Alkali Counterions Research Articles

Synthesis and structural characterization of a hydrated sodium–caesium tetracosatungstate(VI), Na5Cs19[W24O84]·21H2O

Crystal formation of pentasodium nonadecacesium tetracosatungstate(VI) heneikosahydrate, Na5Cs19[W24O84]·21H2O, was successfully achieved by the conversion of [H2W12O42]10− through the addition of excess Cs+. The crystal structure comprising the toroidal isopolyoxidometalate is presented, as well as its Raman spectrum. Na5Cs19(H2O)21W24O84 crystallizes in the rhombohedral space group R\overline{3} with an obverse centering. The title compound represents the addition of a new member to the isopolytungstate family with mixed alkali counter-ions and contains rarely observed five-coordinate tungsten(VI) atoms in the [W24O84]24− anion (site symmetry C 3i ) arising from the conversion mediated by Cs+ counter-ions.

A trade-off between hardness and stretchability of associative networks during the sol-to-gel transition

The trade-off between hardness and stretchability is a cornerstone of materials science. Balancing this trade-off is important in the molecular design of both chemical and physical networks. In this study, we report the quantitative trade-off at the molecular level for physical networks. Namely, we analyze, based on the reversible gelation model, a scaling relationship between the characteristic terminal relaxation modulus Gc in linear viscoelasticity and the stretch ratio λmax at the stress overshoot during the nonlinear elongation flow for unentangled randomly associative polymers, i.e., λmax ∼ Gc−0.17 and λmax ∼ Gc−0.33 in the mean-field and critical-percolation regimes, respectively. We use sulfonated polystyrene having different alkali counterions as a model system to test the relationship. The exponent of λmax ∼ Gc−0.25 seen in the experiment is in between the two theoretical values. We also discuss the quantitative deviation with respect to the size distribution of the network strands.

Dependency of the mobility of carboxymethylated cellulosic polyions on the nature of their counterions

In this paper, we propose a new approach in order to interpret the dependency of the conductibility of carboxymethylated cellulosic polyelectrolytes XCMC1, XCMC2, XCMC3 in water at 25 °C, with their alkali counterions (X ≡ Li+, Na+ and K+) or ammonium counterions (X ≡ Et4N+ and Bu4N+).The 750 ring-blocks of each CMCi chain are randomly substituted by CH2COO groups, of electric charge (eαX), where αX is the degree of dissociation of the counterions X. It is found that for a given CX ≤ 7 10−3 mol·l−1, the experimental equivalent conductivity λP,X of the polyion CMCi decreases as follows: λP, K+ > λP, Na+ ≥ λP,Li+ > λP,Et4N+ > λP,Bu4N+. This behavior cannot be explained either by a specific ionic condensation, or in terms of hydrodynamic friction or ionic friction undergone by the polyion, or in terms of change of configuration of the polyions (ellipsoidal configuration). However, calculations taking into account the dielectric friction effect according to our new general formula, recently published, giving the dielectric friction coefficient, βPdf, for pearl-necklace-like polyelectrolyte chains, allowed to obtain good convergence between experimental and theoretical results. This new approach improves the interpretations given by Manning and Vink in terms of electrophoretic effect only.

Alkali-amide controlled selective synthesis of 7-azaindole and 7-azaindoline through domino reactions of 2-fluoro-3-methylpyridine and aldehydes

An alkali counterion controlled chemoselective synthesis of 7-azaindole and 7-azaindoline has been demonstrated.

Planar Hypercoordinate Carbons in Alkali Metal Decorated CE3 2- and CE2 2- Dianions.

After exploring the potential energy surfaces of Mm CE2 p (E=S-Te, M=Li-Cs, m=2, 3 and p=m-2) and Mn CE3 q (E=S-Te, M=Li-Cs, n=1, 2, q=n-2) combinations, we introduce 38 new global minima containing a planar hypercoordinate carbon atom (24 with a planar tetracoordinate carbon and 14 with a planar pentacoordinate carbon). These exotic clusters result from the decoration of V-shaped CE2 2- and Y-shaped CE3 2- dianions, respectively, with alkali counterions. All these 38 systems fulfill the geometrical and electronic criteria to be considered as true planar hypercoordinate carbon systems. Chemical bonding analyses indicate that carbon is covalently bonded to chalcogens and ionically connected to alkali metals.

