Showing papers on "Ionic bonding published in 2010"

Preparation of Inorganic Materials Using Ionic Liquids

Abstract: Conventional synthesis of inorganic materials relies heavily on water and organic solvents. Alternatively, the synthesis of inorganic materials using, or in the presence of, ionic liquids represents a burgeoning direction in materials chemistry. Use of ionic liquids in solvent extraction and organic catalysis has been extensively studied, but their use in inorganic synthesis has just begun. Ionic liquids are a family of non-conventional molten salts that can act as templates and precursors to inorganic materials, as well as solvents. They offer many advantages, such as negligible vapor pressures, wide liquidus ranges, good thermal stability, tunable solubility for both organic and inorganic molecules, and much synthetic flexibility. In this Review, the use of ionic liquids in the preparation of several categories of inorganic and hybrid materials (i.e., metal structures, non-metal elements, silicas, organosilicas, metal oxides, metal chalcogenides, metal salts, open-framework structures, ionic liquid-functionalized materials, and supported ionic liquids) is summarized. The status quo of the research field is assessed, and some future perspectives are furnished.

Ionicity in ionic liquids: correlation with ionic structure and physicochemical properties

Abstract: Ionic liquids (ILs) are ambient temperature molten salts that have attracted considerable attention because of their negligible volatility, thermal stability, nonflammability, and high ionic conductivity. These remarkable properties result essentially from their ionic nature. Thus, the concept of ionicity of ILs (i.e. how ionic they are) is of great significance for characterising their properties. Here, we show the methodologies used to assess the ionic nature in ILs. On the basis of quantitative estimation of the ionicity, their dependence on ionic structure, polarity scales, and physicochemical properties is reviewed. The ionicity of certain ILs is also predicted from their physicochemical properties. The effects of different classes of ILs (e.g., protic ILs and lithium ILs) and binary systems consisting of ILs and other components on the ionicity are also discussed.

Theory and applications of atomic and ionic polarizabilities

Abstract: Atomic polarization phenomena impinge upon a number of areas and processes in physics. The dielectric constant and refractive index of any gas are examples of macroscopic properties that are largely determined by the dipole polarizability. When it comes to microscopic phenomena, the existence of alkaline-earth anions and the recently discovered ability of positrons to bind to many atoms are predominantly due to the polarization interaction. An imperfect knowledge of atomic polarizabilities is presently looming as the largest source of uncertainty in the new generation of optical frequency standards. Accurate polarizabilities for the group I and II atoms and ions of the periodic table have recently become available by a variety of techniques. These include refined many-body perturbation theory and coupled-cluster calculations sometimes combined with precise experimental data for selected transitions, microwave spectroscopy of Rydberg atoms and ions, refractive index measurements in microwave cavities, ab initio calculations of atomic structures using explicitly correlated wavefunctions, interferometry with atom beams and velocity changes of laser cooled atoms induced by an electric field. This review examines existing theoretical methods of determining atomic and ionic polarizabilities, and discusses their relevance to various applications with particular emphasis on cold-atom physics and the metrology of atomic frequency standards.

Effects of packing fraction and bond valence on microwave dielectric properties of A2+B6+O4 (A2+: Ca, Pb, Ba; B6+: Mo, W) ceramics

Abstract: Microwave dielectric properties of A2+B6+O4 (A2+: Ca, Pb, Ba; B6+: Mo, W) ceramics were investigated as a function of packing fraction and bond valence. For A2+B6+O4 specimens sintered at 800–1100 °C for 3 h, a single phase with a tetragonal scheelite structure was detected, and the theoretical density was higher than 93% throughout the composition. Although the ionic polarizability of Ba2+ ion was larger than that of Ca2+ ion, the dielectric constant (K) of BaB6+O4 showed a smaller value than that of CaB6+O4. These results could be attributed to changes of the packing fraction due to the effective ionic size. The Q·f value was largely dependent on the packing fraction, as well as the percentages of theoretical density. The temperature coefficients of the resonant frequencies (TCFs) of the specimens were affected by the bond valence of oxygen. The specimens of CaMoO4 sintered at 1000 °C for 3 h showed the K of 10.8, Q·f of 76,990 GHz and TCF of −22.8 ppm/°C, respectively.

Stress-relaxation behavior in gels with ionic and covalent crosslinks.

