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.     

Luminescent double-decker type guanine octets with trivalent lanthanide cations: in situ self-assembling and stability evaluation in homogeneous organic media

Silylated guanine formed luminescent double-decker type octet complexes with trivalent lanthanide cations (lanthanide cation : guanine = 1:8) in organic media, while a deaminated guanine derivative gave only 1:2 complexes. The octet formation was evidenced by characteristic UV/Vis absorption changes, CSI-TOF MS and ¹H NMR spectra. The octet with Tb³⁺ showed intense green luminescence with a long lifetime by photo-excitation of the guanine chromophore. The trivalent lanthanie cations stabilized the octets more effectively than common mono- and divalent metal cations.     

Simultaneous separation of common mono- and divalent cations on a calcinated silica gel column by ion chromatography with indirect photometric detection and aromatic monoamines-oxalic acid,containing crown ethers,used as eluent

The application of unmodified silica gel (Super Micro Bead Silica Gel B-5, SMBSG B-5) as a cation-exchange stationary phase in ion chromatography with indirect photometric detection (IC-IPD) for the separation of common mono- and divalent cations (Li+, Na+, NH4+, K+, Mg2+ and Ca2+) was carried out using various aromatic monoamines [tyramine [4-(2-aminoethyl)phenol], benzylamine, phenylethylamine, 2-methylpyridine and 2,6-dimethylpyridine] as eluents. When using these amines as eluents, the peak resolution between these mono- and divalent cations was not quite satisfactory and the peak shapes of NH4+ and K+ were largely destroyed on the SMBSG B-5 silica gel column. Hence, the application of SMBSG B-5 silica gel calcinated at 200, 400, 600, 800 and 1000 degrees C for 5 h in the IC-IPD was carried out. The peak shapes of the monovalent cations were greatly improved with increasing calcination temperature and, as a result, symmetrical peaks of these mono- and divalent cations were obtained on the SMBSG B-5 silica gel calcinated at 1000 degrees C as the stationary phase. In contrast, the peak resolution between these mono- and divalent cations was not improved. Therefore, crown ethers [18-crown-6 (1,4,7,10,13,15-hexaoxacyclooctadecane), 15-crown-5 (1,4,7,10,13-pentaoxacyclopentadecane)] were added to the eluent for the complete separation of these mono- and divalent cations. Excellent simultaneous separation and highly sensitive detection at 275 nm were achieved in 25 min on a column (150x4.6 mm I.D.) packed with SMBSG B-5 silica gel calcinated at 1000 degrees C by elution with 0.75 mM tyramine-0.25 mM oxalic acid at pH 5.0 containing either 1.0 mM 18-crown-6 or 10 mM 15-crown-5.     

Metal cation-induced deformation of DNA self-assembled monolayers on silicon: vibrational sum frequency generation spectroscopy

Nucleic acids possess charged phosphate groups in their backbones, which require counterions to reduce the repulsive Coulombic interactions between the strands. Herein we report how different mono- and divalent metal cations influence the molecular orientations of DNA molecules on silicon surfaces upon immobilization and hybridization. Our sum frequency generation (SFG) spectroscopy studies demonstrated that the degree of conformational variation of DNA self-assembled monolayers on silicon depends on the type of metal cations present. The molecular orientation change of immobilized single-stranded oligonucleotides correlates with DNA-cation affinity (Mg(2+) > Ca(2+) > K(+) approximately Na(+)): metal cations with the strongest affinity disrupt the structure of the underlying linker monolayer the most. Upon hybridization the trend is reversed, which is attributed to the greater ability of divalent cations to mask the negative charges on the DNA backbone. These findings provide useful information for the construction of more sensitive DNA biosensors, particularly the optimization of on-chip hybridization performance.     