Contribution of Different Molecules and Moieties to the Surface Tension in Aqueous Surfactant Solutions. II: Role of the Size and Charge Sign of the Counterions.

Understanding the role of the counterion species in surfactant solutions is a complicated task, made harder by the fact that, experimentally, it is not possible to vary independently bulk and surface quantities. Here, we perform molecular dynamics simulations at constant surface coverage of the liquid/vapor interface of lithium, sodium, potassium, rubidium, and cesium dodecyl sulfate aqueous solutions. We investigate the effect of counterion type and charge sign on the surface tension of the solution, analyzing the contribution of different species and moieties to the lateral pressure profile. The observed trends are qualitatively compatible with the Hofmeister series, with the notable exception of sodium. We point out a possible shortcoming of what is at the moment, in our experience, the most realistic nonpolarizable force field (CHARMM36) that includes the parametrization for the whole series of alkali counterions. In the artificial system where the counterion and surfactant charges are inverted in sign, the counterions become considerably harder. This charge inversion changes considerably the surface tension contributions of the counterions, surfactant headgroups, and water molecules, stressing the key role of the hardness of the counterions in this respect. However, the hydration free energy gain of the counterions, occurring upon charge inversion, is compensated by the concomitant free energy loss of the headgroups and water molecules, leading to a negligible change in the surface tension of the entire system.

Thermal annealing of iridescent cellulose nanocrystal films

The properties of chiral nematic and iridescent cellulose nanocrystal films with different monovalent cations (CNC-X) obtained through evaporation-induced self-assembly (EISA) can be modified by a variety of external stimuli. Here, we study the transformations of their optical and structural properties when the films are thermally annealed at 200 °C and 240 °C for up to 2 days. The chiral nematic structure of the most thermally stable films is not destroyed even after extensive heating due to the thermochemical stability of the cellulose backbone and the presence of surface alkali counterions, which suppress catalysis of early stage degradation. Despite the resilience of the cholesteric structure and the overall integrity of heated CNC-X films, thermal annealing is often accompanied by reduction of iridescence, birefringence, and transparency, as well as formation of degradation products. The versatility, sustainability, and stability of CNC-X films highlight their potential as temperature indicators and photonic devices.

Effect of Counterion Binding to Swelling of Polyelectrolyte Brushes.

The effect of binding strength of counterions with the polyelectrolyte chain to the swelling of polyelectrolyte brushes is studied, by investigating the swelling of both the polycation and polyanion in response to the variation of the salt concentration under the change of counterion's identity. Two polyelectrolyte brushes grafted on solid substrates are adopted: the cationic poly [2-(methacryloyloxy)ethyltrimethyl ammonium] (PMETA-X, X = F, Cl, Br, and I) and the anionic polystyrene sulfonate (M-PSS, M = Li, Na, K, and Cs). The swelling change with the salt concentration is investigated by ellipsometry, quartz crystal microbalance with dissipation, and dielectric spectroscopy. It is discovered that although the thickness of PMETA-X brushes is larger than that of M-PSS brushes of similar grafting density, the former has much less solvent incorporated than the latter. Such a difference is attributed to the weaker interaction between the PMETA+ chain and its halide counterions than that between the PSS- chain and its alkali counterions, discovered by dielectric spectroscopy. This makes the original charges on the PMETA-X chain less neutralized and therefore have a higher charge density, compared with the M-PSS chain. The results demonstrate that the stronger binding of the counterions to the polyelectrolytes makes the main chains less charged, resulting in the weaker inter-chain electrostatic repulsion and less swelling of the brushes. Investigations into the effect of ion identity show the following order of binding strength: for the cationic PMETA+ chain, F- < Cl- < Br- < I- and for the anionic PSS- chain, Li+ < Na+ < K+ < Cs+.

Specificity of Counterion Binding to a Conjugated Polyelectrolyte: A Combined Molecular Dynamics and NOESY Investigation.

