Influence of extra-framework cations on the adsorption properties of X-faujasite systems: microcalorimetry and molecular simulations

Isotherms and differential enthalpies of adsorption are obtained for nitrogen at ambient temperature on monovalent (Li(+), Na(+), K(+)) and divalent (Ca(2+), Ba(2+), Sr(2+), Mn(2+)) substituted X-faujasite systems by microcalorimetry measurements. These experimental data are compared with those obtained by combining grand canonical Monte Carlo simulations and newly derived force fields for describing the interactions between the extra-framework cations and the adsorbates obtained from a simple model based only on the intrinsic properties of the cations. It is the first time that such good qualitative agreement is reported between experiment and simulation for a series of both monovalent and divalent cations.     

CO2 adsorption in mono-, di- and trivalent cation-exchanged metal-organic frameworks: a molecular simulation study

A molecular simulation study is reported for CO(2) adsorption in rho zeolite-like metal-organic framework (rho-ZMOF) exchanged with a series of cations (Na(+), K(+), Rb(+), Cs(+), Mg(2+), Ca(2+), and Al(3+)). The isosteric heat and Henry's constant at infinite dilution increase monotonically with increasing charge-to-diameter ratio of cation (Cs(+) < Rb(+) < K(+) < Na(+) < Ca(2+) < Mg(2+) < Al(3+)). At low pressures, cations act as preferential adsorption sites for CO(2) and the capacity follows the charge-to-diameter ratio. However, the free volume of framework becomes predominant with increasing pressure and Mg-rho-ZMOF appears to possess the highest saturation capacity. The equilibrium locations of cations are observed to shift slightly upon CO(2) adsorption. Furthermore, the adsorption selectivity of CO(2)/H(2) mixture increases as Cs(+) < Rb(+) < K(+) < Na(+) < Ca(2+) < Mg(2+) ≈ Al(3+). At ambient conditions, the selectivity is in the range of 800-3000 and significantly higher than in other nanoporous materials. In the presence of 0.1% H(2)O, the selectivity decreases drastically because of the competitive adsorption between H(2)O and CO(2), and shows a similar value in all of the cation-exchanged rho-ZMOFs. This simulation study provides microscopic insight into the important role of cations in governing gas adsorption and separation, and suggests that the performance of ionic rho-ZMOF can be tailored by cations.     

Effect of exchangeable cation on the swelling property of 2:1 dioctahedral smectite--a periodic first principle study

We used both localized and periodic calculations on a series of monovalent (Li+, Na+, K+, Rb+, Cs+) and divalent (Mg2+, Ca2+, Sr2+, Ba2+) cations to monitor their effect on the swelling of clays. The activity order obtained for the exchangeable cations among all the monovalent and divalent series studied: Ca2+ > Sr2+ > Mg2+ > Rb+ > Ba2+ > Na+ > Li+ > Cs+ > K+. We have shown that, in case of dioctahedral smectite, the hydroxyl groups play a major role in their interaction with water and other polar molecules in the presence of an interlayer cation. We studied both type of clays, with a different surface structure and with/without water using a periodic calculation. Interlayer cations and charged 2:1 clay surfaces interact strongly with polar solvents; when it is in an aqueous medium, clay expands and the phenomenon is known as crystalline swelling. The extent of swelling is controlled by a balance between relatively strong swelling forces and electrostatic forces of attraction between the negatively charged phyllosilicate layer and the positively charged interlayer cation. We have calculated the solvation energy at the first hydration shell of an exchangeable cation, but the results do not correspond directly to the experimental d-spacing values. A novel quantitative scale is proposed with the numbers generated by the relative nucleophilicity of the active cation sites in their hydrated state through Fukui functions within the helm of the hard soft acid base principle. The solvation effect thus measured show a perfect match with experiment, which proposes that the reactivity index calculation with a first hydration shell could rationalize the swelling mechanism for exchangeable cations. The conformers after electron donation or acceptance propose the swelling mechanism for monovalent and divalent cations.     