Voltammetric study of the ion-exchange binding of non-electroactive metal cations to DNA-modified surfaces

We describe a simple electrochemical protocol for studying the ion-exchange binding of non-electroactive ions, specifically mono- and divalent metal cations of biological relevance (Mg(2+), Ca(2+), and K(+)), to DNA-modified surfaces. After incubation in a dilute solution of multiply charged transition metal complex (5.0 microM [Ru(NH(3))(6)]Cl(3)), gold electrodes modified with thiolate-DNA monolayers respond to the presence of these non-electroactive metal cations by producing significant changes in the cyclic voltammograms (i.e., decrease of the integrated charge and shift of formal potential) of the surface-bound redox complex ([Ru(NH(3))(6)](3+)). The divalent cations (particularly Mg(2+)) can be detected at very low concentrations (<10 microM), while the on-set value for K(+) is substantially higher (50 mM). The equilibrium binding constants for Mg(2+) and Ca(2+) to DNA-modified surfaces were calculated.     

Indirect ultraviolet spectrophotometric detection in the ion chromatography of common mono- and divalent cations on an aluminium adsorbing silica gel column with tyramine-containing crown ethers as eluent

The application of laboratory-made aluminium-adsorbing silica gel (Al-Silica) as a cation-exchange stationary phase to ion chromatography-indirect photometric detection (IC-IPD) for common mono- and divalent cations (Li+, Na+, NH+, K+, Mg2+ and Ca2+) was carried out by using protonated tyramine (4-aminoethylphenol) as eluent ion. When using 1.2 mM tyramine-0.2 mM oxalic acid at pH 4.5 as eluent, incomplete separation of the monovalent cations and complete separation of the divalent cations were achieved in 17 min. Then, the addition of crown ethers in the eluent was carried out for the complete separation of the mono- and divalent cations. As a result, when using 1.2 mM tyramine--0.2 mM oxalic acid at pH 4.5 containing either 5 mM 15-crown-5 (1,4,7,10,13-pentaoxacyclopentadecane) or 0.5 mM and 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) as eluent, excellently simultaneous separation of these cations was achieved in 21 min. The proposed IC-IPD was successfully applied to the determination of major cations in natural water samples.     

Influence of solution chemistry on the deposition and detachment kinetics of RNA on silica surfaces

The deposition kinetics of RNA extracted from both virus and bacteria on silica surfaces were examined in both monovalent (NaCl) and divalent (CaCl(2)) solutions under a wide range of environmentally relevant ionic strength and pH conditions by utilizing a quartz crystal microbalance with dissipation (QCM-D). To better understand the RNA deposition mechanisms, QCM-D data were complemented by diffusion coefficients and zeta potentials of RNA as a function of examined solution chemistry conditions. Favorable deposition of RNA on poly-l-lysine-coated (positively charged) silica surfaces was governed by the convective-diffusive transport of RNA to the surfaces. The deposition kinetics of RNA on bare silica surfaces were controlled by classic Derjaguin-Landau-Verwey-Overbeek (DLVO) interactions. The presence of divalent cations (Ca(2+)) in solutions greatly enhanced the deposition kinetics of RNA on silica surfaces. Solution pH also affected the deposition behavior of RNA on silica surfaces. Release experiments showed that detachment of RNA from silica surfaces was significant in NaCl solutions, whereas, the deposited RNA on silica surfaces in CaCl(2) solutions was more likely to be irreversible.     

Simultaneous separation of common mono- and divalent cations on a zirconium-modified silica gel column by ion chromatography with non-suppressed conductimetric detection and tartaric acid–15-crown-5 as eluent

The application of laboratory-made zirconium-modified silica gels (Zr-silicas) as cation-exchange stationary phases to ion chromatography with conductimetric detection (IC–CD) for common mono- and divalent cations (Li⁺, Na⁺, NH₄⁺, K⁺, Mg²⁺ and Ca²⁺) was carried out. Zr-silicas were prepared by the reaction of the silanol group on the surface of silica gel with zirconium tetrabutoxide (Zr(OCH₂CH₂CH₂CH₃)₄) in ethanol. Zr-silica adsorbed on 10 mg zirconium g⁻¹ silica gel was a suitable cation-exchange stationary phase in IC–CD for the separation of these mono- and divalent cations. Excellent simultaneous separation and highly sensitive detection for these cations were achieved in 10 min by IC–CD using a Zr-silica column (150×4.6 mm I.D.) and 10 mM tartaric acid containing 10 mM 15-crown-5 (1,4,7,10,13-pentaoxacyclopentadecane) as the eluent. The proposed IC–CD method was successfully applied to the determination of major mono- and divalent cations in natural water samples.     