Abstract: Long-chained polymers in alginate hydrogels can form networks by either ionic or covalent crosslinks. This paper shows that the type of crosslinks can markedly affect the stress-relaxation behavior of the gels. In gels with only ionic crosslinks, stress relaxes mainly through breaking and subsequent reforming of the ionic crosslinks, and the time scale of the relaxation is independent of the size of the sample. By contrast, in gels with only covalent crosslinks, stress relaxes mainly through migration of water, and the relaxation slows down as the size of the sample increases. Implications of these observations are discussed.

Reversed anionic Hofmeister series: the interplay of surface charge and surface polarity.

Abstract: We describe a two-scale modeling approach toward anion specificity at surfaces of varying charge and polarity. Explicit-solvent atomistic molecular dynamics simulations at neutral hydrophobic (i.e., nonpolar) and neutral hydrophilic (i.e., polar) self-assembled monolayers furnish potentials of mean force for Na+ and the halide anions F−, Cl−, and I− which are then used within Poisson−Boltzmann theory to calculate ionic distributions at surfaces of arbitrary charge for finite ion concentration. On the basis of calculated long-ranged electrostatic forces and coagulation properties, we obtain the direct anionic Hofmeister series at negatively charged hydrophobic surfaces. Reversal takes place when going to negative polar or to positive nonpolar surfaces, leading to the indirect series, while for positive polar surfaces the direct series is again obtained. This is in full accordance with a recent experimental classification of colloidal coagulation kinetics and also reflects the trends of the ion specific sol...

pH effect on the assembly of metal-organic architectures

Abstract: Crystal engineering is the rational design and assembly of solid-state structures with desired properties via the manipulation of intermolecular interactions, hydrogen bonding and metal–ligand complexation in particular. The heart of crystal engineering is to control the ordering of the building blocks, be they molecular or ionic, toward a specific disposition in the solid state. The relatively weak strength of intermolecular forces with respect to chemical bonding renders the assembly of supramolecular constructs sensitive to external physical and chemical stimuli, with pH condition of the reaction mixture being arguably the most prominent and extensively observed. Using selected examples of constructing metal–organic architectures from recent literature, the influences of pH on the specific ligand forms, the generation and metal coordination of hydroxo ligands, ligand transformation promoted by pH condition changes, pH-dependent kinetics of crystallization of a number of metal–organic architectures are discussed. Current status of this particular areas of research in supramolecular chemistry and materials are assessed and personal perspectives as to toward what directions should this chemistry head are elaborated.

Surface Tensions, Surface Potentials, and the Hofmeister Series of Electrolyte Solutions

Abstract: A theory is presented which allows us to accurately calculate the surface tensions and the surface potentials of electrolyte solutions. Both the ionic hydration and the polarizability are taken into account. We find a good correlation between the Jones-Dole viscosity B coefficient and the ionic hydration near the air-water interface. The kosmotropic anions such as fluoride, iodate, sulfate, and carbonate are found to be strongly hydrated and are repelled from the interface. The chaotropic anions such as perchlorate, iodide, chlorate, and bromide are found to be significantly adsorbed to the interface. Chloride and bromate anions become weakly hydrated in the interfacial region. The sequence of surface tensions and surface potentials is found to follow the Hofmeister ordering. The theory quantitatively accounts for the surface tensions of 10 sodium salts for which there is experimental data.

The Zintl Compound Ca5Al2Sb6 for Low‐Cost Thermoelectric Power Generation

Abstract: Understanding transport in Zintl compounds is important due to their unusual chemistry, structural complexity, and potential for good thermoelectric performance. Resistivity measurements indicate that undoped Ca_5Al_2Sb_6 is a charge-balanced semiconductor with a bandgap of 0.5 eV, consistent with Zintl–Klemm charge counting rules. Substituting divalent calcium with monovalent sodium leads to the formation of free holes, and a transition from insulating to metallic electronic behavior is observed. Seebeck measurements yield a hole mass of ∼2m_e, consistent with a structure containing both ionic and covalent bonding. The structural complexity of Zintl compounds is implicated in their unusually low thermal conductivity values. Indeed, Ca_5Al_2Sb_6 possesses an extremely low lattice thermal conductivity (0.6 W mK^(−1) at 850 K), which approaches the minimum thermal conductivity limit at high temperature. A single parabolic band model is developed and predicts that Ca_(4.75)Na_(0.25)Al_2Sb_6 possesses a near-optimal carrier concentration for thermoelectric power generation. A maximum zT > 0.6 is obtained at 1000 K.Beyond thermoelectric applications, the semiconductor Ca_5Al_2Sb_6 possesses a 1D covalent structure which should be amenable to interesting magnetic interactions when appropriately doped.