The chameleon-like nature of zwitterionic micelles: effect of cation binding

Addition of salts, especially perchlorates, to zwitterionic micelles of SB3-14, C(14)H(29)NMe(2)(+)(CH(2))(3)SO(3)(-), induces anionic character and uptake of H(3)O(+) by SB3-14 micelles. Thus, the addition of alkali metal perchlorates accelerates the acid hydrolysis of 2-(p-heptoxyphenyl)-1,3-dioxolane, HPD, in the presence of SB3-14 micelles, which depends on the local proton concentration at the micelle surface. The addition of metal chlorides to solutions of such perchlorate-modified SB3-14 micelles decreases both the negative zeta potential of the micelles and the observed rate constant for acid hydrolysis of HPD. The effect of the monovalent cations Li(+), Na(+), and K(+) is smaller than that of the divalent cations Be(2+), Mg(2+), and Ca(2+), and much smaller than that of the trivalent cations Al(3+), La(3+), and Er(3+). The major factor responsible for this cation valence dependence of these effects is shown to be electrostatic in nature, reflecting the strong dependence of the micellar surface potential on the cation valence. The fact that the salt effects are not identical after correction for the electrostatic effects indicates that additional secondary nonelectrostatic effects may contribute as well.     

Adsorption and Interactions of Methyl Green with Montmorillonite and Sepiolite

The divalent organic cation, methyl green (MG), undergoes a slow transformation (6 h) to a monovalent cation, carbinol (MGOH(+)) upon dilution of its solution (10 mM), or in a buffer at neutral pH. Adsorption isotherms of MG on montmorillonite were determined by two procedures, both of which yield a final pH of suspensions between 7 to 7.4. When the amounts of MG in suspension were lower than the cation-exchange capacity (CEC) of the clay (0.8 mol(c)/kg clay), no measurable amount of MG remained in solution. The maximal amounts of MGOH(+) adsorbed were larger than those of MG(2+), being 1.15 and 0.75 mol MG/kg clay, respectively, corresponding to 140% of the CEC in the first case. On a charge basis the adsorption of added MG(2+) amounts to 185% of the CEC, which raises the possibility that a certain fraction of MG(2+) transformed into the monovalent form during the incubation period, since other divalent organic cations previously studied only adsorbed up to the CEC (paraquat), or slightly above it (diquat). Adsorption of MG on sepiolite (CEC=0.15 mol(c)/kg) further emphasizes the two patterns of its adsorption. The maximal adsorbed amounts of MG(2+) and MGOH(+) were 0.09 and 0.30 mol/kg clay, respectively. X-ray diffraction measurements gave lower values for the basal spacings for montmorillonite-MG(+) than for MGOH(+), suggesting that MG(2+) binds two clay platelets together, as in the case of other divalent cations. A competition for adsorption between MG and the monovalent organic cation, acriflavin (AF), gave lower adsorbed amounts of AF when competing with MG(+), which is interpreted to be due to the smaller basal spacing in this case, which partially inhibits the entry of AF molecules into the interlammelar space. Spectra of montmorillonite-MG particles in the visible range exhibited significant differences between clay-MG and clay-carbinol. Copyright 2000 Academic Press.     

Bipolar reverse osmosis membrane for separating mono-and divalent ions

Bipolar reverse osmosis membranes that have both negatively and positively charged layers have been prepared to enhance the selectivity towards mono- and divalent ions in respect of both cations and anions. Positively charged layers are formed on low pressure reverse osmosis membranes having negative charge (NTR-7410 and 7450) by an adsorption method using polyethyleneimine (PEI) or a quaternary ammonium polyelectrolyte (QAP). These layers attach to the membrane's dense layer, which is made of sulfonated polyether sulfone. The selectivity of mono- and divalent ions is proven by experimental results for single electrolytes (NaCl, Na₂SO₄ and MgCl₂). Although negatively charged membranes repulse divalent anions more strongly than cations and monovalent anions, bipolar reverse osmosis membranes reject both divalent cations and divalent anions better than monovalent ions. An optimal preparation method for bipolar membranes showing selectivity towards mono- and divalent ions were developed. The bipolar membranes showed good ion selectivity for artificial sea water.     

