Solvation energy and gas-phase stability influences on alkali metal cluster ion formation in electrospray ionization mass spectrometry

The ability to observe abundant gas-phase metal cluster ions in electrospray ionization mass spectrometry (ESI-MS) is highly dependent on experimental conditions. Alkali halides (MX) and other alkali metal salts were used to investigate the formation of cluster ions in ESI-MS. All compounds were found to give cluster ions of the form (M(n)(+1)X(n))(+) and (M(n)X(n+1))(-), with only two alkali salts yielding doubly charged cluster ions. In homologous alkali halide series, the relative abundances of cluster ions increased with increasing size of either the cation (positive ion mode) or the anion (negative ion mode). Calculations using an electrostatic model show that the gas-phase stability of cluster ions is greater for smaller cations or anions when a fixed counterion is employed. This stability calculation goes in a direction just opposite to the trend in cluster ion abundances observed in ESI-MS. Studies of equimolar mixtures consisting of two alkali halides reveal two distinct trends. When the equimolar mixture was composed of differing ions that participate in the droplet charge excess with the same counterion, the less solvated ions were found to form more abundant cluster ions. When the ions participating in the charge excess were fixed, the preferred counterion in observed clusters was the one that is more solvated in solution and forms more stable clusters in the gas phase. These observations can be rationalized by an extended form of the charged residue model where the weakly solvated ions that are part of the charge excess are preferentially enriched in offspring droplets during uneven fission. By contrast, transfer of a particular counterion located in the bulk of the droplets to the offspring droplets is not disfavored when this counterion is strongly solvated.     

THERMAL ANALYSIS OF ION-EXCHANGED MONTMORILLONITE

As part of its program in mineral waste management, the Bureau of Mines has investigated the effect of ion exchange on the dewatering potential of clay minerals. Clays of the smectite group possess exchangeable cations to compensate for the overall net negative charge of the clay lattice. The ratio of the valence of the exchange cation to its ionic radius, referred to as the ionic potential, is indicative of the electrostatic influence that a given cation can exert on adjacent molecules. Differential scanning calorimetry (DSC) and thermogravimetric analyses (TG) were performed on eight ion-exchanged montmorillonices (Li⁺, Na⁺, K⁺, NH₄⁺, Ca²⁺, Ba²⁺, Mg²⁺, and ^Sr2+). Results of these analyses indicate that the energy required to dehydrate the ion-exchanged montmorillonites and the associated dehydration water loss are directly related to the ionic potential of the exchange cation. Furthermore, the ordering influence of the exchange ion is responsible for different hydration states within the interlayer water.     

The preparation and ion-exchange properties of zirconium sulphophosphonates

A number of layered zirconium phosphite arylphosphonate and zirconium phosphate arylphosphonate mixed derivatives have been prepared. These compounds were readily sulfonated in fuming sulfuric acid and the resultant sulfonates behaved as strong-acid ion exchangers. In the case where the aryl group bridged across layers, a non-swelling exchanger resulted, and these solids showed ion exclusion towards large complexes. They also exhibited the selectivity sequences Li⁺ > Na⁺ < K⁺ and Mg²⁺ > Ca²⁺ < Ba²⁺. In the exchanger in which the layers were not connected, large degrees of swelling, to the point of colloid formation, were observed. As a consequence, the rates of exchange were very high. The non-crosslinked exchanger showed increased selectivity for larger cations. In the case of coordination complexes the increased selectivity was remarkable and may stem from a combination of favorable electrostatic and steric factors.     

