Ruggero Caminiti

The kinetics of phase transitions observed by energy-dispersive X-ray diffraction

A new energy-dispersive X-ray diffraction (EDXD) method to study the kinetics of the phase transitions (PTs) is reviewed. It enables one to follow the evolution of systems undergoing structural transformations in real time. It is based on a novel approach to PTs and provides several advantages with respect to the conventional techniques used for the same purpose. A detailed treatment of the theory is accomplished to demonstrate the consistency and reliability of this method. Despite the initial complexity of the subject that involves the interactions between X-ray and matter, just very simple measurements are required. Furthermore, the data processing is trivial and the relevant information on the PTs can be straightforwardly obtained from the X-ray spectra. The applications of the EDXD-PT method to the systems studied until now, namely polymeric, biological and mineralogical samples, are finally reported.

Structural properties of 1-alkyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}amide ionic liquids: X-ray diffraction data and molecular dynamics simulations.

X-ray diffraction data for 1-alkyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}amides are reported as a function of the length of the alkyl chain on the imidazolium ring. The measured diffraction patterns have been compared with the theoretical patterns calculated (from the geometries obtained) with molecular dynamics simulations. This provides a detailed description (at the atomistic level) of the morphology in the liquid state of these salts, highlighting the role played by the alkyl chain length. An analysis of the behavior of the hydrogen bonds that are formed between the imidazolium acidic protons and the anion is presented.

Comparing intermediate range order for alkyl- vs. ether-substituted cations in ionic liquids

X-ray scattering data from four pairs of ionic liquids (ILs) are compared. The alkyl-substituted cations show a first sharp diffraction peak between 3 and 4 nm(-1) that is not observed for ILs having cations with ether- or hydroxy-substitutions. These observations indicate a significant difference in the intermediate range order for these liquids.

Liquid Structure of Trihexyltetradecylphosphonium Chloride at Ambient Temperature: An X-ray Scattering and Simulation Study

We report on an experimental and simulation study done on a representative room temperature ionic liquid, namely tetradecyltrihexylphosphonium chloride, at ambient conditions. The study was conducted using small and wide angle X-ray scattering and molecular dynamics simulations. Both approaches converge in indicating that this material is characterized by the existence of strong P-Cl interactions (with characteristic distances between 3.5 and 5.0 A) and by the occurrence of nanoscale segregation, despite the symmetric nature of the cation and similarly to other room temperature ionic liquids. A good agreement is found between the structure factor and pair correlation functions obtained from MD simulations and the corresponding experimental observables, thus strongly validating the interaction potential used in the simulations.

Liquid structure of 1-alkyl-3-methylimidazolium-hexafluorophosphates by wide angle x-ray and neutron scattering and molecular dynamics

We report for the first time joined energy dispersed x-ray and neutron diffraction experiments on a series of (both protiated and selectively deuteriated) 1-alkyl-3-methylimidazolium hexafluorophosphate salts (alkyl = butyl, hexyl, octyl) at ambient conditions. The x-ray experimental data are used to optimize the interaction potential used for running molecular dynamics simulations on these systems. Such a potential leads to a good description of neutron scattering data from the samples without additional refinement, thus further validating the potential definition. The molecular dynamics simulations were used to access microscopic information on the morphology of the proposed systems, thus probing the role played by alkyl chain length on the structure. The comparison of x-ray weighted and neutron-weighted computed diffraction patterns allows the rationalization of several diffraction features. Further insight into cation-anion coordination and alkyl chain conformational equilibrium is provided on the basis of the MD-derived snapshots, confirming and extending previously obtained results on these issues.

Rietveld refinements on laboratory energy dispersive X-ray diffraction (EDXD) data

Rietveld refinements of corundum, a rutile and anatase nanocrystalline synthetic mixture, and gypsum, on laboratory energy dispersive X-ray diffraction (EDXD) data are reported. Cell parameters, positional and displacement parameters are in reasonable agreement with single-crystal reference data, despite the rather poor resolution of EDXD data. In particular, good results were obtained for gypsum (unrestrained refinement) with counting times as short as 1000 s.

Divalent metal–acetate complexes in concentrated aqueous solutions. An x‐ray diffraction and NMR spectroscopy study

Divalent metal–acetate solutions have been investigated by the x‐ray scattering technique and 13C NMR spectroscopy. The aim of this work was to study the complex formation between the divalent metals and a typical organic complexing ligand in concentrated aqueous solutions. The correlation functions and the analysis of the structure functions agree with the literature information that the species present in solution are Me(H2O)2+6 , Me(H2O)6−z(CH3COO)2+−zz , and Me(H2O)6−2z(CH3COO)2+−zz . Good agreement with experimental data is achieved through a model in which the acetate ions are bonded as a monodentate ligand to the Mg, Co, Mn, and Zn cations and as a bidentate to the Cd cation. The cations also possess a second coordination shell of water molecules. Some indications have been obtained supporting the presence of hydration water around the acetate anions.

The monoclinic I2 structure of bassanite, calcium sulphate hemihydrate (CaSO4 · 0.5H2O)

A structural analysis of CaSO 4 · 0.5H 2 O, a dehydration product of gypsum, has been carried out through the Rietveld method on X-ray powder diffraction data. A dehydrated powder of synthetic gypsum has been charged inside a non-hermetically sealed capillary in order to allow a slow rehydration. The starting material has been identified as γ-anhydrite, space group P 6 2 22, cell parameters a = 6.9691(2) A, c = 6.3033(2) A. The final product of the rehydration of γ-anhydrite is CaSO 4 · 0.5H 2 O, space group I 2 (unique axis b ), cell parameters a = 12.0350(5) A, b = 6.9294(3) A, c = 12.6705(4) A, β = 90.266(3)°. The structure of the hemihydrate is strongly pseudo-trigonal, space group P 3 1 21. The symmetry lowering arises from water molecules ordering inside the channels.

Interactions and structure in aqueous NaNO3 solutions

Two concentrated (5 and 7 mol l−1) aqueous solutions of NaNO3 were investigated by x‐ray diffraction. The features of the total radial functions calculated from experimental intensity data show that the positional correlation is not very strong in the systems investigated. However, it is possible to account for experimental data only by using models in which hexahydrated Na+ ions have definite correlations with NO3− ions, even if not through formation of isolated ion pairs. The compatibility of our observations with spectroscopic results is also discussed.

X-ray diffraction study of MgCl2 aqueous solutions

X-ray diffraction data for three aqueous solutions of MgCl2 were analyzed. It is shown that in a very concentrated solution the structure and correlation functions can be reproduced almost completely by using a nearest-neighbor model of ionic hydration, whereas in the more dilute solutions a certain number of interactions between the hydrated cation and external water molecules must also be considered. The mean coordination geometry for the close hydration was found to be octahedral both for cation and anion with mean cation–water distances (2.10–2.12 A) and mean anion–water distances (3.11–3.14 A) in accordance with results found in other cases.