Simple Ionic Systems Research Articles

Bonding mechanism of some simple ionic systems: Bader topological analysis of some alkali halides and hydrides revisited

The ability of Bader's charge density topology analysis to explain various material properties has been examined for simple ionic systems of alkali halides and alkali hydrides. It was established that despite the fact that most of them share the same rock salt crystal structure phase, some of them belong to different topological classes. This fact was used to explain various experimentally observed properties of these materials, and to discuss their deviations from expected trends. Some phase transitions observed in these systems, and their possible relation to changes in the charge density topology have been also investigated from the same point of view. Reasons for anion-anion bond formation in some of them and its features are discussed, as well.

Ion mixing, hydration, and transport in aqueous ionic systems.

The enhancement effect on the ion mobility of fluoride (and that of chloride) in a polycationic system, as the chloride content increases, is shown to also exist in other more simple ionic systems with cations such as the cesium ion and an organic ammonium ion. As the chloride content increases, in addition to the finding that there is more unbound water associated with the cation, we also observe that the average lifetime of a hydrogen bond decreases. This change to the hydrogen bonds is correlated to significant changes to both the structural and dynamical properties of water. The more disordered water structure and faster water dynamics are hypothesized to be also responsible for the enhanced ion mobilities. Furthermore, when either the chloride content or hydration level is changed, the self-diffusion constant of each co-ion changes by almost the same factor, implying the existence of a single universal transport mechanism that determines ion mobilities.

Nonlinear behaviour of simple ionic systems in hydrogel in an electric field.

The stationary behavior of ionic reaction-transport systems contained in hydrogel located between two reservoirs of electrolytes is investigated. The effects of the applied voltage, the composition of the electrolytes in the reservoirs and the distribution of fixed charge in the hydrogel on the spatial patterns of charge, electric potential, temperature and concentrations of the individual components are studied systematically by mathematical modelling. Such systems, containing only several ions (for example, Cl-, K+, OH- and H+) can function as electrolyte diodes and transistors and can exhibit oscillations and hysteresis. The mathematical description of the studied systems is based on balances of species, enthalpy and charge, on Poisson's equation and on the finite rate description of the water dissociation/recombination reaction, without assuming local electroneutrality. The modeling results are compared with experiments, including the system with hydrogel connecting reservoirs of strong base and acid studied by Noszticzius and co-workers. [L.H. Hegedus, N. Kirschner, M. Wittmann and Z. Noszticzius, J. Phys. Chem. A, 1998, 102, 6491 (ref. 1); L. H. Hegedus, N. Kirschner, M. Wittmann, P. Simon and Z. Noszticzius, Chaos, 1999, 9, 283 (ref. 2).]

Polarization effects in ionic systems from first principles

A classical application of the Car-Parrinello method in the computer simulation of ionic systems is demonstrated. The induction effects in the interionic interactions are included in addition to the short-range repulsion and dispersion effects described by effective pair potentials. By representing the induced dipoles as a pair of dynamically variable charges fixed on the ends of a rod and extending the Lagrangian accordingly, the self-consistent induced dipoles at each time step are generated from the values at the previous time step, without the need for explicit minimization. Coulomb-field-induced and overlap-induced polarization effects are included and these are parameterized by ab initio electronic structure calculations. For simple ionic systems it is shown that the neglect of the overlap-induced dipoles leads to a poor representation of real systems.

Glass formation in simple ionic systems via constant pressure molecular dynamics

The utility of constant pressure molecular dynamics (MD) in studies of glass formation is demonstrated by choosing NaCl and ZnCl2 as examples of MX and MX2 salts. Rigid ion pair potentials have been used to model the two salts. Results of these simulations are in good agreement with general glass formation principles. The effect of the interatomic potential on the glass transition and the cooling rate on glass formation in both the salts is reported. The parameter characterizing the inertia of the borders of the MD simulation cell has a strong influence on the glass transition temperature.

Stability and melting of simple ionic systems

A melting criterion for binary ionic compounds is formulated in terms of an instability due to the overlap between the larger ions (generally anions). This model provides a simple explanation of the linear behavior of the ratio of the ionic radii versus the square root of the melting temperature observed for alkali halides and pointed out here for the first time. It is then shown that the variation of the melting temperature with pressure is mainly due to the change of the force constants between the ions and that covalent or polarization effects are essential for the stability of binary superionic conductors like AgI.

Molecular dynamics studies of the vitreous state: Simple ionic systems and silica

Some applications of molecular dynamics calculations to the vitreous state have been examined for simple ionic MX and MX2 type glasses. The MX system which corresponds to a hypothetical vitreous and supercooled amorphous KCl is found to undergo an isobaric transition in the region of 0.3 Tf and an isothermal transition at around 35 kbar (T=Tf). The transition is evidenced by a discontinuity in second order thermodynamic properties and may be associated with a virtual disappearance of translational diffusion. By employing a variety of irreversible stress histories, the vitreous state in this case is shown to be insensitive to the thermodynamic stress history up to a maximum computationally permissible relaxation time of around 10−10 sec. Simulations are also reported for some ionic liquids of MX2 stoichiometry which, in contrast with KCl, have glass-forming ability (BeF2, ZnCl2, and SiO2). Although the relaxation times involved in laboratory glass formation far exceed the long-time limit of computer ’’experiment,’’ the configurationally arrested states which are produced by simulation exhibit many characteristic features of the experimental glasses. Changes of heat capacity consequent on configurational arrest in the case of KCl are similar in magnitude to those observed at the experimental glass transition of simple ionic glasses. The radial distribution for simulated SiO2 ’’glass’’ is very similar to that for vitreous silica, and a simulated compression–vitrification–expansion cycle for this system produces a ’’densified’’ glass, as observed in laboratory experiments. A rattle-and-correlated-jump mechanism appears suitable for the description of diffusion in this liquid. Limitations and further applications of MD calculations pertaining to the vitreous state are discussed.

Influence of isopropanol-water solvent mixtures on the conformation of poly-L-lysine.

AbstractThe helix–coil transition of poly‐L‐lysine (PLL) in water–isopropanol solvent mixtures has been investigated at room temperature by circular dichroism measurements. Within the range of 70%–80% isopropanol concentration (by volume), the polymer undergoes a sharp transition, characterized by the formation of a fully charged α‐helical structure. On the basis of some experimental evidence the role of the organic component in solution appears more complicated than that of strengthening the intramolecular hydrogen bonds in the polymer. By analogy with the distribution of the components of alcohol–water mixtures in simple ionic systems, it is thought that only an high co‐solvent concentration brings about an extensive and possible cooperative depletion of the clusters of firmly‐bound water molecules in the domain of the polylelectrolyte, favoring the transition to the α‐helical structure. On the other hand, CD spectral patterns show that the addition of NaCl in the alcohol‐rich–water mixtures of charged poly‐L‐lysine gives rise to a transition from the α‐helical to a β‐structures conversion obeys a first‐order rate law at all times, with a rate constant dependent on solvent composition and ionic strength. In these conditions, the rate of the process is close to that found for the thermally induced α–β transition. Higher polymer concentration and/or ionic strength cause a phase separation of β‐PLL, suggesting that in this case interchain reactions (where hydrogen bonding should play the major role) predominate. Titration experiments on charged α‐helical poly‐L‐lysine in 85% or 90% isopropanol mixtures confirm the occurrence of a conformational transition, which takes place within a degree of dissociation α of 0.2–0.75. The transition is accompanied by a visible turbidity, which increases as the titration proceeds. Implications of the solvent distribution around the macroion on the observed conformational phenomena are also discussed.