Molecular structures of cation...pi(arene) interactions for alkali metals with pi- and sigma-modalities

The monovalent cations of Na(+), K(+), Rb(+), and Cs(+) derived from the highly electropositive alkali metals represent prototypical charged spheres that are mainly subject to relatively simple electrostatic and solvation (hydration) forces. We now find that the largest of these Rb(+) and Cs(+) are involved in rather strong cation...pi(arene) interactions when they are suitably disposed with the ambifunctional hexasubstituted benzene C(6)E(6). The ether tentacles (E = methoxymethyl) allow these cations to effect eta(1)-bonding to the benzene center in a manner strongly reminiscent of the classical sigma-arene complexes with positively charged electrophiles where Z(+) = CH(3)(+), Br(+), Cl(+), Et(3)Si(+), etc. The somewhat smaller potassium cation is involved in a similar M(+)...pi(arene) interaction that leads to eta(2)-bonding with the aromatic center in the pi-mode previously defined in the well-known series of silver(I)/arene complexes. We can find no evidence for significant Na(+)... pi(arene) interaction under essentially the same conditions. As such, the sigma-structure of the Rb(+) and Cs(+) complexes and pi-structure of the K(+) complex are completely integrated into the continuum of sigma-pi bondings of various types of electrophilic (cationic) acceptors with arene donors that were initially identified by Mulliken as charge-transfer.     

Adsorption of tetracycline onto goethite in the presence of metal cations and humic substances

Adsorption of tetracycline, one of the most widely used antibiotics, onto goethite was studied as a function of pH, metal cations, and humic acid (HA) over a pH range 3-10. Five background electrolyte cations (Li(+), Na(+), K(+), Ca(2+), and Mg(2+)) with a concentration of 0.01 M showed little effect on the tetracycline adsorption at the studied pH range. While the divalent heavy metal cation, Cu(2+), could significantly enhance the adsorption and higher concentration of Cu(2+), stronger adsorption was found. The results indicated that different adsorption mechanisms might be involved for the two types of cations. Background electrolyte cations hardly interfere with the interaction between tetracycline and goethite surfaces because they only form weak outer-sphere surface complexes. On the contrary, Cu(2+) could enhance the adsorption via acting as a bridge ion to form goethite-Cu(2+)-tetracycline surface complex because Cu(2+) could form strong and specific inner-sphere surface complexes. HA showed different effect on the tetracycline sorption under different pH condition. The presence of HA increased tetracycline sorption dramatically under acidic condition. Results indicated that heavy metal cations and soil organic matters have great effects on the tetracycline mobility in the soil environment and eventually affect its exposure concentration and toxicity to organisms.     

A solid-state 23Na NMR study of monovalent cation binding to double-stranded DNA at low relative humidity

We report a solid-state (23)Na NMR study of monovalent cation (Li(+), Na(+), K(+), Rb(+), Cs(+) and NH(4) (+)) binding to double-stranded calf thymus DNA (CT DNA) at low relative humidity, ca 0-10%. Results from (23)Na--(31)P rotational echo double resonance (REDOR) NMR experiments firmly establish that, at low relative humidity, monovalent cations are directly bound to the phosphate group of CT DNA and are partially dehydrated. On the basis of solid-state (23)Na NMR titration experiments, we obtain quantitative thermodynamic parameters concerning the cation-binding affinity for the phosphate group of CT DNA. The free energy difference (DeltaG degrees ) between M(+) and Na(+) ions is as follows: Li(+) (-1.0 kcal mol(-1)), K(+) (7.2 kcal mol(-1)), NH(4) (+) (1.0 kcal mol(-1)), Rb(+) (4.5 kcal mol(-1)) and Cs(+) (1.5 kcal mol(-1)). These results suggest that, at low relative humidity, the binding affinity of monovalent cations for the phosphate group of CT DNA follows the order: Li(+) > Na(+) > NH(4) (+) > Cs(+) > Rb(+) > K(+). This sequence is drastically different from that observed for CT DNA in solution. This discrepancy is attributed to the different modes of cation binding in dry and wet states of DNA. In the wet state of DNA, cations are fully hydrated. Our results suggest that the free energy balance between direct cation-phosphate contact and dehydration interactions is important. The reported experimental results on relative ion-binding affinity for the DNA backbone may be used for testing theoretical treatment of cation-phosphate interactions in DNA.     

