 26 atoms for 0.3~ML of Fe, and 35 atoms for 0.4~ML of Fe. The temperature stability of the clusters obtained with 0.4~ML Fe is investigated from 300~K to 600~K. It is established that Smoluchowski ripening takes place as the annealing temperature increases and the superlattice structure has completely disappeared after annealing at 600~K. Temperature programmed desorption of molecularly adsorbed N

_2

at 50~K on a sample with 0.4~ML of Fe deposited on graphene/Ir(111) reveals 3 desorption peaks at 120~K, 92~K, and 60~K. In addition, slow dissociation of nitrogen molecules at 50~K adsorbed on the higher energy adsorption sites is demonstrated.

EPFL2021

Complexes of lanthanide (III) ions have been used in structural studies of biomolecules and as contrast agents (CA). Clinically approved CAs have only one water molecule in their first coordination sphere. Structure and water exchange rate of these complexes have been very well studied, however, water exchange kinetics of lanthanide complexes with two coordinated water molecules have not been investigated widely. In this regard, in the present work, a comprehensive study of water exchange kinetics of selected Ln3+ complexes with different ligands having two inner-sphere water molecules was conducted. In chapter III, selected lanthanide complexes of DO3A and DTTA-Me as representatives for macrocyclic and acyclic ligands have been studied by 17O NMR spectroscopy and 1H nuclear magnetic relaxation dispersion (NMRD). Water exchange rate constants measured on both complexes show a maximum at dysprosium and are much faster on DTTA-Me complexes than on the DO3A complexes. A change in water exchange mechanism is detected depending on both lanthanide and ligand structure. When analyzing the water exchange rates of [Ln(L)(H2O)2]x complexes, a unique rate constant for the exchange of the two water molecules was considered, however, the individual rate constants can be either very similar or very different. In order to investigate this, in chapter IV, the replacement of coordinated water molecule(s) in [Gd(DTTA-Me)(H2O)2]- by fluoride anions using multinuclear NMR spectroscopy was studied. Variable pressure 17O NMR measurements were also conducted for mechanistic assignment of the exchange reactions. It was found that fluoride binding facilitate the departure of the coordinated water molecule following a dissociative mechanism and accordingly cause a marked acceleration of the water exchange. In chapter V, the water exchange properties of lanthanide complexes of AAZTAPh-NO2 ligand was studied by 1H NMR spectroscopy. Water exchange rate constants were found to change more than two orders of magnitude along the series. Moreover, [Dy(AAZTAPh-NO2)(H2O)2]- was found to potentially be a very effective negative contrast agent for high magnetic fields imaging applications. Chapter VI is dedicated to the water exchange kinetics of selected lanthanide perchlorate and chloride aqua ions at different concentrations of 0.5 m to 2 m in order to establish the extent to which the nature of the counter-ion and the concentration of the Ln3+ change the rate and mechanism of the water exchange. Our results measured on neodymium ion are the first direct experimental proof for the maximum of water exchange rate constant along the lanthanide series as well as the change of the mechanism for water exchange from a dissociative mechanism for aqua ions of the early lanthanides to an associative mechanism for those of the late lanthanides.

EPFL2016

In the last two decades, Magnetic Resonance Imaging (MRI) has evolved into one of the most powerful techniques in diagnostic clinical medicine and biomedical research. MRI is primarily used to produce anatomical images, but may also give information on the physical-chemical state of tissues, flow diffusion and motion. The strong expansion of medical MRI has prompted the development of a new class of pharmacological products, called contrast agents, designed for administration to patients in order either to enhance the contrast between normal and diseased tissue or to indicate organ function or blood flow. Nowadays, around 30 % of all MRI investigations use a contrast medium, which number will certainly further increase with the development of new agents and applications. Since the first clinical application of MRI contrast agents in the eighties, many attempts have been made to ameliorate their efficiency. The aim of this work was to enlarge our knowledge on the physical chemistry of MRI related paramagnetic metal chelates in the perspective of contributing to the development of more efficient, more tissue specific and safer contrast agents. Thermodynamic stability and kinetic inertness of the metal complexes are crucial features for their safe biomedical application. We have carried out detailed kinetic studies on the dissociation of various Gd(III) chelates which have been recently proved to have optimal water exchange rate. The ligands involved both acyclic and macrocyclic molecules. In order to approach the biologically relevant conditions, we studied exchange reactions between these Gd(III) complexes and the endogenously most abundant Zn2+ ion. For the macrocyclic compounds, formation kinetic studies have also been performed for various lanthanides. Lanthanide and transition metal complexes of several novel chelators have been subject of thermodynamic stability studies, using pH-potentiometry, UV-Vis spectrophotometry or 1H NMR. Among the ligands, we find a series of linear, pyridine and phosphonate containing molecules, and diverse macrocyclic, dimeric structures. Their Gd(III) or Mn(II) complexes were characterized with regard to MRI contrast agent application. The rate and mechanism of the water exchange, as well as the rotational dynamics of the molecules have been assessed by variable temperature, variable pressure and multiple field 17O NMR and 1H NMRD measurements. On the Gd(III) complexes of xylene-cored DO3A dimers, we have identified an unexpected aggregation phenomenon. With one of the pyridine and phosphonate containing complexes, we have demonstrated that water exchange rates as high as that on the aqua ion are possible to achieve.

EPFL2006

10^{-6}