Influence of monovalent ion size on colloidal forces probed by Monte Carlo simulations

The present work studies the role of ionic size in the interactions between the electrical double layers of colloids immersed into electrolyte solutions of monovalent ions. Such interactions are studied by means of Monte Carlo (MC) simulations and the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Despite the omission of the steric effects and some other features of real electrolyte solutions, DLVO theory is known to work qualitatively well for 1:1 electrolyte solutions. However, this affirmation is based on previous tests where an ionic diameter around 0.4 nm was taken for all ionic species. In contrast, some experimental studies suggest that larger hydrated ions should be considered and even specified for each type of ion. In this work, the importance of ionic size is analyzed by applying the primitive model of electrolyte to the intermediate region between a pair of equally charged infinite planar surfaces. The double layer interactions were calculated from the ionic densities at the distance of closest approach to the charged surfaces, this method constitutes an alternative to the traditional calculations at the midplane. Our MC simulations predict the existence of negative net pressures for monovalent electrolytes in the case of zero charge density. In addition, MC simulations reveal some disagreements with theoretical predictions for ionic diameters larger than 0.4 nm. These discrepancies can become significant if surface charge density is large enough due to the restructuration of the double layer. The physical mechanisms for these deviations are also discussed.     

Indirect photometric detection of monovalent cations via postsuppressor ion replacement in microcolumn ion chromatography

Summary Monovalent cations were indirectly detected via postcolumn suppressor ion replacement in microcolumn ion chromatography. The ion-repalcement column loaded by chromophoric ions was connected to the suppressor column. The eluent, nitric acid, was converted to water through the suppressor and anion-replacement columns, while the analyte cations were coeluted with the chromophoric anions. The analyte cations were indirectly detected by measuring UV absorptions of the chromophoric anions.     

Quantifying the effect of ionic strength on colloidal fouling potential in membrane filtration

Ultrafiltration experiments were conducted to study the fouling potential of colloidal suspensions under different ionic strengths and colloid concentrations. A linear relationship was found relating the colloidal fouling potential to the logarithm of the Debye-Huckel parameter, a characteristic for electrical double layers of colloids. This finding provided a useful quantitative linkage between the colloidal fouling potential and the water chemistry. Considering the linear dependence of colloidal fouling potential on the colloid concentration, a bilinear model was proposed to explain the coupling effects of colloid concentration and ionic strength of the suspension on the fouling potential. The model predictions of fouling potential were found to fit accurately with experimentally determined fouling potential values. Further analysis of the model showed that ionic strength can significantly affect colloidal fouling, for example, a 10-fold increase in ionic strength from 0.001 to 0.01 M for a given feed concentration has the same membrane fouling effect as doubling the feed concentration. The model allows for a quick and reliable assessment of fouling potential without even performing any experiments. This could then be used to design the membrane process or pretreatment stages required to mitigate membrane fouling.     

Electrodialysis of calcium and carbonate high concentration solutions and impact on composition in cations of membrane fouling

Fouling, which is the accumulation of undesired solid materials at the phase interfaces of permselective membranes, is one of the major problems in electrodialysis. The objectives of the present work were to investigate the effect of the composition in calcium and carbonate of a model solution to be treated by conventional electrodialysis on their migration kinetics and the composition in cations of the membrane fouling. In the absence of sodium carbonate in the solution, no fouling was visually observed on anion-exchange membranes (AEM) and fouling was observed only at 1600 mg/L CaCl2 on cation-exchange membrane (CEM), while at only 800 mg/L CaCl2 with sodium carbonate, a deposit was observed on both membranes. This difference could be explained by the fact that carbonate has a high buffer capacity, and the time to reach pH 4.0 was then longer than the one without carbonate. Consequently, the migration of the ionic species was carried out over a longer period of time during ED treatment with sodium carbonate addition and in extent the demineralization rates were higher: 43 vs 86%. For treatment with sodium carbonate and 1600 mg/L CaCl2, the higher migration during ED treatment, increased the concentration of calcium, from 14.24 to 93.38 mg/g dry membrane and from 0.74 to 10.27 mg/g dry membrane for CEM and AEM, respectively. Due to the basic pH on the side of the membrane in contact with the NaCl solution, the calcium would precipitate to form calcium hydroxide on CEM while the calcium migrated through the CEM was blocked by the AEM where it formed another fouling.     