Effects of cations on the hydrogen bond network of liquid water: new results from X-ray absorption spectroscopy of liquid microjets

Oxygen K-edge X-ray absorption spectra (XAS) of aqueous chloride solutions have been measured for Li(+), Na(+), K(+), NH(4)(+), C(NH(2))(3)(+), Mg(2+), and Ca(2+) at 2 and 4 M cation concentrations. Marked changes in the liquid water XAS are observed upon addition of the various monovalent cation chlorides that are nearly independent of the identity of the cation. This indicates that interactions with the dissolved monovalent cations do not significantly perturb the unoccupied molecular orbitals of water molecules in the vicinity of the cations and that water-chloride interactions are primarily responsible for the observed spectral changes. In contrast, the addition of the divalent cations engenders changes unique from the case of the monovalent cations, as well as from each other. Density functional theory calculations suggest that the ion-specific spectral variations arise primarily from direct electronic perturbation of the unoccupied orbitals due to the presence of the ions, probably as a result of differences in charge transfer from the water molecules onto the divalent cations.     

Monovalent ion adsorption at the muscovite (001)-solution interface: relationships among ion coverage and speciation, interfacial water structure, and substrate relaxation

The interfacial structure between the muscovite (001) surface and aqueous solutions containing monovalent cations (3 × 10(-3) m Li(+), Na(+), H(3)O(+), K(+), Rb(+), or Cs(+), or 3 × 10(-2) m Li(+) or Na(+)) was measured using in situ specular X-ray reflectivity. The element-specific distribution of Rb(+) was also obtained with resonant anomalous X-ray reflectivity. The results demonstrate complex interdependencies among adsorbed cation coverage and speciation, interfacial hydration structure, and muscovite surface relaxation. Electron-density profiles of the solution near the surface varied systematically and distinctly with each adsorbed cation. Observations include a broad profile for H(3)O(+), a more structured profile for Li(+) and Na(+), and increasing electron density near the surface because of the inner-sphere adsorption of K(+), Rb(+), and Cs(+) at 1.91 ± 0.12, 1.97 ± 0.01, and 2.26 ± 0.01 ?, respectively. Estimated inner-sphere coverages increased from ~0.6 to 0.78 ± 0.01 to ~0.9 per unit cell area with decreasing cation hydration strength for K(+), Rb(+), and Cs(+), respectively. Between 7 and 12% of the Rb(+) coverage occurred as an outer-sphere species. Systematic trends in the vertical displacement of the muscovite lattice were observed within ~40 ? of the surface. These include a <0.1 ? shift of the interlayer K(+) toward the interface that decays into the crystal and an expansion of the tetrahedral-octahedral-tetrahedral layers except for the top layer in contact with solution. The distortion of the top tetrahedral sheet depends on the adsorbed cation, ranging from an expansion (by ~0.05 ? vertically) in 3 × 10(-3)m H(3)O(+) to a contraction (by ~0.1 ?) in 3 × 10(-3) m Cs(+). The tetrahedral tilting angle in the top sheet increases by 1 to 4° in 3 × 10(-3) m Li(+) or Na(+), which is similar to that in deionized water where the adsorbed cation coverages are insufficient for full charge compensation.     

Cation-specific interactions with carboxylate in amino acid and acetate aqueous solutions: X-ray absorption and ab initio calculations

Relative interaction strengths between cations (X = Li (+), Na (+), K (+), NH 4 (+)) and anionic carboxylate groups of acetate and glycine in aqueous solution are determined. These model systems mimic ion pairing of biologically relevant cations with negatively charged groups at protein surfaces. With oxygen 1s X-ray absorption spectroscopy, we can distinguish between spectral contributions from H 2O and carboxylate, which allows us to probe the electronic structure changes of the atomic site of the carboxylate group being closest to the countercation. From the intensity variations of the COO (-) aq O 1s X-ray absorption peak, which quantitatively correlate with the change in the local partial density of states from the carboxylic site, interactions are found to decrease in the sequence Na (+) > Li (+) > K (+) > NH 4 (+). This ordering, as well as the observed bidental nature of the -COO (-) aq and X (+) aq interaction, is supported by combined ab initio and molecular dynamics calculations.     

