Dilute Ionic Solutions Research Articles

Specific iron binding to natural sphingomyelin membrane induced by non-specific co-solutes

HypothesisSphingomyelin (SPM), a crucial phospholipid in the myelin sheath, plays a vital role in insulating nerve fibers. We hypothesize that iron ions selectively bind to the phosphatidylcholine (PC) template within the SPM membrane under near-physiological conditions, resulting in disruptions to membrane organization. These interactions could potentially contribute to the degradation of the myelin sheath, thereby playing a role in the development of neurodegenerative diseases. ExperimentsWe utilized synchrotron-based X-ray spectroscopy and diffraction techniques to study the interaction of iron ions with a bovine spinal-cord SPM monolayer (ML) at the liquid-vapor interface under physiological conditions. The SPM ML serves as a model system, representing localized patches of lipids within a more complex membrane structure. The experiments assessed iron binding to the SPM membrane both in the presence of salts and with additional evaluation of the effects of various ion species on membrane behavior. Grazing incidence X-ray diffraction was employed to analyze the impact of iron binding on the structural integrity of the SPM membrane. FindingsOur results demonstrate that iron ions in dilute solution selectively bind to the PC template of the SPM membrane exclusively at near-physiological salt concentrations (e.g., NaCl, KCl, KI, or CaCl2) and are pH-dependent. In-significant binding was detected in the absence of these salts or at near-neutral pH with salts. The surface adsorption of iron ions is correlated with salt concentration, reaching saturation at physiological levels. In contrast, multivalent ions such as La3+ and Ca2+ do not bind to SPM under similar conditions. Notably, iron binding to the SPM membrane disrupts its in-plane organization, suggesting that these interactions may compromise membrane integrity and contribute to myelin sheath damage associated with neurological disorders.

Reexpansion of charged nanoparticle assemblies in concentrated electrolytes

Electrostatic forces in solutions are highly relevant to a variety of fields, ranging from electrochemical energy storage to biology. However, their manifestation in concentrated electrolytes is not fully understood, as exemplified by counterintuitive observations of colloidal stability and long-ranged repulsions in molten salts. Highly charged biomolecules, such as DNA, respond sensitively to ions in dilute solutions. Here, we use non-base-pairing DNA-coated nanoparticles (DNA-NP) to analyze electrostatic interactions in concentrated salt solutions. Despite their negative charge, these conjugates form colloidal crystals in solutions of sufficient divalent cation concentration. We utilize small-angle X-ray scattering (SAXS) to study such DNA-NP assemblies across the full accessible concentration ranges of aqueous CaCl2, MgCl2, and SrCl2 solutions. SAXS shows that the crystallinity and phases of the assembled structures vary with cation type. For all tested salts, the aggregates contract with added ions at low salinities and then begin expanding above a cation-dependent threshold salt concentration. Wide-angle X-ray scattering (WAXS) reveals enhanced positional correlations between ions in the solution at high salt concentrations. Complementary molecular dynamics simulations show that these ion-ion interactions reduce the favorability of dense ion configurations within the DNA brushes below that of the bulk solution. Measurements in solutions with lowered permittivity demonstrate a simultaneous increase in ion coupling and decrease in the concentration at which aggregate expansion begins, thus confirming the connection between these phenomena. Our work demonstrates that interactions between charged objects continue to evolve considerably into the high-concentration regime, where classical theories project electrostatics to be of negligible consequence.

Metallo-Supramolecular Complexation Behavior of Terpyridine- and Ferrocene-Based Polymers in Solution-A Molecular Hydrodynamics Perspective.

The contribution deals with the synthesis of the poly(methacrylate)-based copolymers, which contain ferrocene and/or terpyridine moieties in the side chains, and the subsequent analysis of their self-assembly behavior upon supramolecular/coordination interactions with Eu3+ and Pd2+ ions in dilute solutions. Both metal ions provoke intra and inter molecular complexation that results in the formation of large supra-macromolecular assembles of different conformation/shapes. By applying complementary analytical approaches (i.e., sedimentation-diffusion analysis in the analytical ultracentrifuge, dynamic light scattering, viscosity and density measurements, morphology studies by electron microscopy), a map of possible conformational states/shapes was drawn and the corresponding fundamental hydrodynamic and macromolecular characteristics of metallo-supramolecular assemblies at various ligand-to-ion molar concentration ratios () in extremely dilute polymer solutions were determined. It was shown that intramolecular complexation is already detected at (), while at solution/suspension precipitates. Extreme aggregation/agglomeration behavior of such dilute polymer solutions at relatively “high” metal ion content is explained from the perspective of polymer-solvent and charge interactions that will accompany the intramolecular complexation due to the coordination interactions.