Physicochemical Reasons for the Use of TiOHPO4 for Treatment of Liquid Radioactive Waste

Heterogeneous exchange of the proton of hydrated titanyl hydrogen phosphate with Li, Na, K, Rb, and Cs cations in aqueous solutions was studied potentiometrically. The sorption equations were proposed. The concentration and thermodynamic equilibrium constants were calculated. The optimal pH range of the solution providing effective sorption of radioactive cesium on hydrated titanyl hydrogen phosphate was determined. The decrease in the efficiency of radiocesium sorption from liquid waste with a high content of competing nonradioactive alkali metal cations is due to a decrease in the pH of this solution in the course of the sorption. To provide efficient sorption removal of radiocesium, the pH should be maintained in the range 4-7, or the sodium form of the sorbent should be used. The selectivity of radiocesium sorption on TiOHPO₄ in the presence of excess sodium cation was estimated.     

Identification of metal cations, metal complexes, and anions by electrospray mass spectrometry in the negative ion mode

Pneumatically assisted electrospray mass spectrometry (ES-MS) is used in the negative ion mode for aqueous metal (M) solutions in an excess of hydrochloric or nitric acid, where the major anion X = Cl- or NO3-. A collision energy of approximately 20 eV removes anion-solvent clusters for most elements and leaves negative complex ions of the general form (Mn+Xn+1)-. Complexation with anions prevents charge reduction reactions at least to n = 3, even in cases where the third ionization energy of M greatly exceeds the first ionization energy of the solvent. These negative ions thus preserve the charge state of the metal cation from the solution and allow identification of both cations and anions in a single set of electrospray conditions. Cations such as Fe3+ or Cu2+ that have a lower oxidation state in solution produce a distribution of negative ions, each with a single negative charge overall; e.g., an Fe3+ solution produces both Fe(III)X4- and Fe(III)X3-. This distribution of FeIII and FeII species is attributed to electrochemical reduction of Fe3+ at the negatively charged ES needle. "Native" anions such as perrhenate or molybdate produce singly charged analogues such as ReO4- or HMoO4-. Metal-EDTA complexes are seen as M(III)Y- or M(II)HY-. The sensitivity for these "native" anions is suppressed by competition with the excess chloride or nitrate used to produce the metal-containing complex ions.     

Ion and lithium isotope selectivity of monoclinic antimonic acid

Antimonic acid with the monoclinic structure, M-SbA, which had a high selectivity for the lithium ion among alkali metal ions, was prepared by extracting lithium ions from LiSbO₃ through ion exchange with protons and its lithium isotope selectivity has been studied. M-SbA underwent the reversible monoclinic-orthorhombic structural change upon desorption-sorption of lithium ions while it showed the irreversible monoclinic-cubic structural change upon sorption of sodium ions. Isotopically, M-SbA was a ⁶Li-specific ion exchanger. The ⁷Li-to-⁶Li isotopic separation factor was slightly dependent on the kinds of counterion in the solution phase of the batch experiments and the maximum S value obtained at 25°C was 1.025.     

MoS2 Passivated Multilayer Graphene Membranes for Li-Ion Extraction From Seawater

Abundant Li resources in the ocean are promising alternatives to refining ore, whose supplies are limited by the total amount and geopolitical imbalance of reserves in Earth's crust. Despite advances in Li⁺ extraction using porous membranes, they require screening other cations on a large scale due to the lack in precise control of pore size and inborn defects. Herein, MoS₂ nanoflakes on a multilayer graphene membrane (MFs-on-MGM) that possess ion channels comprising i) van der Waals interlayer gaps for optimal Li⁺ extraction and ii) negatively charged vertical inlets for cation attraction, are reported. Ion transport measurements across the membrane reveal ≈6- and 13-fold higher selectivity for Li⁺ compared to Na⁺ and Mg²⁺, respectively. Furthermore, continuous, stable Li⁺ extraction from seawater is demonstrated by integrating the membrane into a H₂ and Cl₂ evolution system, enabling more than 10⁴-fold decrease in the Na⁺ concentration and near-complete elimination of other cations.     