A new normalization method for determination of colloidal fouling potential in membrane processes

Normalization of permeate flux data is widely used to characterize membrane fouling under different experimental conditions. The main intention of normalization is to allow a fair comparison of feed water fouling potentials by eliminating the effects of different operational parameters used in the experiments, such as net driving pressure and clean-membrane resistance. However, it was demonstrated that the commonly used intuitive normalization methods usually could not serve their intended purpose. In this study, a new normalization method was proposed for characterizing water-fouling potential based on fundamental principles of membrane fouling. The intention of this normalization method was to define a fouling potential for feed water that was independent of, or at least, not strongly affected by operational conditions. Laboratory-scale ultrafiltration fouling tests were conducted under different colloid sizes, concentrations, and driving pressures. The experiments showed that the fouling potentials defined by the newly proposed normalization method were linearly related to the colloid concentration of the feed water and that the effect of operational conditions used in the fouling experiments on the fouling potential was minimal.     

Measurement of the effect of monovalent cations on RNA hairpin stability

Using optical tweezers, we have measured the effect of monovalent cation concentration and species on the folding free energy of five large (49-124 nt) RNA hairpins, including HIV-1 TAR and molecules approximating A.U and G.C homopolymers. RNA secondary structure thermodynamics are accurately described by a model consisting of nearest-neighbor interactions and additive loop and bulge terms. Melting of small (<15 bp) duplexes and hairpins in 1 M NaCl has been used to determine the parameters of this model, which is now used extensively to predict structure and folding dynamics. Few systematic measurements have been made in other ionic conditions or for larger structures. By applying mechanical force, we measured the work required to fold and unfold single hairpins at room temperature over a range of cation concentrations from 50 to 1000 mM. Free energies were then determined using the Crooks fluctuation theorem. We observed the following: (1) In most cases, the nearest-neighbor model accurately predicted the free energy of folding at 1 M NaCl. (2) Free energy was proportional to the logarithm of salt concentration. (3) Substituting potassium ions for sodium slightly decreased hairpin stability. The TAR hairpin also misfolded nearly twice as often in KCl, indicating a differential kinetic response. (4) Monovalent cation concentration affects RNA stability in a sequence-dependent manner. G.C helices were unaffected by changing salt concentration, A.U helices were modestly affected, and the hairpin loop was very sensitive. Surprisingly, the U.C.U bulge of TAR was found to be equally stable in all conditions tested. We also report a new estimate for the elastic parameters of single-stranded RNA.     

Metastable ion studies of various C3H6 radical cations

Metastable abundance ratios have been measured involving four decomposition reactions of C₃H₆ radical cations formed from a variety of precursors. The ratios are quite similar in accord with extensive isomerization to a propene structure prior to fragmentation. Small, yet constant differences are observed for those C₃H₆ ions which have been shown to be formed as cyclic ions by ion cyclotron resonance studies. The differences are interpreted to reflect internal energy variations, which result because the initially formed ions have two different structures. The abundance ratios are shown to depend on ionizing energy, repeller voltage and accelerating voltage, but are independent of the degrees-of-freedom in the precursor as well as the number of steps necessary to produce the [C₃H₆]^+· Despite small variations in metastable ratios, the classification of various [C₃H₆]^+· ions can be achieved under a variety of conditions which affect the internal energy of the decomposing ions.     

Influence of membrane surface properties on initial rate of colloidal fouling of reverse osmosis and nanofiltration membranes

Recent studies have shown that membrane surface morphology and structure influence permeability, rejection, and colloidal fouling behavior of reverse osmosis (RO) and nanofiltration (NF) membranes. This investigation attempts to identify the most influential membrane properties governing colloidal fouling rate of RO/NF membranes. Four aromatic polyamide thin-film composite membranes were characterized for physical surface morphology, surface chemical properties, surface zeta potential, and specific surface chemical structure. Membrane fouling data obtained in a laboratory-scale crossflow filtration unit were correlated to the measured membrane surface properties. Results show that colloidal fouling of RO and NF membranes is nearly perfectly correlated with membrane surface roughness, regardless of physical and chemical operating conditions. It is further demonstrated that atomic force microscope (AFM) images of fouled membranes yield valuable insights into the mechanisms governing colloidal fouling. At the initial stages of fouling, AFM images clearly show that more particles are deposited on rough membranes than on smooth membranes. Particles preferentially accumulate in the “valleys” of rough membranes, resulting in “valley clogging” which causes more severe flux decline than in smooth membranes.     

