The structure of concentrated aqueous solutions of chromium nitrate and cerium chloride studied by X-ray diffraction and Raman spectroscopy

Abstract

Concentrated aqueous solutions of chromium nitrate and cerium chloride were investigated by X-ray diffraction and Raman spectroscopy. The X-ray diffraction patterns of two sets of solutions (1.60 up to 2.48 mol dm − 3 for chromium nitrate and 0.98 up to 2.81 mol dm − 3 for cerium chloride) display an intense maximum, prepeak, in the angular region about Q 0 ∼ 0.9 Å − 1 (Q = 4 π sin θ / λ, θ being the diffracting angle). The value of Q 0 corresponding to these maxima is proportional to the power 1/3 of the molar concentration. The proportionality coefficient has the value that should be observed for a close packing (fcc network) if assumed for cations. This study demonstrates, once more, the existence of an intermediate range order (up to ∼10 Å) in very concentrated aqueous solutions of salts constituted by small ions of different valences, range order previously reported by these authors in many articles. Raman experiments are in consonance with X-ray diffraction results.

NO3–Coordination in Aqueous Solutions by15N/14N and18O/natO Isotopic Substitution: What Can We Learn from Molecular Simulation?

“…This overlapping behavior, caused by the weak hydrogen scattering, hinders not only the XRD determination of the water orientation around the nitrate ion but also the interpretation of the coordinating environment from heavy-water NDIS. In fact the experiments involving 14 N/ 15 N substitution in aqueous NaNO 3 solutions revealed strong hydrogen bonding (i.e., D•••N interactions at r ND ∼ 2 Å), 17 while similar experiments on aqueous NaNO 3 and ND 4 NO 3 18−20 exhibited no prominent hydrogen-bonding peaks.…”

“…Given this state of affairs in the determination of nitrate hydration behavior, here we suggest an alternative approach based on the interplay between NDIS ( 14 N/ 15 N and nat O N / 18 O N ) experiments and molecular simulations of aqueous nitrates as an effective tool to gain crucial insight into the microstructure of the nitrate hydration shell. The novelty behind the proposed methodology is 2-fold: (a) it takes advantage of the newest findings regarding the scattering-length contrast between 18 O and nat O, i.e., 0.204 ± 0.003 fm 62 i.e., nearly a factor of 6 greater than the earlier tabulated value 63  making feasible NDIS experiments using nat O/ 18 O substitution, 64 and (b) it involves the use of heavy and null water as a way to extract coordination information not only for the waterhydrogen (not possible from XRD due to the extremely weak hydrogen scattering) but also for the water-oxygen around the nitrate sites which has been only possible from XRD experiments.…”

“…Moreover, this interplay provides rigorous ways (i) to identify the individual partial RDF's contributions to the total neutron-weighted distribution function (peak size and location), (ii) to isolate and assess the ion-pair formation, and (iii) to determine the actual nitrate−water coordination and corresponding hydrogen-bonding structure. 65−67 Therefore, the goal of this work is to explore the potential deconvolution of the contributions from water−nitrate correlations using neutron diffraction involving successive heavy-and null-aqueous solutions of KNO 3 under 14 N/ 15 N and nat O N / 18…”

“…achieve a full characterization of the first water coordination around the nitrate ion. For that purpose, in section 2 we discuss the fundamentals underlying the first-order difference approach involving 14 N/ 15 N and nat O N / 18 O N substitutions in aqueous KNO 3 solutions when the aqueous environments comprise either heavy or null water comple-mented with a brief description of the Poirier−DeLap 68 approach to the determination of the degree of ion-pair association according to the extracted K + •••NO 3 − distribution function. In section 3 we describe the intermolecular potential models used in the simulation of the aqueous KNO 3 and the corresponding molecular dynamics simulation methodology, while in section 4 we discuss the microstructural behavior of the aqueous KNO 3 and its characterization of the coordinating water environments according to the proposed interplay.…”

Toward the understanding of hydration phenomena in aqueous electrolytes from the interplay of theory, molecular simulation, and experiment

“…has been characterized as exhibiting mild interactions with water, i.e., weakly hydrated as a result of its small charge and relatively large effective ionic that induce limited distortion of the hydrogen bonding of the surrounding water [172][173] . These features might have contributed to the large disparity in the reported water coordination of the nitrate anion from different experimental sources, including NMR [174][175][176][177][178][179][180] , IR [181][182][183][184][185][186][187] , Raman [173][174][188][189][190][191][192][193][194][195][196][197] , XRD 172-173, 190, 193, 195-204 , as well as NDIS [205][206][207][208][209][210] , and point to unresolved concerns regarding the determination and interpretation of the hydration microstructure of nitrates in a variety of aqueous environments 63 .…”