Poly(thiophen-3-ylacetic acid) (PTAA) is a representative of conjugated polyelectrolytes which are used in many optoelectronics devices. The performance of these devices is affected by the polymer conformation, which, among others, depends on the nature of the counterion. In this study, the binding of tetrabutylammonium counterions (TBA+) on PTAA was determined using a combination of nuclear Overhauser effect spectroscopy (NOESY) and molecular dynamics (MD) simulation. It was found that TBA+ ions specifically bind on the hydrophobic main chain of PTAA, while, according to MD simulations, alkali counterions predominantly bind in the vicinity of negatively charged carboxylic groups located on side chains. The MD trajectories were used to compute the relaxation matrices and the NOESY spectra. With the help of these latter calculations, the changes of intensities in experimental NOESY spectra upon binding of TBA+ ions to PTAA were interpreted.

Thermoelectric Performance of n‐Type Poly(Ni‐tetrathiooxalate) as a Counterpart to Poly(Ni‐ethenetetrathiolate): NiTTO versus NiETT

AbstractThe synthesis of the metal‐coordination polymer poly(nickel tetrathiooxalate) (NiTTO) is presented, which represents an alternative route to n‐type thermoelectric materials similar in nature to nickel ethenetetrathiolate (NiETT) polymers. The TTO monomer is synthesized through an electrochemical reduction of carbon disulfide, followed by coordination polymerization with a nickel(II) salt to yield a coordination complex polymer with a neutral repeat unit. An alkali metal counterion exchange and polymerization optimization are performed, and a thermoelectric power factor of 6 µW m−1 K−2 (electrical conductivity of over 10 S cm−1) is achieved for NiTTO composite films in a polyvinylidene fluoride (PVDF) matrix. This is significantly higher than other reported n‐type polymers (and to within a factor of two of NiETT) and demonstrates the potential of NiTTO as a new thermoelectric material. Elemental analysis indicates a nearly neutral backbone structure in NiTTO with minimal charge balancing alkali ions, indicating that these polymers are more oxidized than NiETTs. X‐ray photoelectron spectroscopy reveals a different coordination environment for NiTTO compared to NiETTs with the same alkali counterion, indicating that—despite their structural similarity in theory—these two materials are indeed different.

Uranyl-Peroxide Capsule Self-Assembly in Slow Motion.

Uranyl-peroxide capsules are the newest family of polyoxometalates. Although discovered 13 years previously with over 70 topologies reported, there is a lack in the fundamental understanding of assembly mechanisms, particularly the role of the alkali counterions. Herein, the reaction pathway and assembly of uranyl peroxide capsules is reported by tracking the conversion from K+ uranyl triperoxide monomer to the K+ uranyl-peroxide U28 capsule by means of small-angle X-ray scattering and Raman spectroscopy. For the first time, the K+ uranyl-peroxide pentamer face is isolated and structurally characterized, giving credence to the long-held belief that these geometric faces serve as building blocks to the fully formed capsules. Once isolated and re-dissolved, the pentamer face undergoes rapid conversion to capsule forms, underlining its high reactivity that challenges its isolation. Calorimetric measurements of the studied species confirms the pentamer lies on the energy landscape between the monomer and capsule.

Cyclodextrin-Driven Formation of Double Six-Ring (D6R) Silicate Cage: NMR Spectroscopic Characterization from Solution to Crystals

Identification and isolation of secondary building units (SBUs) from synthesis media of zeolites still represent a challenging task for chemists. The cage structure anion Si12O3012− known as the double six-ring (D6R) was synthesized from α-cyclodextrin (α-CD) mediated alkaline silicate solutions and conditions of its stability and reactivity in aqueous solution were studied by using nuclear magnetic resonance (NMR) spectroscopy. A single crystal X-ray diffraction (XRD) analysis revealed a novel polymorph of the hybrid complex K12Si12O30·2α-CD·nD2O (n ≈ 30–40), which crystallizes in the orthorhombic C2221 space group symmetry with a = 14.841(4) Å, b = 25.855(6) Å, and c = 41.91(1) Å. The supramolecular adduct of the silicate anion sandwiched by two α-CDs forms a perfect symmetry matching the H-bonding donor-acceptor system between the organic macrocycle and the D6R unit. The driving force of such a hybrid assembly has found to be strongly dependent on the nature of the cation present as large alkali counter ions (K+, Rb+ and Cs+), which stabilize the D6R structure acting as templates. Lastly, we provided the first 29Si MAS NMR measurement of 3Q Si in an isolated D6R unit that allows the verification of the linear correlation between the chemical shift and &lt;SiOSi&gt; bond angle for 3Q Si species in DnR cages (n = 3, 4, 6).