Electrostatic bonding models: A test on group 1 and 2 metal complexes with H2O,NH3, H2S,PH3, and related ligands

Electrostatic models frequently proposed to describe ion–molecule interactions have been tested on the adducts formed by Group 1 and 2 cations with H₂O, NH₃, H₂S, PH₃, their methyl analogs, and their anions. The results from the model calculations were compared with all-electron calculations (geometry optimized, MP2, TZP basis sets) carried out on adducts formed with Li⁺, Na⁺, K⁺, Ca²⁺, and Mg²⁺. The electrostatic potential model was utilized in two ways: The attraction of the point charge was calculated with and without relaxation of the ligand. A third model allowed relaxation of the ligand but treated the cation as a frozen core. The final model was the crude point charge/point dipole approximation. At long range, the models satisfactorily track the effects on energy of gross changes in the ion–ligand interaction (monovalent versus divalent ions, neutral ligands versus anions, parent ligands versus methyl derivatives), but correlation at close range is poor, especially for binding by divalent cations. The hypothesis that the calculated strength of cation–dipole binding is dependent on calculated dipole moment could not be verified. © 1995 by John Wiley & Sons, Inc.     

Classical simulations with the POLIR potential describe the vibrational spectroscopy and energetics of hydration: divalent cations, from solvation to coordination complex

POLIR, a polarizable water potential optimized for vibrational and intermolecular spectroscopy in pure water but not optimized for solvation, is used to describe solutions of the divalent metal cations Ca(2+), Mg(2+), and Cu(2+). The spectral shifts in the O-H stretch region obtained from classical simulations are in agreement with experiment. The water-ion binding energies are dominated by classical electrostatics, even though the Cu(2+) case might be considered to involve an intermediate-strength chemical bond. Three-body energies of the ion with the first solvation shell are in agreement with ab initio calculations. Our results indicate the importance of polarization in the development of accurate, transferable, force fields and the power of classical methods when it is carefully included.     

Forces between Carboxylic Acid Surfaces in Divalent Electrolyte Solutions

The behavior of self-assembled monolayers of thiohexadecanoic acid adsorbed onto gold interacting in asymmetric 2:1 electrolytes has been studied with direct force measurements. The effects of two divalent cations (Mg(2+) and Ca(2+)) were studied at concentrations ranging from 1 μM to 10 mM. As compared to interactions in the presence of Na(+), the divalent ions adsorb strongly to the surfaces, with the effect of lowering the surface potential and decreasing the double-layer repulsion. At concentrations above 10 μM, the Ca(2+) ions were found to adsorb stronger than Mg(2+). Ca(2+) ions cause charge reversal at high concentrations, and the net interactions at 10 mM were attractive over the measurable range. Copyright 2000 Academic Press.     

Complexation of metal ions in Langmuir films formed with two amphiphilic dioxadithia crown ethers