Hydrogen substitution in lithium-aluminosilicates

Quartz and keatite are SiO₂ polymorphs with relatively dense frameworks containing small voids. Aluminosilicates with small charge compensating cations such as lithium, magnesium and zinc can adopt the same types of structures with the cations occupying the voids. It 1s shown that the lithium content in both quartz- and keatite-type (Li, Mg_0.5, Zn_0.5)AlSi₂O₆ solid solutions can be ion exchanged against hydrogen from hot concentrated H₂SO₄. This ion exchange can be reversed. At high temperatures water can irreversibly be driven off the hydrogen forms. Neither ion exchange nor thermal dehydration affects space group symmetry or framework linkage, but lattice parameter variations indicate considerable framework distortions. Thermal expansion also varies strongly with cation content. The data indicate that during dehydration aluminium ions leave the framework and enter the voids thereby providing charge compensation for the remaining framework aluminium content.[/p]     

New tobermorite cation exchangers

Tobermorite minerals, calcium silicate hydrates substituted with Al³⁺ and alkali, exhibit cation exchange and selectivity properties. The total cation exchange capacities of the Al³⁺ and alkali substituted tobermorites synthesized here range from 128 to 197 meq per 100g. These substituted tobermorites also have high selectivity for caesium and rubidium. For examples, a tobermorite synthesized from Na₂SiO₃, AlCl₃ and CaO has a caesium adsorption coefficientK _d of 7200 while another one synthesized from a zeolite and CaO has a caesium adsorptionK _d of 12 850 in 0.02N CaCl₂ containing 0.0002N CsCl. Pure tobermorites and tobermorites with only Al³⁺ substitution do not exhibit much cation exchange capacity or selectivity because of cation hydration effects and steric factors of tobermorites. The cation exchange and selectivity properties of the Al³⁺ and alkali substituted tobermorites fall between those of clay minerals and zeolites. Unlike clay minerals and zeolites, the new group of cation exchangers is expected to be thermodynamically stable in cement and concrete which have a similar chemical composition. Therefore, the new cation exchangers will be suitable for inexpensive solidification in cement after their use, for example, in decontamination of caesium from low-level nuclear wastes.     

Liquid-liquid extraction of alkali metal ions with photochromic crowned spirobenzopyrans

On the liquid-liquid extraction using 1,2-dichloroethane as an organic solvent, the crowned spirobenzopyrans exhibited textractability in the following order: Li+ > Na+ > K+ > or = tetramethylammonium ion (TMA+), Li+ > Na+ > K+ > TMA+, and Na+ > K+ > Li+ > TMA+ for spirobenzopyran derivatives bearing monoaza-12-crown-4, 1; monoaza-15-crown-5, 2; and monoaza-18-crown-6, 3; respectively, under dark conditions. The ion selectivity of 1 depends on the metal-ion complexing ability of monoaza-12-crown-4. Even 2, which carries a 15-crown-5 moiety, showed Li+ selectivity because of the much stronger interaction of Li+ with the phenolate ion of the merocyanine form of 2 than that of Na+. The Na+ selectivity of 3 is also attributed to the ionic interaction with the phenolate ion of the merocyanine form, since the ionic interaction prefers Na+ to K+ regardless of the higher affinity of the 18-crown-6 ring itself to the latter ion. The Li+ extraction into the organic phase with 1 was enhanced by UV irradiation (300-400 nm), while some depression in the extraction was found by visible irradiation (>500 nm). The effect of visible irradiation on the Li+ complexing ability of 1 was also examined with electrospray ionization mass spectrometry.     

Extraction of cesium ions from aqueous solutions using calix[4]arene-bis(tert-octylbenzo-crown-6) in ionic liquids

Solvent extraction of cesium ions from aqueous solution to hydrophobic ionic liquids without the introduction of an organophilic anion in the aqueous phase was demonstrated using calix[4]arene-bis(tert-octylbenzo-crown-6) (BOBCalixC6) as an extractant. The selectivity of this extraction process toward cesium ions and the use of a sacrificial cation exchanger (NaBPh(4)) to control loss of imidazolium cation to the aqueous solutions by ion exchange have been investigated.     