Toward understanding the origin of positive effects of ionic liquids on catalysis: formation of more reactive catalysts and stabilization of reactive intermediates and transition states in ionic liquids.

Abstract: Over the past decade, ionic liquids have received a great deal of attention as a new means for catalyst immobilization. Large numbers of catalysts having polar or ionic character have been successfully immobilized in ionic liquids, thus allowing their recovery and recycling. However, catalyst immobilization is not the only benefit of ionic liquids in catalysis, of greater importance are the positive effects of ionic liquids on catalytic rates. In this Account, we highlight our work in elucidating the origin of the accelerating effects of ionic liquids in a range of catalytic reactions. Lewis acidic metal triflates often become much more reactive in ionic liquids containing noncoordinating anions as a result of "anion exchange." Consequently, the more electrophilic Lewis acidic species generated in situ accelerate the catalytic reactions dramatically. In some cases, highly reactive intermediates, such as vinyl cations, arenium cations, oxygen radical anions, and so forth, can be stabilized in the presence of ionic liquids, thus increasing the reactivity and selectivity of the reactions. Concerted processes such as S(N)2 and Diels-Alder reactions can also be accelerated through the cooperative activation of both the nucleophile and the electrophile by ionic liquids. In transition metal-catalyzed reactions, certain catalytically active oxidation states can be stabilized in ionic liquids against deactivation to catalytically inactive species. Thus it is clear that gaining an understanding of the origin of these "positive ionic liquid effects" is highly important, not only for predicting the effects of ionic liquids on other organic reactions but also for designing new catalytic reactions. Ionic liquids, by virtue of (typically) having a synthetically accessible carbon backbone, are amenable to tailoring by the organic chemist. Accordingly, their molecular structures can be subtly varied to give "tunable" properties, which can then be used to rationally examine the fundamental reasons that they accelerate catalyzed reactions. Although the origins of enhanced catalytic rates by ionic liquids have been elucidated in many areas, other undiscovered ionic liquid phenomena remain to be unearthed. Developing a better understanding of these modularly tunable liquid salts will foster new discoveries of catalytic reactions that are accelerated by ionic liquids as solvents or additives.

Cellular Uptake of Densely Packed Polymer Coatings on Gold Nanoparticles

Abstract: A variety of functional polymer chains prepared by RAFT were directly grafted onto 5, 10, and 20 nm gold nanoparticles (AuNPs). The polymer shell coating the AuNPs was densely packed because of the strong binding between the trithioester groups on the polymer chain-ends and gold. It was found that due to the densely packed nature of the shell the polymer chains were significantly stretched compared to their usual Gaussian coil conformation in water. This was even evident for polymer chains where ionic repulsion between neighboring chains should be significant. Therefore, with such high grafting densities the surface properties and size of the hybrid nanoparticles should be the only contributing factors in cellular uptake in epithelial Caco-2 cells. This study has provided valuable insight into the effects of charge and size of NPs for the application of NPs in the delivery of therapeutic agents across the intestine. Our results showed that the negatively charged AuNPs were taken up by the cells with great...

Crystal Structures and Properties of Large Protonated Water Clusters Encapsulated by Metal−Organic Frameworks

Abstract: A large ionic water cluster H(H2O)28+, consisting of a water shell (H2O)26 and an encaged species H(H2O)2+ as a center core, was trapped in the well-modulated cavity of a porous metal−organic frame...

