Neutron scattering study of the proton dynamics in aqueous solutions of sulphuric acid and caesium sulphate

Abstract

The quasielastic neutron scattering spectra of aqueous solutions of sutphuric acid have been recorded and compared to those of pure water and of caesium sulphate solutions in order to study the proton dynamics in these systems and in particular the abnormal proton diffusivity expected to occur in acidic solution. Mean transport coefficients for the water molecules' translational and rotational diffusive motions have first been determined in the three systems. They indicate a marked slowing down of the translational diffusion when salt or acid are added to water. The rotational motion is much less affected. In H2SO 4 solutions corresponding to the maximum of specific conductivity, some evidence is obtained for a weak and broad quasielastic component, not present in pure water or in Cs2SO~ solutions. Its analysis is reasonably accurate only for momentum transfer values Q lower than 0.5 A-1. In this low Q limit, the rotational contributions can be neglected and the spectra of the acidic solutions can simply be reproduced by the weighted sum of two lorentzian translational profiles. The more intense and narrower one corresponds to normal diffusion of solvation water molecules. We assign the broader additional component to the abnormal proton diffusivity. It contains about 12 per cent of the total intensity in agreement with the percentage of protons involved in n30 + species. The width of both components increases linearly with Q2 below Q = 0"5 A-1. In a H2SO 4 3.2 M solution at 263 K, mean water diffusion can be characterized by a diffusion constant D t = (0"53 "4-0"03) 10-9m2s 1 whereas the fast proton diffusion process is characterized by D § = (2.2 _ 0-4)10-8 m 2 s-1. The corresponding activation energies are respectively AE t = (18-4 + 0.4)kJ mol-1 and AE § = (8.4 _ 2)kJ mol-1. Thus, for the first time, a signature of the abnormal proton mobility is observed by neutron scattering.

Dynamics of water in LiCl and CaCl2 aqueous solutions confined in silica matrices: A backscattering neutron spectroscopy study

“…Quasielastic neutron scattering (QENS) is often a technique of choice for studying mobility of hydrogen-containing species and has been used in numerous studies of confined water. On the other hand, there have been few QENS studies of aqueous solutions in the bulk form [1][2][3][4][5] and almost none in confinement [6,7]. The relative lack of QENS studies of aqueous solutions could possibly be attributed to the fact that even the dynamic properties of pure water, which represents the limiting case of an infinitely dilute solution, are not well understood, while the presence of ions adds another layer of complexity [8].…”

Dynamics in Glass Forming Sulfuric and Nitric Acid Hydrates

“…Overall, these data display the same general temperature dependence. In Figure we also included diffusion data from quasi-elastic neutron scattering. − Using this technique, c = 3.2 and 5 M aqueous H 2 SO 4 solutions (corresponding to n ≈ 14 and n ≈ 8, respectively) and a 6.1 M aqueous HNO 3 solution (corresponding to n ≈ 7) were investigated . At lower temperatures, the neutron data recorded for these more highly diluted acid hydrates indicate a somewhat faster diffusion than we observe via NMR.…”

“…Translational diffusion coefficients of viscous acid hydrates studied using field gradient diffusometry (filled symbols, this work) and quasi-elastic neutron scattering (open symbols, refs and ).…”

“…However, some stoichiometric compositions, particularly in the sulfuric acid system, are good glass formers, too. Glass forming aqueous sulfuric acid solutions were already studied by specific heat, , viscosity, − conductivity, − as well as diffusion measurements. − Apart from transport coefficients, also vibrational properties − and structural aspects , of these hydrates were examined. Owing to a generally much stronger crystallization tendency as compared to the sulfuric systems, the supercooled and glassy states of the nitric acid hydrates were studied much less. ,− The calorimetric glass transition temperatures T g,cal of the presently investigated acid hydrates are in the range from 153 to 164 K. , Glass transitions as well as transport properties were also investigated for various other acid hydrates. ,− Particular attention was directed to phosphoric and phosphorus acids that hold promising potential for use in solid electrolyte devices. − …”

Quasielastic neutron scattering study of acidic solutions

“…In a previous paper [2] we have started an investigation of aqueous acidic solutions in order to characterize by quasielastic neutron scattering (QNS) not only the usual transport processes occurring in aqueous solution but also the eventual contribution of fast proton kinetics in highly conducting solutions.…”