Atomic force microscope: a tool for studying ionophores

The aim of the investigations was to show the analytical use of an atomic force microscopy (AFM) tip coated with an ion-selective membrane and to show that the ion-selective boundary potential is detectable as a force induced by ion-selective electrostatic interactions, which are more pronounced than double-layer forces. This new technique allows the area-specific ion exchange over boundaries to be displayed with a destruction-free technique by AFM in an aqueous buffer. From experiments with ISEs (ion-selective electrodes), a boundary potential for valinomycin was assumed to be established in close vicinity to a K+-releasing surface. To trace this boundary potential, an AFM tip was coated with a potassium-selective polymer film containing valinomycin as used in preparing ISEs. The K+-releasing substrate was prepared by incorporating a lipophilic potassium salt into a plasticized PVC layer. In contact with an electrolyte such as sodium chloride solution, an ion exchange takes place. Analogue experiments were performed using a sodium-selective ionophore, DD16C5, incorporated into the coating of the AFM tip, with a Na+-releasing substrate. The boundary potential was traced and investigated with the help of force vs distance curves. The resulting adhesion forces for a valinomycin-coated tip in a 150 mM NaCl solution were 9.8+/-3.275 nN using a blank PVC substrate and 330.15+/-113.0 nN using a substrate containing 10 wt % potassium tetrakis(4-chlorophenyl) borate. The selectivity of the ion-selective AFM tips was measured on sodium relative to potassium-releasing substrates and studied in different salt solutions with concentrations between 10 mmol L(-1) and 1 mol L(-1). For valinomycin, a force selectivity coefficient log Kf(K,Na) of -2.5+/-0.5 for K+ against Na+ and a selectivity coefficient log Kf(Na,K) of -4 +/- -0.5 for DD16C5 were measured. In addition, the surface of the polymer substrate was imaged by AFM in contact mode with and without lipophilic potassium salt. The modulation of the force-distance curves induced by the ion exchange was compared to the experimental change in elasticity of the blank and ion-exchanging substrate. The Young's moduli measured with strain force analysis on a potassium-containing substrate were 5 times smaller than the ones registered with nanoindentation and did not explain the modulation of the force vs distance curves.     

A mechanism of separation in electrostatic ion chromatography

The retention mechanism of electrostatic ion chromatography (EIC) is currently under debate and is the focus of this paper. A comprehensive set of retention data has been obtained on a C18 column coated with the zwitterionic surfactant 3-(N,N-dimethylmyristylammonio)propanesulfonate used with a range of mobile phases in which both the mobile-phase anion and cation have been varied systematically. Electro-osmotic flow measurements were also obtained on fused-silica capillaries coated with the zwitterion (and also some monofunctional surfactants) and were used to evaluate the nature of the surface charge on the layer of adsorbed surfactant in the presence of various background electrolytes. A new retention mechanism for EIC was developed on the basis of these data. This mechanism proposes that equilibration of the bound zwitterions with a mobile phase containing a suitable electrolyte causes the establishment of a charged layer created by the terminal sulfonate groups of the zwitterion, which acts as a Donnan membrane. The magnitude and polarity of the charge on this membrane depends on the nature of the mobile-phase ions. The Donnan membrane exerts weak electrostatic repulsion or attraction effects on analyte anions. A second component of the retention mechanism is chaotropic interaction of the analyte anion with the quaternary ammonium functional group of the zwitterion. This interaction exerts the major effect on the separation selectivity of EIC, such that analyte anions are eluted in order of increasing chaotropic interactions in accordance with the Hofmeister series.     

Solvatation en milieu organique des cations intercales dans les xerogels de V2O5

V₂O₅ xerogels (V₂O₅, 1.6H₂O) undergo topotactic reversible exchange reaction at room temperature in organic solvents containing monovalent alkali or divalent (Mn²⁺) cations. Basal spacings are dependent on solvent type and charge to radius ratio of guest cations. From interlayer distances, two solvation stages have been inferred, depending on the nature of the solvent and of the cation, except with Cs⁺ for which no intracrystalline swelling by organic solvents is observed.     