The two new crown ethers presented in this study were synthesized in order to investigate two important features of ionophores, namely metal cation complexation and interfacial properties, and the way in which they interrelate. The two derivatives were conceived as analogs of membrane phospholipids with respect to their amphiphilicity and geometry. They contain a hydrophilic 1,1'-dioxo-3,3'-dithio-14-crown ether headgroup and bear two myristoyl or stearoyl lateral chains. The length of the myristoyl and stearoyl derivatives in an extended conformation is comparable with the thickness of the individual leaflets of cell membranes. The membrane-related and complexation properties of the two crown ether derivatives were studied in monomolecular films spread on pure water and on aqueous solutions of mono-, di-, and trivalent metal salts. The properties of the monolayers are described quantitatively using thermodynamic models. The compression isotherms of the monolayers formed on different subphases show a clear-cut differentiation of the monovalent and di- or trivalent cations with both ligands. This differentiation was interpreted in terms of conformational changes occurring in the crown ether derivatives upon complexation. Molecular modeling indicates that the mono- and divalent cations are coordinated differently by the ligands, yielding complexes with different conformations. The differences of the conformations of the mono- and di- or trivalent cation complexes may be important from the point of view of the interactions with lipid membranes and the biological activity of these potential ionophores.     

Solvation forces between silica bodies in supercritical carbon dioxide

We report Monte Carlo simulations of the solvation pressure between two planar surfaces, which represent the interface of spherical silica nanoparticles in supercritical carbon dioxide. Carbon dioxide (CO2) was modeled as an atomistic dumbbell or a spherical Lennard-Jones particle. The interaction between CO2 molecules and silica surfaces was characterized by the standard Steele potential with energetic heterogeneities representing the hydrogen bonds. The parameters for the solid-fluid interaction potentials were obtained by fitting our simulations to the experimental isotherms of CO2 sorption on mesoporous siliceous materials. We studied the dependence of the solvation force on the distance between planar silica surfaces at T = 318 K, at equilibrium bulk pressures p(bulk) ranging from 69 to 200 atm. At 69 atm, we observed a long-range attraction between the two surfaces, and it vanished when the pressure was increased to 102 and then 200 atm. The results obtained with different fluid models were consistent with each other. According to our observations, energetic heterogeneities of the surface have negligible influence on the solvation pressure. Using the Derjaguin approximation, we calculated the solvation forces between spherical silica nanoparticles in supercritical CO2 from the solvation pressures between the planar surfaces.     

Adsorption of codeine on hydrophilic silica and silica surface modified by hydrophobic groups in the presence of electrolytes

The adsorption of codeine from aqueous solution onto colloidal silica and silica surface-modified with chemiadsorbed octadecyl dimethyl silane (ODDMS) or dimethyl silane (DMS) groups was studied in the presence of neutral electrolytes at different pH values. From codeine-hydrochloride solutions codeine cations are strongly bound to negatively charged silica surfaces. Inorganic salts (NaCl, NaNO₃) reduce the adsorption of the organic cation. On silica modified by ODDMS (10% of surface silanol groups are occupied), codeine cations are adsorbed to a higher extent at pH 6, while at pH 8 the adsorbed amounts are lower than on the bare silica surface. Neutral electrolytes reduce codeine adsorption on the ODDMS modified silica. On the hydrophobic silica, completely covered by DMS groups, codeine adsorption is considerably lower than on the bare silica, but neutral salts increase the adsorption. The adsorption of codeine is compared with the adsorption of aggregating surfactant ions. Common and different features of their interactions with silica surfaces are outlined.     

Thermopower composition dependence in ferrospinels

The composition dependence of thermoelectric power (Seebeck coefficient) in ferrospinels with fixed valence foreign cations has been calculated via combined small polaron and cation distribution models. Satisfactory agreement with experimental data is achieved assuming cation distribution thermodynamic constants to be independent of foreign cation concentration. Data are analyzed for a trivalent foreign cation (Al³⁺) at elevated temperature and for divalent foreign cations (Ni²⁺, Mg²⁺, Zn²⁺) at lower temperatures.     