Competition between Li+ and Mg2+ in metalloproteins. Implications for lithium therapy

Lithium is used (in the form of soluble salts) to treat bipolar disorder and has been considered as a possible drug in treating chronic neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's diseases. One of the proposed mechanisms of Li(+) action involves a competition between the alien Li(+) and native Mg(2+) for metal-binding sites and subsequent inhibition of key enzymes involved in specific neurotransmission pathways, but not vital Mg(2+) proteins in the cell. This raises the following intriguing questions: Why does Li(+) replace Mg(2+) only in enzymes involved in bipolar disorder, but not in Mg(2+) proteins essential to cells? In general, what factors allow monovalent Li(+) to displace divalent Mg(2+) in proteins? Specifically, how do the composition, overall charge, and solvent exposure of the metal-binding site as well as a metal-bound phosphate affect the selectivity of Li(+) over Mg(2+)? Among the many possible factors, we show that the competition between Mg(2+) and Li(+) depends on the net charge of the metal complex, which is determined by the numbers of metal cations and negatively charged ligands, as well as the relative solvent exposure of the metal cavity. The protein itself is found to select Mg(2+) over the monovalent Li(+) by providing a solvent-inaccessible Mg(2+)-binding site lined by negatively charged Asp/Glu, whereas the cell machinery was found to select Mg(2+) among other competing divalent cations in the cellular fluids such as Ca(2+) and Zn(2+) by maintaining a high concentration ratio of Mg(2+) to its biogenic competitor in various biological compartments. The calculations reveal why Li(+) replaces Mg(2+) only in enzymes that are known targets of Li(+) therapy, but not in Mg(2+) enzymes essential to cells, and also reveal features common to the former that differ from those in the latter proteins.     

Competitive cesium-133 NMR spectroscopic study of complexation of different metal ions with dibenzo-21-crown-7 in acetonitrile-dimethylsulfoxide and nitromethane-dimethylsulfoxide mixtures

(133)Cs NMR spectroscopy was used to determine the stoichiometry and stability of the Cs(+) ion complex with dibenzo-21-crown-7 (DB21C7) in acetonitrile-dimethylsulfoxide (96.5:3.5, w/w) and nitromethane-dimethylsulfoxide (96.5:3.5, w/w) mixtures. A competitive (133)Cs NMR technique was also employed to probe the complexation of Na(+), K(+), Rb(+), Ag(+), Tl(+), NH(4)(+), Mg(2+), Ba(2+), Hg(2+), Pb(2+) and UO(2)(2+) ions with DB21C7 in the same solvent systems. All the resulting 1:1 complexes in nitromethane-dimethylsulfoxide were more stable than those in acetonitrile-dimethylsulfoxide solution. In both solvent systems, the stability of the resulting complexes was found to vary in the order Rb(+)>K(+) approximately Ba(2+)>Tl(+)>Cs(+)>NH(4)(+) approximately Pb(2+)>Ag(+)>UO(2)(2+)>Hg(2+)>Mg(2+)>Na(+).     

Comparison of cation adsorption by isostructural rutile and cassiterite

Macroscopic net proton charging curves for powdered rutile and cassiterite specimens with the (110) crystal face predominant, as a function of pH in RbCl and NaCl solutions, trace SrCl(2) in NaCl, and trace ZnCl(2) in NaCl and Na Triflate solutions, are compared to corresponding molecular-level information obtained from static DFT optimizations and classical MD simulations, as well as synchrotron X-ray methods. The similarities and differences in the macroscopic charging behavior of rutile and cassiterite largely reflect the cation binding modes observed at the molecular level. Cation adsorption is primarily inner-sphere on both isostructural (110) surfaces, despite predictions that outer-sphere binding should predominate on low bulk dielectric constant oxides such as cassiterite (ε(bulk) ≈ 11). Inner-sphere adsorption is also significant for Rb(+) and Na(+) on neutral surfaces, whereas Cl(-) binding is predominately outer-sphere. As negative surface charge increases, relatively more Rb(+), Na(+), and especially Sr(2+) are bound in highly desolvated tetradentate fashion on the rutile (110) surface, largely accounting for enhanced negative charge development relative to cassiterite. Charging curves in the presence of Zn(2+) are very steep but similar for both oxides, reflective of Zn(2+) hydrolysis (and accompanying proton release) during the adsorption process, and the similar binding modes for ZnOH(+) on both surfaces. These results suggest that differences in cation adsorption between high and low bulk dielectric constant oxides are more subtly related to the relative degree of cation desolvation accompanying inner-sphere binding (i.e., more tetradentate binding on rutile), rather than distinct inner- and outer-sphere adsorption modes. Cation desolvation may be favored at the rutile (110) surface in part because inner-sphere water molecules are bound further from and less tightly than on the cassiterite (110) surface. Hence, their removal upon inner-sphere cation binding is relatively more favorable.     