Aggregation of nanosized colloidal silica in the presence of various alkali cations investigated by the electrospray technique

The slow aggregation process of a concentrated silica dispersion (Bindzil 40/220) in the presence of alkali chlorides (LiCl, NaCl, KCl, RbCl, and CsCl) was investigated by means of mobility measurements. At intervals during the aggregation, particles and aggregates were transferred from the liquid phase to the gas phase via electrospray (ES) and subsequently size selected and counted using a scanning mobility particle sizer (SMPS). This method enables the acquisition of particle and aggregate size distributions with a time resolution of minutes. To our knowledge, this is the first time that the method has been applied to study the process of colloidal aggregation. The obtained results indicate that, independent of the type of counterion, a sufficient dilution of the formed gel will cause the particles to redisperse. Hence, the silica particles are, at least initially, reversibly aggregated. The reversibility of the aggregation indicates additional non-DLVO repulsive steric interactions that are likely due to the presence of a gel layer at the surface. The size of the disintegrating aggregates was monitored as a function of the time after dilution. It was found that the most stable aggregates were formed by the ions that adsorb most strongly on the particle surface. This attractive effect was ascribed to an ion-ion correlation interaction.     

Colloid vibration potential and ion vibration potential in a dilute suspension of spherical colloidal particles

A general electroacoustic theory is presented for the macroscopic electric field in a dilute suspension of spherical colloidal particles in an electrolyte solution, which consists of the colloid vibration potential (CVP) and the ion vibration potential (IVP), induced by an oscillating pressure gradient field due to an applied sound wave. This is a unified theory that unites previous theories for CVP and those for IVP. Approximate analytic expressions are derived for CVP and IVP. The obtained IVP expression agrees with Debye's formula that is corrected by taking into account the force acting on the electrolyte ions as a result of the pressure gradient in the sound wave. The obtained CVP expression is correct to the first order of the particle zeta potential and applicable for arbitrary kappaalpha, where kappa is the Debye-Hückel parameter and alpha is the particle radius. It is found that an Onsager relation holds between CVP and dynamic electrophoretic mobility. It is also shown that the CVP from particles with very small kappaalpha approaches IVP; that is, in the limit of very small kappaalpha a particle behaves like an ion.     

Thermal stability of aluminium hydroxycarbonates with monovalent cations

Dawsonite-type compounds of formula MAl(OH)₂CO₃ (M = Na. K, NH₄) as well as a laminar hydrotalcite-type hydroxycarbonate of composition [Al₂Li(OH)₆]₂CO₃·4H₂O. have been hydrothermally synthesized The thermal decomposition of these compounds was monitored by DTA and TG, and the resulting products have been studied by X-ray and IR techniques. Sodium and potassium dawsonites are destroyed at 335°C. yielding a poorly crystalline compound in which part of the overall carbonate is present; the remaining carbonate is lost between 600 and 700°C, yielding NaAlO₂ and KAlO₂, respectively Ammonium dawsonite and lithium hydrotalcite are less stable, their thermal decomposition occurring at about 240°C. The ammonium dawsonite heated at 680°C shows the presence of A1₂O₃ with a poorly ordered structure, while lithium hydrotalcite yields poorly crystalline γ-Al₂O₃ at 500°C and a mixture of γ-LiAlO₂ and LiAl₅O₈ when the compound is heated at higher temperatures ( ~ 1000°C).     

Influence of aluminum on the uptake of various cations from a solution into carrots

The influence of Al on the absorption of various elements by a carrot (U.S. harumakigosun) was investigated using a multitracer technique. An uptake experiment was conducted within the range of 0.0–2.0 ppm AlCl₃ in culture solution. By the addition of AlCl₃, uptake of elements such as Be, V, Zn and rare earth elements (REE) into roots was increased. For Be and V an approximately three-fold increase was observed. The degree of uptake enhancement of nonessential elements by AlCl₃ was generally very high, whereas some of the essential or beneficial elements exhibited a decrease in uptake with the increase of AlCl₃ concentration. This suggests that the uptake of nonessential elements might be increased through transporters with decreased selectivity due to Al. From the viewpoint of the acid rain problem, it is suggested that one of the detrimental effects of Al on plants is the imbalanced elemental absorption.     

Influence of the sodium and calcium non-framework cations on the adsorption of hexane isomers in zeolite BEA

We have performed configurational-bias Monte Carlo simulations to compute pure component adsorption isotherms of n-hexane, 3-methylpentane and 2,2-dimethylbutane in BEA-Polymorphs A and B at 423?K. The effect of the density and nature of influence of non-framework cations was systematically analyzed. Our results show that differences in the type and concentration of the non-framework cations lead to differences in adsorption loading. We found that this behavior is directly related to the preferential adsorption sites of the isomers as well as to the amount and location of the non-framework cations.     