Study on hydration layers near nanoscale silica dispersed in aqueous solutions through viscosity measurement

On the basis of the Einstein theory of viscosity of dispersion, a parameter, termed as solvation factor, is presented to evaluate the solvation degree of nanoscale particles dispersed in a liquid in this work. The value of the parameter is obtained through the measurements of relative viscosity of the dispersions as a function of the volume fraction of dry particles. The solvation factor has been used to study the hydration layers near nanoscale silica particles dispersed in water and aqueous electrolyte (NaCl and CaCl2) solutions in this work. The experimental results have shown that a strong hydration indeed applied to the silica surfaces in aqueous solutions, leaving a large volume of hydration layers on the surfaces. Also, it has been found that the hydration of the nanoscale silica particles could be greatly enhanced if they were dispersed in aqueous NaCl or CaCl2 solutions, which might be attributed to that the hydrated cations (Na+ or Ca2+) bind onto the silica/ water interface and thus increase the volume of the hydration layers.     

Cation-mediated interaction of dextran sulfate with phospholipid vesicles: binding, vesicle surface polarity, leakage and fusion

 The interaction of dextran sulfate (DS) with dimyristoylphosphatidylcholine (DMPC) large unilamellar vesicles was investigated. DS of different molecular weights (1, 8, 40 and 500 kDa) and divalent cations (Ca²⁺, Mg²⁺ and Mn²⁺) and the trivalent cation La³⁺ were used in the experiments. Binding of DS was studied by use of the microelectrophoresis and monolayer technique. Binding depends strongly on cation and NaCl concentrations in the medium and does not occur in the absence of multivalent cations. Binding is modulated by the molecular weight of the polymers; DS with lower molecular weights lead to less negative zeta potentials at identical concentrations. A comparable monomer of DS, glucose-6-sulfate, does not change the zeta potential of DMPC vesicles. Monolayer experiments revealed a decrease in surface pressure after addition of multivalent cations and DS, indicating a stronger interaction of the cation–polymer complex with the phosphatidylcholine headgroups than its penetration into the phospholipid (PL) bilayer. The cation-mediated binding of DS to the vesicles leads to aggregation of the vesicles. The tendency to promote aggregation of DMPC vesicles is La³⁺>Ca²⁺>Mn²⁺≥ Mg²⁺. The aggregated vesicles show a stacklike arrangement of the bilayers as shown by freeze-fracture electron microscopy. The strong aggregation is accompanied by lipid mixing measured by the 1,4-nitrobenzo-2-oxa-1,3-diazole–phosphatidylethanolamine (PE)/lissamine rhodamine B sulfonyl-PE assay. At low ionic strength substantial lipid mixing can be observed in the previously mentioned order of the cations. This lipid mixing is accompanied by an increase in the permeability of the vesicles as revealed by the 1-aminonaphthalene-3,6,8-trisulfonic acid/p-xylenebis (pyridiium bromide) assay. The extent of leakage is determined by the cation used and the DS molecular weight. These interaction processes between the opposing bilayers are connected with a decrease in the water content in the gap between the opposing PL bilayers. As a measure for the change of the polar properties of the vesicle surface the shift of the emission wavelength of the fluorescent probe dansylphosphatidylethanolamine was measured. The effectiveness of divalent/trivalent cations to decrease the surface dielectric constant of DMPC vesicles also followed the sequence of ions as found for binding, PL mixing and leakage. The results are discussed in terms of the changed hydration at the bilayer surface induced by DS in the presence of multivalent ions. Received: 16 December 1998/Accepted: 17 December 1999     

Laws and mechanism of adsorption of cations by different ion-exchange forms of silica gel

Using radioactive tracer method, the regularities of adsorption of over 30 mono-, di- and trivalent cations including transition metal and lanthanide ions on the H, Ca- and Al-forms of silica gel are established. It has been shown that the affinity of cations of the same charge to the silica gel surface depends both on the nature of the adsorbing ion and the nature of exchangeable cation on the surface. Adsorption of alkali earth metal ions on the Ca-form of silica gel increases with a decrease of their radius i.e. an inversion of the sequence of adsorption compared to H- or Al-form of this adsorbent or polymeric cation-exchange resins takes place. For lanthanide ions the sequence of adsorption is the same for all ion-exchange forms of the silica gel studied, namely, an increase of adsorption with a decrease of their crystallographic radius, i.e. from La ³⁺ to Lu ³⁺ takes place. The laws observed are explained by taking into account the fact that adsorption of cations by silica gel is determined by both electrostatic interactions and additional covalent/donor–acceptor interactions between the surface and cations. The latter is due to formation of π-bonds between the electron pair in surface oxygen and vacant p-, d- or f-orbital of adsorbing cations.     