Enhancing the interaction between cellulose and dilute aqueous ionic liquid solutions and its implication to ionic liquid recycling and reuse

Cellulose-dissolving ionic liquids (ILs) have been used in biomass pretreatment for over a decade. Cellulose solubility in the ILs is strongly inhibited by water, which has negative impacts on IL pretreatment and reuse of the recycled ILs. Here, a distillation and aeration apparatus was used as the reactor for biomass pretreatment in dilute aqueous IL solutions and in recycled IL liquor without drying or purification. Four biomass types, switchgrass, miscanthus, sorghum and pine, were studied. X-ray diffraction (XRD) was used to measure the interaction between biomass and the IL. Small angle neutron scattering (SANS) was applied to monitor the changes of the pore structure in wet biomass samples. Satisfactory enzymatic hydrolysis results were obtained among all the pretreated samples.

Capacitance response and concentration fluctuations close to ionic liquid-solvent demixing

We investigate the concentration fluctuations in a simple model of electrolyte (two positively and negatively charged Lennard-Jones spheres in a solution of an uncharged Lennard-Jones liquid) confined between electrodes formed by parallel graphene layers. Using constant potential molecular dynamics simulations and extensive constant charge simulations the effect of the proximity to the demixing transition on the electric double layers is analyzed and compared to the results of our continuum mean field theory. In agreement with our previous theoretical findings, we observe a considerable enhancement of the capacitance when temperature is lowered approaching the demixing transition for dilute ionic solutions. This enhancement is less visible when the ionic concentration is increased. Moreover, we observe the new shape of the capacitance, with a maximum at potential of zero charge, and symmetric maxima for positive and negative voltages.

Naphthalimide-Based Hydrazone Derivatives: Synthesis, Mechanochromism in the Solid State and Response to Ions in Dilute Solutions.

Molecules showing mechanochromic luminescence (MCL) are promising for use in the in the fields of sensing and probes. We report the design and synthesis of new naphthalimide-based hydrazone derivatives, NI-TPE and NI-3BA. Both the luminogens are weakly emissive with s Φf =0.3 % and 0.5 % respectively when aggregated in amorphous states as strong π-π stacking and intermolecular interaction prevent luminescence. On the contrary, in the crystalline state, single crystal analysis of two derivatives shows that nonradiative decay is reduced or inhibited by molecular stacking modes and intermolecular interactions. Increases of fluorescence emission intensity to s Φf =5.5 % and 6.0 % upon solvent evaporation are attributed to weak π-π overlapping and hydrogen bonding (N-H ⋅⋅⋅ O, distance 2.99 Å), which are beneficial to the formation of molecules with a loose packing. At the same time, the packing modes that the two derivatives adopt in the crystal lattice are destroyed to result in a low solid-state fluorescence quantum yield and a bathochromic shift of 23-25 nm upon grinding. All these factors cause the two derivatives show an unusual "turn off" MCL phenomenon. The fluorescence emission, its pH reversibility, and selective response to fluoride and acetate ions of up to 91-93 % in dilute solutions were also demonstrated.

Combining soft electrode and ion exchange membranes for increasing salinity difference energy efficiency

A large variety of routes have been envisaged for improving the efficiency of methods aimed at producing clean, renewable energy. One of them includes the so-called capmix techniques, whereby clean electrical energy is obtained by alternately contacting salty and fresh solutions with capacitive electrodes. Investigations in this area have focused on optimizing materials, cycle implementation, up-scaling, and so on. One possibility that has been explored regards the use of ion exchange membranes for producing the necessary voltage differences between electrodes when in contact with concentrated and dilute ionic solutions. A related, but different approach consists in treating the electrodes with charged layers, notably polyelectrolytes. In this work, we propose an original combination of the two most efficient capmix techniques by simultaneously coating the carbon electrodes with membranes and polyelectrolytes. With the objective of analyzing the benefit of this new proposal in terms of energy and power, CAPMIX cycles were performed and carefully examined. These results will lead to the first insight into the ensemble behavior of polyelectrolyte layers and exchange membranes which will be key for the overall improvement of the CAPMIX methods.