Alkali Counterions Impact Crystallization Kinetics of Apatite Nanocrystals from Amorphous Calcium Phosphate in Water at High pH

Crystallization from amorphous precursors emerges as a major principle in biomineralization, but the mechanisms and kinetics involved remain unclear for many materials. Herein, we used in situ synchrotron X-ray diffraction to study crystallization of apatite from amorphous calcium phosphate at a pH where the solution is dominated by phosphate to circumvent the formation of acidic calcium phosphates. The initial crystals were almost isometric. Highly anisotropic growth resulted in needle shaped crystals at later times. Changing the counterion from sodium to potassium lead to changes in nucleation time, growth speed, and nanocrystal shape but preserved the overall crystallization mechanism.

Alkali-Driven Disassembly and Reassembly of Molecular Niobium Oxide in Water.

Counterions are deemed "spectators" in aqueous solutions of cationic or anionic molecular metal-oxo clusters. While pH and concentration drive aqueous metal speciation as a first approximation, the important effect of counterions is usually overlooked and never considered in standard Pourbaix databases. Alkali counterions for polyoxometalate (POM) clusters control solubility with distinct periodic trends, but evidence for alkali control over speciation is ambiguous. Here we show that a simple Nb-POM, [Nb10O28]6- ({Nb10}), converts to oligomers of (H xNb24O72)(24- x)- ({Nb24}) upon adding only alkali chloride salts, even in buffered neutral solutions. Raman and X-ray scattering reveal that the rate of {Nb10} to {Nb24} conversion increases with alkali cation radius and cation concentration. Cation-bridged oligomers of {Nb24} y ( y = 2,4) are defined by comparing experimental to computed small-angle X-ray scattering spectra. Computational studies and mass spectrometry indicate that the alkalis open the compact {Nb10} cluster in conjunction with protonation of a heptamer {Nb7} intermediate, in which alkali-{Nb10} association at key locations on the cluster initiates the reaction. Computation also explains the alkali periodic trend for {Nb10} to {Nb24} conversion; larger alkalis more effectively destabilize {Nb10}. This periodic trend asserts the hypothesis that Nb-cluster speciation near neutral pH is driven by the alkali cations in the absence of added base or acid. The extremely high solubility of these 3.5 nm polyoxoanion assemblies-2 M Nb at near neutral pH-is both surprising and exploitable for aqueous synthesis of niobate thin films or nanomaterials used in energy and microelectronics applications.

Role of Counterion in the Adsorption Behavior of 1:1 Ionic Surfactants at Fluid Interfaces-Adsorption Properties of Alkali Perfluoro-n-octanoates at the Air/Water Interface.

Equilibrium surface tension (σe) versus bulk concentration (c) isotherms of aqueous, surface-chemically pure solutions of various alkali perfluoro-n-octanoates were measured at 295 K. These 1:1 ionic surfactant systems belong to the pseudo nonionic ones. Evaluating the different σe vs log c isotherms by basic adsorption equations reveals that they follow ideal surface behavior. The novelty of this investigation exists in the fact that the surface area demand per molecule adsorbed calculated from the experimental σe vs log c isotherms is identical to that of the hydrated alkali cation. Thus, as long as the counterion's cross-sectional area is greater than that of its amphiphilic anion, the amphiphile's total surface area demand will exclusively be governed by that of its alkali counterion. This, in turn, means that the counterion is nonrandomly bound to the amphiphilic anion in the adsorption layer. Furthermore, the size of the hydrated alkali counterion in the adsorption layer does not differ from that in the bulk phase.

A Spontaneous Structural Transition of {U24 Pp12 } Clusters Triggered by Alkali Counterion Replacement in Dilute Solution.