Effect of Structural Variation within Lipophilic Lariat Ether Phosphonic Acid Monoesters on the Selectivity and Efficiency of Competitive Alkali Metal Cation Extraction into Chloroform

Lipophilic lariat ether phosphonic acid monoethyl esters with systematic crown ether ring size variation from 12-crown-4 to 24-crown-8 are utilized for competitive alkali metal cation extractions from aqueous solutions into chloroform. Effective alkali metal cation extraction from weakly acidic, neutral, and basic aqueous solutions is achieved. With 4, 5, and 6 oxygens in the crown ether rings, selectivities for Li(+), Na(+), and K(+), respectively, are observed. An 18-crown-6 phosphonic acid monoester exhibits excellent extraction selectivity for K(+) with K(+)/Li(+) and K(+)/Na(+) > 100. The lipophilic group attachment site, as well as the crown ether ring size, is shown to influence the extraction selectivity for the lariat ether phosphonic monoesters.     

An analogy of an ion-selective electrode sensor based on the voltammetry of microcrystals of tetracyanoquinodimethane or tetrathiafulvalene adhered to an electrode surface

The voltammetry of solid 7,7,8,8-tetracyanoquinodimethane (TCNQ) and tetrathiafulvalene (ITF) at an electrode-microparticle-aqueous (electrolyte) interface generates characteristic current-potential profiles associated with solid-solid-phase transformations. During the reactions, electrolyte ions are included into the TCNQ (cations) and TTF (anions) lattice sites as part of the charge neutralization process. Consequently, electrolyte ion concentration is associated with the reversible potential of the TCNQ0/- and TTF0/+ reactions, making these processes candidates for the development of novel voltammetric cation and anion sensors, respectively. Electrode potential-analyte ion concentration dependence studies exhibited highly reproducible potential shifts of 45 (+/- 1) mV/decade change in ion analyte concentration for both the TCNQ cation sensor and the TTF anion sensor. When presented with mixed-analyte solutions, both ion-sensing systems exhibited a degree of ion selectivity. Ion selectivity trends may be modeled using equations based on a Nicolsky-type selectivity relationship, in accordance with the concept that these are the voltammetric analogies of potentiometric ion-selective membrane electrodes.     

The nature of 'overexchanged' copper and platinum on zeolites

The uptake of platinum and copper tetra-ammine (PTA and CTA, [(NH(3))(4)Pt](2+) and [(NH(3))(4)Cu](2+)) into zeolites was compared over silica and three zeolites (Y, MOR and MFI) with different points of zero charge and aluminium content. Adsorption was determined as a function of pH at several metal concentrations, and pH shifts relative to metal free control experiments were carefully monitored. The uptake of both metal ammine complexes onto silica is well described by electrostatic adsorption. We suggest that the metal cations interact with zeolites by two mechanisms, ion exchange at the Al exchange sites and electrostatic adsorption at silanol groups. The former is the dominant mechanism at low to mid pH, and the latter at high pH. This effect is most clearly manifested in zeolites with low aluminium content such as ZSM5; electrostatic adsorption at high pH in ZSM5 yields metal loadings much in excess of the ion exchange capacity and so gives rise to 'overexchange'. Differences between PTA and CTA can be explained by the weaker stability of the CTA complex and its response to the decrease in local pH near the adsorption plane of low PZC zeolites. This change in local pH near oxide surfaces is characteristic of electrostatic adsorption. As the local pH decreases, the CTA ion is probably converted to a dimerized copper complex, perhaps Cu(2)(OH)(2)(2+). A portion of the released ammonia is protonated, increasing the solution pH. In high PZC, high aluminium zeolites with high ion exchange capacity, there is relatively little contribution from electrostatic adsorption.     