Benzo-15-crown-5 (2,3,5,6,8,9,11,12-octahydrobenzo[b][1,4,7,10,13]pentaoxacyclopentadecine) and dibenzo-15-crown-5 (6,7,9,10,17,18-hexahydrodibenzo[b,h][1,4,7,10,13]pentaoxacyclopentadecine as fluorescent probes for physiologically important potassium ion

Cation recognition plays a vital role in defining advanced functions of macromolecules in nature. An example of such an interaction is the action of a natural antibiotic, valinomycin, that behaves as a potassium ionophore. It encages the cation to transport it across a cell membrane and easily releases it inside the cell. Macrocyclic complexes of some crown ethers mimick alkali ion interactions with natural ionophores. We have synthesized complexes of two crown ethers namely benzo-15-crown-5 (2,3,5,6,8,9,11,12-octahydrobenzo[b][1,4,7,10,13]pentaoxacyclopentadecine) and dibenzo-15-crown-5 (6,7,9,10,17,18-hexahydrodibenzo[b,h][1,4,7,10,13]pentaoxacyclopentadecine with potassium halides (fluoride, chloride, bromide and iodide) in acetonitrile and characterized them by IR, UV, ESI-MS, 1H NMR and 13C NMR techniques. The effect of the anion on the stability of the complexes was observed by 1H NMR studies. The alkali metal ion is held to the oxygen donor atoms of the macrocyclic ring by ion-dipole interactions. The potential of the small ring oxacrown ethers, benzo-15-crown-5 and dibenzo-15-crown-5, to act as probes for potassium ions in dilute solutions (4.1 × 10−4 M) was investigated by recording the variation in the fluorescence spectra of benzo-15-crown-5 and dibenzo-15-crown-5 on complexation with potassium fluoride in acetonitrile and chloroform. The rigidity of the macrocyclic ring and proximity of fluorophore units affected the fluorescence intensity of the complexes.

Additional Arguments for a Correction of the Debye-Hückel, Maxwell-Boltzmann Equations for Dilute Electrolyte Equilibria

Peter Debye and Erich Huckel had developed a theory for the ionic activity coefficients in dilute solutions of strong electrolytes some 95 years ago [1]. Their limiting law still stands and is confirmed as close to reality in many experiments. In a previous article [2], it is shown that these limiting activity coefficients arise because the electrical contribution in the electrochemical potential of ionic species is overestimated traditionally with a factor 2. The smaller value removes inconsistencies in the models and complies better with the basic electrostatic principles. In this article further evidence is given in support of this alternative description. As consequence the dilute activity coefficients become unity, e.g. are removed, which means that the electrochemical potential of ions in dilute solutions is expressed directly in concentration, instead of activity, which simplifies modelling in such dilute solutions.

Comparison of electrodialysis and reverse electrodialysis processes in the removal of Cu(II) from dilute solutions

Electrodialysis (ED) and electrodialysis reversal (EDR) processes have been often used for separation of ions in dilute solutions. In this study, the performance of ED and EDR processes has been examined in the removal of copper from the dilute solutions. First, applied voltage, initial concentration, flow rate, type of electrolyte and the effect of concentration were determined for both processes. Then, separation efficiency, current efficiency, energy requirement and material flux of the processes were calculated, and the performances of the processes were compared. The separation efficiency and energy consumption of EDR process were higher compared to ED process under equal operating conditions. Also, the current efficiency (39.58%) of EDR process was lower than the current efficiency (67.46%) of ED process. It can be said that the ED process is more suitable in terms of energy consumption for separation in the low flow rate and concentration.

RAFT polymerization and dually responsive behaviors of terpyridine-containing PNIPAAm copolymers in dilute solutions

Introduction of new components is an effective way to gain new functionalities and to regulate the properties of responsive polymers. In the present work, a RAFT polymerization approach to terpyridine-containing poly(N-isopropyl acrylamide) (PNIPAAm) copolymers was developed and the dually responsive behavior of the obtained polymer to temperature variation and metal ions in dilute solutions were investigated. By utilizing RAFT polymerization, the molecular weight polydispersity index of the polymer can be reduced compared with the traditional free radical polymerization method. Due to the coexistence of thermosensitive component and terpyridine, the synthesized polymer is thermosensitive and metal ion sensitive. Further titration investigation revealed that this polymer may be utilized as a water-soluble and colorimetric sensor for Fe2+ and Fe3+ and the obvious color change from a colorless state to a purple state makes the polymer a potential sensor to detect Fe2+ and Fe3+ macroscopically by naked eye. Overall, the terpyridine-containing poly(N-isopropyl acrylamide) copolymers may enrich the family of terpyridine-containing polymers and provide some clues for developing new multi-responsive and multi-functional polymers.