Role of acid–base interactions on the adhesion of oral streptococci and actinomyces to hexadecane and chloroform—influence of divalent cations and comparison between free energies of partitioning and free energies obtained by extended DLVO analysis

Calcium is an abundantly present, divalent cation in the oral cavity and plays a crucial role in the adhesion of oral microorganisms to tooth surfaces as well as in coaggregation and coadhesion among the oral microflora. The aim of this study was to determine the effects of divalent cation (Ca²⁺, Mg²⁺, Ba²⁺) adsorption on the adhesion of two actinomyces and two streptococcal strains to hexadecane (MATH) and chloroform (MATS) in order to detect changes in acid–base character of the cell surfaces. Initial removal rates of the organisms by hexadecane, lacking an acid–base interaction with the organisms, were always smaller than those by chloroform. Furthermore, adsorption of divalent cations generally increased the initial removal rates of the microorganisms, but no statistically significant differences among different cations were observed. Gibbs energies of partitioning calculated from the stationary end-point adhesion of the organisms ranged from −2 to −4 kT for adhesion to hexadecane and were about twofold more negative for adhesion to chloroform. Contact angles on lawns of microorganisms with and without adsorbed divalent cations were similar. Zeta potentials of all microorganisms were slightly negative under the conditions of MATH and MATS and became only 4 mV more positive upon divalent cation adsorption. Hexadecane had a zeta potentials of −21 mV in the potassium phosphate solution used, which became 13 mV less negative upon Ca²⁺ adsorption. An extended DLVO approach of microbial adhesion to hexadecane, based on microbial contact angles and zeta potentials, taking into account Lifshitz–van der Waals, acid–base and electrostatic interactions did not show any potential energy barrier and demonstrated a deep primary interaction minimum at close approach due to acid–base attraction. As the Gibbs energy of partioning was only −2 to −4 kT, it is concluded that for the collection of organisms studied here, the final contactable surface area is small and structural features on the cell surfaces like fibrils and fimbriae, maintain a distance of ca. 10–15 nm between the hexadecane and the overall cell surface and therewith prevent acid–base interactions to become operative to a significant extend. Furthermore, from the lack of influence of divalent cations on macroscopic cell surface contact angles and zeta potentials, it is suggested that cation adsorption is minor and localized to the fibrils and fimbriae.     

Transport behavior of humic acid-modified nano-hydroxyapatite in saturated packed column: effects of Cu, ionic strength, and ionic composition

The surfaces of nano-hydroxyapatite (nHAP) used for contaminated soil and groundwater remediation may be modified to render nHAP highly mobile in the subsurface. Humic acid (HA) is widely used to modify and stabilize colloid suspensions. In this work, column experiments were conducted to determine the effects of contaminant (e.g., Cu) concentration, ionic strength (IS), and ion composition (IC) on the transport behavior of HA-modified nHAP in saturated packed columns. IS and nature of the cation had strong effects on the deposition of nHAP, and the effect was greater for divalent than for monovalent cations. Divalent cations have a greater capacity to screen the surface charge of nHAP, and Ca(2+) bridges the HA-modified nHAP colloidal particles, which causes greater deposition. Moreover, Cu(2+) had a greater effect on the transport behavior than Ca(2+) due to their strong exchange with Ca(2+) of nHAP and its surface complexation with nHAP. The relative travel distance L(T), of the injected HA-modified nHAP colloids, ranges from less than one to several meters at varying Cu concentrations, ISs, and ICs in saturated packed columns. The results are crucial to evaluate the efficacy of nHAP on the remediation of contaminated soil and groundwater environments.