Reagent anions for charge inversion of polypeptide/protein cations in the gas phase

Various reagent anions capable of converting polypeptide cations to anions via ion/ion reactions have been investigated. The major charge inversion reaction channels include multiple proton transfer and adduct formation. Dianions composed of sulfonate groups as the negative charge carriers show essentially exclusive adduct formation in converting protonated peptides and proteins to anions. Dianions composed of carboxylate groups, on the other hand, show far more charge inversion via multiple proton transfer, with the degree of adduct formation dependent upon both the size of the polypeptide and the spacings between carboxylate groups in the dianion. More highly charged carboxylate-containing anions, such as those derived from carboxylate-terminated polyamidoamine half-generation dendrimers show charge inversion to give anion charges as high in magnitude as -4, with the degree of adduct formation being inversely related to dendrimer generation. All observations can be interpreted on the basis of charge inversion taking place via a long-lived chemical complex. The lifetime of this complex is related to the strengths and numbers of the interactions of the reactants in the complex. Calculations with model systems are fully consistent with sulfonate groups giving rise to more stable complexes. The kinetic stability of the complex can also be affected by the presence of electrostatic repulsion if it is multiply charged. In general, this situation destabilizes the complex and reduces the likelihood for observation of adducts. The findings highlight the characteristics of multiply charged anions that play roles in determining the nature of charge inversion products associated with protonated peptides and proteins.     

Influence of structural variation in room-temperature ionic liquids on the selectivity and efficiency of competitive alkali metal salt extraction by a crown ether

An improved method for the preparation of 1-alkyl-3-methylimidazolium hexafluorophosphates provides a series of room-temperature ionic liquids (RTILs) in which the 1-alkyl group is varied systematically from butyl to nonyl. For competitive solvent extraction of aqueous solutions of alkali metal chlorides with solutions of dicyclohexano-18-crown-6 (DC18C6) in these RTILs, the extraction efficiency generally diminished as the length of the 1-alkyl group was increased. Under the same conditions, extraction of alkali metal chlorides into solutions of DC18C6 in chloroform, nitrobenzene, and 1-octanol was undetectable. The extraction selectivity order for DC18C6 in the RTILs was K+ > Rb+ > Cs+ > Na+ > Li+. As the alkyl group in the RTIL was elongated, the K+/ Rb+ and K+/Cs+ selectivities exhibited general increases with the larger enhancement for the latter. For DC18C6 in 1-octyl-3-methylimidazolium hexafluorophosphate, the alkali metal cation extraction selectivity and efficiency were unaffected by variation of the aqueous-phase anion from chloride to nitrate to sulfate.     

Ion/molecule reactions to chemically deconvolute the electrospray ionization mass spectra of synthetic polymers

A new approach has been developed to analyze synthetic polymers via electrospray ionization mass spectrometry. Ion/molecule reactions, a unique feature of trapping instruments such as quadrupole ion trap mass spectrometers, can be used to chemically deconvolute the molecular mass distribution of polymers from the charge-state distribution generated by electrospray ionization. The reaction involves stripping charge from multiply charged oligomers to reduce the number of charge states. This reduces or eliminates the overlapping of oligomers from adjacent charge states. 15-Crown-5 was used to strip alkali cations (Na+) from several narrow polydisperse poly(ethylene glycol) standards. The charge-state distribution of each oligomer is reduced to primarily one charge state. Individual oligomers can be resolved, and the average molecular mass and polydispersities can be calculated for the polymers examined here. In most cases, the measured number-average molecular mass values are within 10% of the manufacturers' reported values obtained by gel permeation chromatography. The polydispersity was typically underestimated compared to values reported by the suppliers. Mn values were obtained with 0.5% RSD and are independent, over several orders of magnitude, of the polymer and cation concentration. The distributions that were obtained fit quite well to the Gaussian distribution indicating no high- or low-mass discriminations.