Dramatic effect of the metal cation in dealkylation reactions of phosphinic esters promoted by complexes of polyether ligands with metal iodides

Metal ion electrophilic catalysis has been revealed in dealkylation reactions of phosphinic esters 1-4 promoted by complexes of polyether ligands 5-7 with metal iodides MI(n) (M[n+] = Li(+), Na(+), K(+), Rb(+), Ca(2+), Sr(2+), Ba(2+)) in low polarity solvents (chlorobenzene, 1,2-dichlorobenzene, and toluene) at 60 degrees C. The catalytic effect increases with increasing the Lewis acid character of the cation, in the order Rb(+)< K(+)< Na(+)< Li(+) and Ba(2+)< Sr(2+)< Ca(2+). The results are interpreted in terms of a transition state where the complexed cation (M[n+] subset Lig) assists the departure of the leaving group Ph(2)P(O)O(-) and, at the same time, favors the attack at carbon of the nucleophile I(-) ("push-pull" mechanism). The rate sequence found for 1-4 (Me > Et > i-Pr and t-Bu) shows that this reaction can be utilized for the selective dealkylation of these substrates.     

Metal-controlled assembly of uranyl diphosphonates toward the design of functional uranyl nanotubules

Two uranyl nanotubules with elliptical cross sections were synthesized in high yield from complex and large oxoanions using hydrothermal reactions of uranyl salts with 1,4-benzenebisphosphonic acid or 4,4'-biphenylenbisphosphonic acid and Cs(+) or Rb(+) cations in the presence of hydrofluoric acid. Disordered Cs(+)/Rb(+) cations and solvent molecules are present within and/or between the nanotubules. Ion-exchange experiments with A(2){(UO(2))(2)F(PO(3)HC(6)H(4)C(6)H(4)PO(3)H)(PO(3)HC(6)H(4)C(6)H(4)PO(3))}·2H(2)O (A = Cs(+), Rb(+)), revealed that A(+) cations can be exchanged for Ag(+) ions. The uranyl phenyldiphosphonate nanotubules, Cs(3.62)H(0.38)[(UO(2))(4){C(6)H(4)(PO(2)OH)(2)}(3){C(6)H(4)(PO(3))(2)}F(2)]·nH(2)O, show high stability and exceptional ion-exchange properties toward monovalent cations, as demonstrated by ion-exchange studies with selected cations, Na(+), K(+), Tl(+), and Ag(+). Studies on ion-exchanged single crystal using scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM/EDS) provide evidence for chemical zonation in Cs(3.62)H(0.38)[(UO(2))(4){C(6)H(4)(PO(2)OH)(2)}(3){C(6)H(4)(PO(3))(2)}F(2)]·nH(2)O, as might be expected for exchange through a diffusion mechanism.     

Oxide-dependent adsorption of a model membrane phospholipid, dipalmitoylphosphatidylcholine: bulk adsorption isotherms