Permeability of dilute ionic liquid solutions through a nanofiltration membrane – Effect of ionic liquid concentration, filtration pressure and temperature

Abstract Nanofiltration is studied to separate glucose from dilute ionic liquid [emim][OAc]–water solutions. The aim of the research is to evaluate the effect of filtration pressure, temperature and ionic liquid (IL) concentration on the separation of ionic liquid and glucose. This kind of separation challenge can be formed in the regeneration of cellulose fibres in a wet spinning bath. The separation tests are done with an NF270 nanofiltration membrane. The tolerance of the NF membrane in ionic liquid is also estimated. Effective osmotic pressures and viscosity were found to have a major role in the permeability of the membrane when filtration was done at dilute IL solutions. Temperature was found to have a significant effect on the flux due to reduced viscosity at higher temperatures. The flux was increased by about 50% when the temperature increased from 20 °C to 40 °C. The best possible separation factor values were achieved when the feed solution contained 1 wt% [emim][OAc] at 20 °C temperature and the filtration pressure was 6 bars. On the basis of the IL concentration and temperature test series, a good separation window for this kind of solution would be at permeate flux values from 50 L m−2 h−1 to 100 L m−2 h−1. After a test series in concentrations of IL from 0.5 wt% to 10 wt%, the pure water permeability (PWP) was reduced by 23% from the original. The decline of PWP during the temperature tests series was 33%.

Light-scattering characteristics of hydrated ions in dilute solutions of major sea salts

The major ions in seawater include Na+, Mg2+, Cl−, Ca2+, and K+, altogether accounting for >96% of the total mass of sea salts. The radii of ions in solution depend on the number of the solvent molecules in their solvation shells, and so would have an influence on the light-scattering of the aqueous solution. Taking in to account density fluctuation theory of pure water and Rayleigh scattering of hydrated ions, a theoretical model was developed estimating the light-scattering characteristics of the dilute ionic aqueous solution. The results show that there is a decreasing contribution due to density fluctuation, in the same salinity of solution, the most contribution to scattering due to density fluctuation is CaCl2 solution and MgCl2>KCl>NaCl. There is an increasing contribution due to hydrated ions for the light scattering. For each kind of hydrated ion, the contribution is Cl−>Mg2+>K+>Ca2+>Na+, and for each kind of solution, the contribution is MgCl2>CaCl2>NaCl>KCl. Comparing the two contribution for the light scattering in one solution, we can get the latter effect dominating.

Water response to intense electric fields: A molecular dynamics study.

This paper investigated polarization properties of water molecules in close proximity to an ionic charge in the presence of external electric fields by using an approach based on simulations at the atomic level. We chose sodium and chloride ions in water as examples of dilute ionic solutions and used molecular dynamics simulations to systematically investigate the influence of an external static electric field on structural, dipolar, and polarization properties of water near charged ions. Results showed that a threshold electric field higher than 10(8) V/m is needed to affect water polarization and increase mean dipole moment of water molecules close to the ion. A similar threshold holds for water permittivity profiles, although a field 10× higher is needed to ensure that water permittivity is almost constant independently of the position close to the ion. Electric fields of such intensities can greatly enhance polarizability of water in hydration shells around ions.

A pillar-layer MOF for detection of small molecule acetone and metal ions in dilute solution

A pillared-layer MOFs as luminescent probes has been covered which can be used to detect metal ions and small molecules. The results exhibit that its luminescent intensity is highly sensitive to Ni2+ and Cd2+ ions and acetone molecules.

Self-assembled polyelectrolyte surfactant nanocomposite membranes for pervaporation separation of MeOH/MTBE

Pervaporation separation of polar/non-polar mixtures is one of the attractive areas in membrane science and technology. In the present study, membranes with highly cross-linked nanometric selective layers were developed through layer-by-layer (LBL) assembly of polyanionic and cationic surfactants and explored for separation of methanol (MeOH)/methyl tert-butyl ether (MTBE) mixtures. Pristine polyelectrolyte surfactant complex (PELSC) composite membranes were fabricated by deposition of dilute and concentrated ionic solutions of sodium cellulose sulfate (NaCS) and hexadecylpyridinium chloride (HDPC) onto the polyacrylonitrile (PAN) ultrafiltration (UF) membrane supports. In addition, nanocomposite PELSC membranes containing different loadings of nano-sized SiO2 particles were fabricated, for the first time, by solution casting method. Morphological and Fourier transform infra-red (FTIR) characterizations confirmed homogeneous morphology of membranes and successful incorporation of nano-silica particles into nanocomposite precipitate. The effects of several fabrication (i.e., number of deposition steps and nano-silica loading) and operational parameters (i.e., feed temperature and concentration) on the separation performance of MeOH/MTBE were investigated. The results revealed that increasing feed temperature enhanced both flux and selectivity in case of pristine PELSC membranes. It was also observed that increasing nano-silica loading from 2wt% to 10wt% in nanocomposite PELSC membranes led to improvement in flux at the expense of selectivity. The highest flux (1.62kg/m2h) was obtained in nanocomposite PELSC membrane containing 10wt% nano-silica particles. This study demonstrates successful development of unique structures of PELSC and nanocomposite PELSC membranes with considerable potential application for pervaporation separation of polar/non-polar mixtures.