Low‐Temperature Superionic Conductivity in Strained Yttria‐Stabilized Zirconia

Abstract: Very high lateral ionic conductivities in epitaxial cubic yttria-stabilized zirconia (YSZ) synthesized on single-crystal SrTiO3and MgO substrates by reactive direct current magnetron sputtering are reported. Superionic conductivities (i.e., ionic conductivities of the order similar to 1 Omega(-1)cm(-1)) are observed at 500 degrees C for 58-nm-thick films on MgO. The results indicate a superposition of two parallel contributions - one due to bulk conductivity and one attributable to conduction along the film substrate interface. Interfacial effects dominate the conductivity at low temperatures (andlt;350 degrees C), showing more than three orders of magnitude enhancement compared to bulk YSZ. At higher temperatures, a more bulk-like conductivity is observed. The films have a negligible grain-boundary network, thus ruling out grain boundaries as a pathway for ionic conduction. The observed enhancement in lateral ionic conductivity is caused by a combination of misfit dislocation density and elastic strain in the interface. These very high ionic conductivities in the temperature range 150-500 degrees C are of great fundamental importance but may also be technologically relevant for low-temperature applications.

Nanoscale morphology in precisely sequenced poly(ethylene-co-acrylic acid) zinc ionomers.

Abstract: The morphology of a series of linear poly(ethylene-co-acrylic acid) zinc-neutralized ionomers with either precisely or randomly spaced acid groups was investigated using X-ray scattering, differential scanning calorimetry (DSC), and scanning transmission electron microscopy (STEM). Scattering from semicrystalline, precise ionomers has contributions from acid layers associated with the crystallites and ionic aggregates dispersed in the amorphous phase. The precisely controlled acid spacing in these ionomers reduces the polydispersity in the aggregate correlation length and yields more intense, well-defined scattering peaks. Remarkably, the ionic aggregates in an amorphous, precise ionomer with 22 mol % acid and 66% neutralization adopt a cubic lattice; this is the first report of ionic aggregate self-assembly onto a lattice in an ionomer with an all-carbon backbone. Aggregate size is insensitive to acid content or neutralization level. As the acid content increases from 9.5 to 22 mol % at ∼75% neutralizati...

Ion-Specific Effects under Confinement: The Role of Interfacial Water

Abstract: All-atom molecular dynamics simulations were employed for the study of the structure and dynamics of aqueous electrolyte solutions within slit-shaped silica nanopores with a width of 1067 A at ambient temperature All simulations were conducted for 250 ns to capture the dynamics of ion adsorption and to obtain the equilibrium distribution of multiple ionic species (Na+, Cs+, and Cl−) within the pores The results clearly support the existence of ion-specific effects under confinement, which can be explained by the properties of interfacial water Cl− strongly adsorbs onto the silica surface Although neither Na+ nor Cs+ is in contact with the solid surface, they show ion-specific behavior The differences between the density distributions of cations within the pore are primarily due to size effects through their interaction with confined water molecules The majority of Na+ ions appear within one water layer in close proximity to the silica surface, whereas Cs+ is excluded from well-defined water layers

Pyrazoles and imidazoles as ligands: Part I. Some simple metal(II) perchlorates and tetrafluoroborates solvated by neutral pyrazole and imidazole

Abstract: A number of new solvates containing pyrazole (Pz) and imidazole (Iz) as ligands, of the general formula M(ligand)n(anion)2 is reported. In these compounds M = Mg, Mn, Fe, Co, Ni, Cu, Zn and Cd, the anions are ClO4− and BF44−, and n is usually 6; n = 4 in the case of the Cu compounds and for Zn in combination with Iz. The compounds are characterized and identified by chemical analysis and physical measurements. With the aid of ligand-field spectra in combination with X-ray powder patterns, octahedrally coordinated metal ions are found in compounds of stoichiometry M(ligand)6(anion)2. Both Pz and Iz are placed in the spectro-chemical and nephelauxetic series. The spectrochemical series appears to be (with increasing Dq): CH3CN NH3 > Iz > Pz > en. The magnetic moments for the transition-metal compounds fall within the range usually observed for octahedrally coordinated ions. Both paramagnetic resonance measurements and the deviation of the magnetic moments from the spin-only value, indicate a considerable amount of covalency in the metal-ligand bond. Infrared spectroscopy evidently shows pure ionic ClO4− and BF4− for all solvates, except those of Cu(II) where the anions are coordinated to the metal ion. Except for the N-H modes, no large shifts occur in the ligand infrared absorptions with respect to the free ligand. Compared with the gaseous ligands, the N-H stretch in the solvates is decreased, whereas the bending modes are increased in frequency. This is attributed to - weak - hydrogen bonding with the anions in the lattice of these solvates. Metal-ligand stretching frequencies for both ligands occur in the 165-330 cm−1 range, and follow the Irving-Williams sequence of metal ions.

