Maurizio Romanelli

The dynamics of a nitroxide radical in water adsorbed on porous supports studied by ESR

Abstract The ESR of the neutral spin probe 2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl in the study of its dynamics of water adsorbed onto silica gels with pore diameters in the range 4 to 100 nm is described. The correlation times for the motion and their activation energies were calculated from the ESR linewidth at different temperatures. The mobility of the spin probe decreases with decreasing pore size, thus reflecting an increase in the average viscosity of the adsorbed water. Different behaviors are shown by water in silica gels with pore diameters 10–100 nm and in silica gels with p.d. 4 nm. The results obtained with the nitroxide are compared with those obtained with Cu(II) and Mn(II) on the same supports, and discussed in terms of the nature of the adsorbed water.

ESR study on the interaction between a positively charged spin probe and the silica gel surface

Abstract The electron spin resonance lineshape of the positively charged 4-trimethylammonium-2,2,6,6-tetramethylpiperidine-1-oxyl nitroxide radical has been used for an investigation of the structural properties of water adsorbed on narrow-pore (4 nm diameter) silica gel. Different spectra were obtained from the radical in the different regions inside the pores, as a function of the distance from the surface. The radical showed slow-motion conditions in the first water layers near the surface (primary interaction region), while fast-motion conditions applied in the secondary interaction region. The partition constant of the radical between the two regions and the correlation times for the radicals motion have been calculated through a computer simulation in slow- and fast-motion conditions. The results have been compared with those obtained with other spin probes in similar systems.

First evidence of natural superconductivity covellite

The singular conductive properties of natural covellite were investigated through magnetic AC susceptibility measurements. Two natural hexagonal covellite lamellae coming from Calabona (Italy) and Butte (USA), respectively, were investigated from 30 to 1.5 K. At 1.63(5) K, an abrupt increase of the diamagnetism (Meissner effect) was observed, thus evidencing a transition from a poor conducting to a superconducting state. The transition is independent on small applied DC fields (< 50 Oe), whereas larger values prevent the superconducting state to be attained. These features confirm covellite as a Class I superconductor. This peculiar property of covellite, never observed before on natural materials, can be mainly related to its structural features. The layered framework of covellite, where CuS3 planes are isolated by covalently bonded S2 planes, provides an electron excess, which can be easily involved in the bulk conduction.

Genetic evolution of nanocrystalline Fe oxide and oxyhydroxide assemblages from the Libiola mine (eastern Liguria, Italy) structural and microstructural investigations

The Libiola Fe-Cu-sulphide mine, near Sestri Levante (eastern Liguria), represents one of the most extensively exploited sulphide deposits in Italy. In this area, active Acid Rock Drainage (ARD) processes are evident. The major resulting mineral phases are Fe oxides and oxyhydroxides, occurring in varicoloured crusts on the surface of waste rocks and in unconsolidated muds. In this study, the Fe assemblages of the waste rock were investigated by microchemical (SEM), structural (XRD), microstructural (TEM) and spectroscopic (DRS, IR, μ-Raman) techniques, in order to determine the phase composition, the textural relations among the minerals and their genetic evolution. They are characterized by intimate intergrowths of hematite and goethite with minor quartz and lepidocrocite; in some samples, the presence of very minor schwertmannite was detected. TEM and HR-TEM observations revealed that hematite is present within pseudo-elliptical bodies as pseudo-hexagonal to subrounded nanocrystalline lamellae (from 18.9 to 26.5 nm in diameter), whereas goethite occurs either as parallel intergrowths of acicular crystals (from 10 to 16.3 μm in length) or as sheaf-like assemblages. On the basis of the present data, the studied Fe oxide and oxyhydroxide assemblages are found to represent distinct spatial and temporal stages of a nano-scale evolution process.