Probing the mechanical stability of DNA in the presence of monovalent cations

We examine the interaction between monovalent cations and DNA using several different assays that measure the stability of double-stranded DNA (dsDNA). The thermal melting of dsDNA and the mechanical separation of dsDNA into two single strands both depend on the stability of dsDNA with respect to ssDNA and are sensitive to the interstrand phosphate repulsion. We find that the experimentally measured melting temperatures and unzipping forces are approximately the same for all of the ions considered in this study. Likewise, the force required to transform B-DNA into the overstretched form is also similar for all of the ions. In contrast, for a given ion concentration, the force at which the overstretched state fully relaxes back to the canonical B-DNA form depends on the cation; however, for all cations, the overstretching force decreases with decreasing ion concentration, suggesting that this force is sensitive to screening. We observe a general trend for smaller ions to produce more efficient relaxation. Finally, for a given cation, the relaxation can also depend on the anion.     

Hydrated copper and gold monovalent cations: Ab initio study

To understand the hydration phenomena of noble transition metals, we investigated the structures, hydration energies, electronic properties, and spectra of the Cu(+)(H(3)O)(1-6) and Au(+)(H(2)O)(1-6) clusters using ab initio calculations. The coordination numbers of these clusters are found to be only two, which is highly contrasted to those of Ag(+)(H(2)O)(n) (which have the coordination numbers of 3-4) as well as the hydrated alkali metal ions (which have the coordination numbers of approximately 6). For the possible identification of their interesting hydration structures, we predict their IR spectra for the OH stretch modes.     

Bifunctional sulfur-silicon podands as new nucleophilic ionophores in acyl transfer reactions. Influence of monovalent cations on the reaction kinetics

Two bifunctional sulfur-silicon nucleophilic ionophores 3-(trimethoxysilyl)-propanethiol (Nu1) and 3-(tri-2-methoxyethoxysilyl)-propanethiol (Nu2) were used as reagents in the acyl transfer reaction studied by kinetic methods. Nuclear magnetic resonance (NMR) and electro-spray ionisation mass spectrometry (ESI MS) were used for determination of the stoichiometry and stability constants of the complexes made by the podands and lithium or sodium ions in acetonitrile solution.     

Effect of monovalent and divalent cations on the photoinactivation of bacteria with meso-substituted cationic porphyrins

It is well established that for successful photoinactivation (PI) of gram-negative bacteria a cationic photosensitizer is required. This requirement suggests a charge-dependent interaction between the photosensitizer and the gram-negative bacterium, which may be influenced by the presence of ions in the suspending medium. The aim of the present study was to investigate the effect of cations Na+ and Ca2+ on the efficacy of the PI of the gram-negative Pseudomonas aeruginosa and the gram-positive Staphylococcus aureus. The bacteria were suspended in buffer containing either meso-tetra(N-methyl-4-pyridyl)-porphyrin or meso-mono-phenyl-tri(N-methyl-4-pyridyl)-porphyrin as photosensitizer and various concentrations of Na+ or Ca2+. The cell suspensions were exposed to a broadband light dose of 9 J/cm2. In buffer without added cations, P. aeruginosa and S. aureus were equally sensitive to PI. Addition of cations strongly decreased the sensitivity of both bacteria to PI, with the PI of P. aeruginosa being much more decreased than that of S. aureus, and Ca2+ being more effective than Na+. The decreased sensitivity was accompanied by a reduced binding of the photosensitizers to the bacteria.     

Removal of divalent cations reduces fouling of ultrafiltration membranes

The effect of divalent ions on hydraulic irreversible fouling of ultrafiltration membranes was studied. Not only the effect of removing divalent ions by pretreatment of raw water with ion exchange is quantitatively studied, but also the effects of different types of backwash water are considered. By replacing divalent ions with sodium in cation exchange, the amount of hydraulic irreversible fouling (remaining fouling after backwashing) is reduced by at least 60%. When adding either calcium or magnesium to water treated with cation exchange, a linear relation is found between the ion concentration and the irreversible fouling rate. The effects of calcium and magnesium are identical when the concentrations are expressed in mol/L. Removing divalent ions from the backwash water does not affect irreversible fouling, but when using MilliQ water as backwash water, irreversible fouling can (almost) completely be prevented.