Diffusion Limit of Kinetic Equations for Multiple Species Charged Particles

In ionic solutions, there are multi-species charged particles (ions) with different properties like mass, charge etc. Macroscopic continuum models like the Poisson–Nernst–Planck (PNP) systems have been extensively used to describe the transport and distribution of ionic species in the solvent. Starting from the kinetic theory for the ion transport, we study a Vlasov–Poisson–Fokker–Planck (VPFP) system in a bounded domain with reflection boundary conditions for charge distributions and prove that the global renormalized solutions of the VPFP system converge to the global weak solutions of the PNP system, as the small parameter related to the scaled thermal velocity and mean free path tends to zero. Our results may justify the PNP system as a macroscopic model for the transport of multi-species ions in dilute solutions.

SolITC: A rapid method to determine solubility products and heats of solution of hydrophobic ionic liquids from in situ synthesis in isothermal titration calorimeter

This work presents a solubility isothermal titration calorimetry (solITC), a new calorimetric method that allows the determination of solubility product (Ksp) and heat of solution (ΔsolH°) of hydrophobic ionic compound in water. Based on a single titration of dilute ionic solution into its counterionic solution the method consists of observing the abrupt change of the heat flow signal due to hydrophobic phase separation and fitting the integrated heats to exact thermodynamic model. A priori analysis shows that the applicability range spans for Ksp determination over five orders of magnitudes (typically 10-9<Ksp<10-4 for |ΔsolH°|≈30kJ·mol-1) keeping at least 5% precision in both Ksp and ΔsolH° values. For Ksp<10-10, only ΔsolH° can be precisely determined. The method performance was validated on forming three different types of precipitate with different levels of solubility and heat of solution at T=298.15K. The new method is applied to explore 18 cation–anion combinations for possible ionic liquid formation.

Stern potential and Debye length measurements in dilute ionic solutions with electrostatic force microscopy

We demonstrate the ability to measure Stern potential and Debye length in dilute ionic solution with atomic force microscopy. We develop an analytic expression for the second harmonic force component of the capacitive force in an ionic solution from the linearized Poisson–Boltzmann equation. This allows us to calibrate the AFM tip potential and, further, obtain the Stern potential of sample surfaces. In addition, the measured capacitive force is independent of van der Waals and double layer forces, thus providing a more accurate measure of Debye length.

Image Charge Solvation Model (ICSM) with Improved Boundary Conditions

In previous work the Image Charge Solvation Model (ICSM) [1] was introduced as an alternative way to simulate bulk properties of solutions. The ICSM consists of a spherical cavity of explicit solvent embedded in a continuum dielectric medium, and solves the electrostatic interactions accurately and more efficiently than the Particle Mesh Ewald (PME) method on large scales. In spite of the success of the model in simulating a pure water system, and extremely dilute ionic solutions [2], periodic boundary conditions create unphysical charge-charge correlations at finite ion concentrations. Although expanding the system size can reduce these extraneous correlations to negligible levels in principle, the PME method remains more efficient on large scales. In this work we re-evaluate the applied boundary conditions at the cavity wall to correct the problem at its origin, which is related to torques on water molecules due to the electrostatic reaction field [3]. Here, we apply a mean field force representing bulk media outside the cavity, and a self-correcting torque field is introduced to enforce translational symmetry. Our modified ICSM maintains accuracy, performs more efficiently than previously and removes the primary cause of spatial correlations between ions.[1] Y. Lin, A. Baumketner, S. Deng, Z. Xu, D. Jacobs, W. Cai, An image-based reaction field method for electrostatic interactions in molecular dynamics simulations of aqueous solutions. J. Chem. Phys. 131, (2009).[2] Y. Lin, A. Baumketner, W. Song, S. Deng, D. Jacobs, and W. Cai, Ionic solvation studied by image-charge reaction field method. J. Chem. Phys. 134, (2011).[3] W. Song, Y. Lin, A. Baumketner, S. Deng, W. Cai and D. Jacobs, Effect of the reaction field on molecular forces and torques revealed by an Image-Charge Solvation Model, Comm. Comp. Phys. 13:129-149 (2013).