Chemical speciation of Ag in galena by EPR spectroscopy

Abstract Electron paramagnetic resonance (EPR) spectroscopy has been used to study the valence state of silver in “argentiferous” galena samples from the Apuane Alps (Tuscany, Italy) mining district. This method was used to reveal primary metallic silver (Ag0) in galena. Both thermodynamic data and experimental studies suggest that galena and native silver can stably coexist, but have not been reported as a primary (hypogene) assemblage in natural samples. EPR spectroscopy proved to be a suitable tool to solve this problem, because this technique is capable of detecting paramagnetic species down to the ppb level, even in a highly absorbent matrix such as galena. A detailed SEM-EDS investigation could not detect metallic silver (or gold) in galena samples, but did reveal small (few micrometers) inclusions of Ag-bearing phases, in which silver has a formal valence of +1. On the other hand, EPR spectra indicated the presence in galena of pairs and clusters of elemental silver atoms, which may be associated with pairs of metallic gold, or with silver-gold hetero-atomic pairs. Therefore, SEM/EDS and EPR are complementary techniques, revealing the presence of both Ag+ and Ag0. The Ag(Au) metallic species were apparently deposited on the galena surface during its growth from mineralizing fluids. Their scarcity, and the presence of larger amounts of Ag1+ phases, suggest that the assemblage galena-metallic silver was stable only under peculiar physical and chemical conditions. The formation of Ag0 was presumably linked to local and rare chemical fluctuations of the hydrothermal environment, characterized by low activities of S, Sb, Bi (Cu…) and high activity of Ag in the fluids. The occurrence of both homo- and hetero-atomic pairs suggests either different kinetics of pair formation, or possible fluctuations in the composition of the hydrothermal fluids, which alternatively carried Ag or Au species, or both

ESR and crystal‐field anisotropy in 6S spin‐state ions in solution

The nature of the disortion of the crystal‐field symmetry acting on symmetrically coordinated Fe(III) and Mn(II) complexes is investigated. Fine splitting components in the spectra of viscous solutions and of frozen solutions denote the presence of a wide distribution of sites with different zero‐field splitting terms, part of which are comparable to, or greater than, the Zeeman term. In low‐viscous solutions the mean zero‐field splitting term, which accounts for the linewidth of the ESR spectra, arises from the averaging of essentially the same crystal‐field distribution.

Electron spin relaxation of manganese (II) in solution

Abstract The electron spin resonance of Mn(II)in liquids and its implication as a paramagnetic probe are discussed. A motional model is proposed to account for

Electron spin relaxation and hyperfine line shape of manganese (II) in mixed‐solvent systems

The X‐band ESR line shape of the six hyperfine components of manganese (II) has been investigated both in pure and in mixed solvents. The procedure used by Luckhurst and Pedulli [G. R. Luckhurst and G. F. Pedulli, Mol. Phys. 22, 931 (1971)] to account for the removal of the spin degeneracy by the nonsecular terms in the spin Hamiltonian has been used to compute the line shape as a function of (D:D) and of τc. In mixed solvents, large deviations from the theoretical line shape are due to the presence of differently coordinated complexes. A criterion for the determination of the selective solvation has been established.

Arsenic-bearing calcite in natural travertines: evidence from sequential extraction, μXAS, and μXRF.

Recent studies demonstrated that synthetic calcite may host considerable amounts of arsenic (As). In this paper, the concentration of As in natural calcite was determined using two novel, specifically designed, sequential extraction procedures. In addition, the oxidation state of As and its distribution between calcite and coexisting Fe-oxyhydroxides was unravelled by μXRF elemental mapping and As K-edge μXAS spectroscopy. Our results conclusively demonstrate that arsenic can be found in natural calcite up to 2 orders of magnitude over the normal crustal As abundances. Because of the large diffusion of calcite in the environment, this phase may exert an important control on As geochemistry, mobility, and bioavailability.

Surface effects toward the mobility of charged spin probes: ESR study of nitroxide solutions adsorbed on zeolites

Abstract A study concerning the surface effects of zeolites X and Y toward the mobility of charged nitroxides is reported. Because of their critical dimensions, the nitroxides were only able to enter the large faujasite cavities. The relaxation analysis allowed us to establish the existence of a decreased mobility of the water solutions of nitroxide adsorbed in the zeolitic cavities with respect to mobility in bulk water. The negative radical Tempyo − gave results on the status of water near the cavity center whereas the ESR spectra of the positive radical TempTMA + were largely affected by water layers near the cavity walls.