The importance of substrate chemistry and structure on supported phospholipid bilayer design and functionality is only recently being recognized. Our goal is to investigate systematically the substrate-dependence of phospholipid adsorption with an emphasis on oxide surface chemistry and to determine the dominant controlling forces. We obtained bulk adsorption isotherms at 55 degrees C for dipalmitoylphosphatidylcholine (DPPC) at pH values of 5.0, 7.2, and 9.0 and at two ionic strengths with and without Ca(2+), on quartz (alpha-SiO(2)), rutile (alpha-TiO(2)), and corundum (alpha-Al(2)O(3)), which represent a wide a range of points of zero charge (PZC). Adsorption was strongly oxide- and pH-dependent. At pH 5.0, adsorption increased as quartz < rutile approximately corundum, while at pH 7.2 and 9.0, the trend was quartz approximately rutile < corundum. Adsorption decreased with increasing pH (increasing negative surface charge), although adsorption occurred even at pH > or = PZC of the oxides. These trends indicate that adsorption is controlled by attractive van der Waals forces and further modified by electrostatic interactions of oxide surface sites with the negatively charged phosphate ester (-R(PO(4)-)R'-) portion of the DPPC headgroup. Also, the maximum observed adsorption on negatively charged oxide surfaces corresponded to roughly two bilayers, whereas significantly higher adsorption of up to four bilayers occurred on positively charged surfaces. Calcium ions promote adsorption beyond a second bilayer, regardless of the sign of oxide surface charge. We develop a conceptual model for the structure of the electric double layer to explain these observations.     

Selective binding of monovalent cations to the stacking G-quartet structure formed by guanosine 5'-monophosphate: a solid-state NMR study

We report a solid-state multinuclear ((23)Na, (15)N, (13)C, and (31)P) NMR study on the relative affinity of monovalent cations for a stacking G-quartet structure formed by guanosine 5'-monophosphate (5'-GMP) self-association at pH 8. Two major types of cations are bound to the 5'-GMP structure: one at the surface and the other within the channel cavity between two G-quartets. The channel cation is coordinated to eight carbonyl oxygen atoms from the guanine bases, whereas the surface cation is close to the phosphate group and likely to be only partially hydrated. On the basis of solid-state (23)Na NMR results from a series of ion titration experiments, we have obtained quantitative thermodynamic parameters concerning the relative cation binding affinity for each of the two major binding sites. For the channel cavity site, the values of the free energy difference (Delta G degrees at 25 degrees C) for ion competition between M(+) and Na(+) ions are K(+) (-1.9 kcal mol(-1)), NH(4)(+) (-1.8 kcal mol(-1)), Rb(+) (-0.3 kcal mol(-1)), and Cs(+) (1.8 kcal mol(-1)). For the surface site, the values Delta G degrees are K(+) (2.5 kcal mol(-1)), NH(4)(+) (-1.3 kcal mol(-1)), Rb(+) (1.1 kcal mol(-1)), and Cs(+) (0.9 kcal mol(-1)). Solid-state NMR data suggest that the affinity of monovalent cations for the 5'-GMP structure follows the order NH(4)(+) > Na(+) > Cs(+) > Rb(+) > K(+) at the surface site and K(+) > NH(4)(+) > Rb(+) > Na(+) > Cs(+) > Li(+) at the channel cavity site. We have found that the cation-induced stability of a 5'-GMP structure is determined only by the affinity of monovalent cations for the channel site and that the binding of monovalent cations to phosphate groups plays no role in 5'-GMP self-ordered structure. We have demonstrated that solid-state (23)Na and (15)N NMR can be used simultaneously to provide mutually complementary information about competitive binding between Na(+) and NH(4)(+) ions.     

Uptake of cesium, strontium and europium by a poly(sodium acrylate-acrylic acid) hydrogel

The uptake of cesium, strontium and europium from dilute nitric acid solutions by a poly(sodium acrylate-acrylic acid) PAA hydrogel has been investigated. pH variations are consistent with cation exchange processes: COO(-), Na (+)H (+), COO(-), Na (+)M (m+) ( M (m+) = Cs (+)and Sr (2+)) and COOH Eu (3+). Saturation of the gel is achieved for metal/carboxylate ratios R = 0.5. The swelling ratios of gels loaded with metal cations are those of uncharged, shrunk gels (Sr, Eu) or of charged, swollen gels (Cs) in agreement with the formation of uncharged (COO)(2)Sr, (COO)(2)EuX (X = NO(3) or OH) type complexes and (COO(-), Cs(+)) ion pairs. The metal cations are extracted in the gels following the order of their affinities with carboxylic groups Eu(3+) > Sr(2+) > Cs(+). An increase of the ionic strength of the metal aqueous solution up to 0.5M NaNO(3) leads to slightly decrease the europium uptake by the PAA hydrogel, but 0.1M NaNO(3) is sufficient to prevent the Sr and Cs extractions.