Nanostructured Liquid Crystals Combining Ionic and Electronic Functions

Abstract: New molecular materials combining ionic and electronic functions have been prepared by using liquid crystals consisting of terthiophene-based mesogens and terminal imidazolium groups. These liquid crystals show thermotropic smectic A phases. Nanosegregation of the pi-conjugated mesogens and the ionic imidazolium moieties leads to the formation of layered liquid-crystalline (LC) structures consisting of 2D alternating pathways for electronic charges and ionic species. These nanostructured materials act as efficient electrochromic redox systems that exhibit coupled electrochemical reduction and oxidation in the ordered bulk states. For example, compound 1 having the terthienylphenylcyanoethylene mesogen and the imidazolium triflate moiety forms the smectic LC nanostructure. Distinct reversible electrochromic responses are observed for compound 1 without additional electrolyte solution on the application of double-potential steps between 0 and 2.5 V in the smectic A phase at 160 degrees C. In contrast, compound 2 having a tetrafluorophenylterthiophene moiety and compound 3 having a phenylterthiophene moiety exhibit irreversible cathodic reduction and reversible anodic oxidation in the smectic A phases. The use of poly(3,4-ethylenedioxythiophene)-poly(4-styrene sulfonate) (PEDOT-PSS) as an electron-accepting layer on the cathode leads to the distinct electrochromic responses for 2 and 3. These results show that new self-organized molecular redox systems can be built by nanosegregated pi-conjugated liquid crystals containing imidazolium moieties with and without electroactive thin layers on the electrodes.

Ion Conduction in Imidazolium Acrylate Ionic Liquids and their Polymers

Abstract: Polymerizable imidazolium acrylates and their polymers with pendant imidazolium cations were synthesized with hexafluorophosphate and bis(trifluoromethanesulfonyl)imide counterions and characterized using calorimetry and dielectric spectroscopy. The ionic polymers containing a diethyleneoxy unit as an N-substituent on the imidazolium cation display higher ionic conductivities than the analogous N-n-butyl polymers. Using a physical model of electrode polarization, we separate the conductivity of single-ion conductors into number density of conducting ions p and their mobility μ. The monomers invariably possess higher conducting ion number density than the polymers, owing to the cation being part of the polymer, but p is insensitive to the N-substituent. In contrast, the diethyleneoxy N-substituent imparts higher mobility than the n-butyl N-substituent, for both monomers and polymers, owing to a lower binding energy between the imidazolium and the counteranions, which is not directly reflected in glass tran...

A Low-Temperature Bonding Process Using Mixed Cu–Ag Nanoparticles

Abstract: A low-temperature bonding process to form joints with high strength and ionic migration resistance using mixed Cu–Ag nanoparticles was studied Although it was difficult to obtain strong joints using Cu nanoparticles, with the addition of Ag nanoparticles to the Cu nanoparticles the bonding strength of the Cu-to-Cu joints increased The joints formed by the mixed Cu–Ag nanoparticles at 350°C exhibited a high bonding strength of ~50 MPa Counterelectrodes made of the mixed Cu–Ag nanoparticles had four times higher ionic migration resistance compared with counterelectrodes made only of Ag nanoparticles

Preparation of protic ionic liquids with minimal water content and 15N NMR study of proton transfer

Abstract: Low-molecular-weight Bronsted acids and amine bases were used to reproducibly prepare very dry, high-purity room-temperature protic ionic liquids (PILs). A series of eight amine bases and six Bronsted acids were combined to produce 48 mixtures, of which 18 were liquid at room temperature. The phase transitions and thermal decomposition temperatures were determined for each mixture; whereas viscosity, density and conductivity were determined for the room-temperature liquids. By utilising 15N NMR it was possible to distinguish between neutral and ionised amine bases (ammonia vs. ammonium-type ion), which indicated that the protic ionic liquids were completely ionised when made as a stoichiometric mixture. However, a Walden plot comparison of fluidity and molar conductivity indicated the majority of PILs had much lower conductivity than predicted by viscosity unless the base contained excess proton-donating groups. This disparity is indicative of protic ionic molecules forming neutral aggregates or non-Newtonian fluid hydrogen-bonded networks with a secondary Grotthuss proton-hopping mechanism arising from polyprotic bases.