A transition between two isomeric clusters involving the change of the main skeleton structure of a well-defined, rigid molecular cluster [(UO2 )24 (O2 )24 (P2 O7 )12 ]48- , {U24 Pp12 }, is achieved by simply introducing proper alkali cations into its dilute aqueous solution. While the unique structural transition can be triggered by introducing any of the Na+ /K+ /Rb+ /Cs+ alkali ions, the two isomers, Li/Na-{U24 Pp12 } and Na/K-{U24 Pp12 }, as typical macroions, can accurately choose among different alkali counter-cations based on their hydrated sizes, and the ion selectivity process clearly showed endothermic features. The preferred K+ and Rb+ ions have suitable sizes to be incorporated into the proper windows on {U24 Pp12 } nanocapsules, as supported by the transition points in both ITC studies and IR measurements.

Preliminary investigations on a simple polyelectrolyte derived from (CH2CH2C(COOH)2)n: Unexpected solubility-insolubility pattern controlled selectively by the nature of the alkali counterion

Monodisperse samples of alkali poly(trimethylene-1,1-dicarboxylate) polyelectrolytes of (CH2CH2C(COO−)2)n structure and of low to moderate molecular weights have been synthesized and characterized. These polymers are densely and evenly decorated by carboxylate groups, with the anionic moieties extending symmetrically in transverse directions on every third carbon alongside the backbone. In sharp contrast to other polyelectrolytes, they easily precipitate with sodium and potassium ions in water. The precipitation is selective, the solubility varying with the nature of the alkali cation in the order: Li+ > K+ > Na+. Precipitation is fast and reversible, the solubility-insolubility transition being rapidly crossed in both directions when the proper alkali ion is added in excess to the solution/suspension. Experimental investigations (WAXS, FTIR) on the precipitated polymer and DFT theoretical calculations on model 1:1 complexes of malonate subunits with alkali cations have been carried out in order to cast some light on the above phenomena. In addition, a model molecule (CH3C(COOH)2CH2CH2C(COOH)2CH3) has been prepared, supporting the structure of the obtained polyelectrolyte as well as the formation of specific structures at half-deprotonation of the poly(carboxylic acid).

Factors influencing the photocatalytic activity of alkali Nb[sbnd]Ta perovskites for hydrogen production from aqueous methanol solutions

The activity of ATa0.5Nb0.5O3 (A: Li, Na or K) photocatalysts for H2 production has been studied using methanol aqueous solutions. Previous studies showed that photocatalysts with 1:1 combination of Ta and Nb showed greatly improved activity compared to the single-metal oxides. In this work, compositional variations have shown that sodium suits best as alkali counterion for H2 evolution. Variation of reaction parameters like pH and methanol concentration shows that neutral pH and 10 vol.% methanol lead to the best activity of NaNb0.5Ta0.5O3 perovskite. In addition, a notable increase in the H2 evolution is observed by the use of Pt as co-catalyst, and the variation of its loading amount has also a great influence on the photocatalytic performance. The highest H2 production rate is obtained with the NaNb0.5Ta0.5O3 catalyst with 0.125 wt.% of Pt loading under UV irradiation.

Self-Assembly of Uranyl-Peroxide Nanocapsules in Basic Peroxidic Environments.

A wide range of uranyl-peroxide nanocapsules have been synthesized using very simple reactants in basic media; however, little is known about the process to form these species. We have performed a density functional theory study of the speciation of the uranyl ions under different experimental conditions and explored the formation of dimeric species via a ligand exchange mechanism. We shed some light onto the importance of the excess of peroxide and alkali counterions as a thermodynamic driving force towards the formation of larger uranyl-peroxide species.

Dynamics of Uranyl Peroxide Nanocapsules

Discrete aqueous metal oxide polyionic clusters that include aluminum polycations, transition-metal polyoxometalates, and the actinyl peroxide clusters have captivated the interest of scientists in the realm of both their fundamental and applied chemistries. Yet the counterions for these polycations or polyanions are often ignored, even though they are imperative for solubility, crystallization, purification, and even templating cluster formation. The actinyl peroxide clusters have counterions not only external, but internal to the hollow peroxide capsules. In this study, we reveal the dynamic behavior of these internal alkali counterions via solid-state and liquid NMR experiments. These studies on two select cluster geometries, those containing 24 and 28 uranyl polyhedra, respectively, show that the capsules-like clusters are not rigid entities. Rather, the internal alkalis both have mobility inside the capsules, as well as exchange with species in the media in which they are dissolved. The alkali mobilities are affected by both what is inside the clusters as well as the composition of the dissolving medium.