Stable Tetra‐ and Penta‐Anions in the Gas Phase

The quest for stable gas‐phase anions in highly negative charge states has been a great challenge. While multiply charged anions are stabilized in solids and liquids by compensating cations and solvation cells, respectively, stable anions containing less than a hundred atoms in the gas phase and capable of carrying charge beyond ?3 is unknown. Here, we report the discovery of thermodynamically stable tetra‐ and penta‐anions, containing less than 50 and 80 atoms, respectively, in the gas phase. A universal model is developed that explains their stability in terms of the synergy between closed shell, high electron affinity, and size and predicts new highly‐charged anions by using the known charged clusters as building blocks. Synthesis of these species can open a new chapter in materials chemistry.     

The role of electrostatic interactions on the rejection of organic solutes in aqueous solutions with nanofiltration

The effects of electrostatic interactions on the rejection of organic solutes with nanofiltration membranes were investigated. For two different membranes, the rejection of selected organic acids, positively and negatively charged pharmaceuticals and neutral pharmaceuticals was investigated at different feed water chemistries (different ionic strengths and pH conditions, with and without the presence of NOM and divalent cations). It was concluded that for negatively charged membranes, electrostatic repulsion leads to an increase of the rejection of negatively charged solutes and electrostatic attraction leads to a decrease of the rejection of positively charged solutes, compared to neutral solutes. Neutral and positively charged solutes engage in hydrophobic interactions with negatively charged membranes, whereas negatively charged solutes do not engage in hydrophobic interactions since they cannot approach the membrane surface. This provides proof for the theory of an increased concentration of positively charged organic solutes and a decreased concentration of negatively charged organic solutes at the membrane surface compared to the bulk fluid. This concept may be denoted as “charge concentration polarisation”. The concept was further used as a modelling tool to predict the effects of electrostatic interactions on the rejection of trace organic solutes.     

Charge-selective recognition at fibroin-modified electrodes for analytical application

A novel fibroin-modified electrode with charge recognition is reported. The characteristics of silk fibroin membranes have been exploited for analytical applications. The membrane, with an isoelectric point of pH 4.5, was applied to graphite and carbon-fiber electrodes. The modified electrode was negatively charged in solutions of pH>4.5, and so rejected anions and attracted cations. In solutions of pH<4.5 the electrode was positively charged, and so rejected cations and attracted anions. The pH-responsive charge recognition of the modified electrode was investigated for some neurocompounds. A fibroin carbon-fiber electrode was used for in-vivo determination of the concentration of the cationic neurotransmitter dopamine (DA).This revised version replaces the article published online on April 2005.     

Streaming potential measurements as a characterization method for nanofiltration membranes

The streaming potentials of two different nanofiltration membranes were studied with several electrolyte solutions to investigate the influence of salt type and concentration on the zeta potential and kinetic surface charge density of the membranes. The zeta potentials decreased with increasing salt concentration, whereas the kinetic surface charge densities increased. The kinetic surface charge densities could be described by Freundlich isotherms, except in one case, indicating that the membranes had a negligible surface charge. The kinetic surface charge density observed was caused by adsorbed anions. Salt retention measurements showed different mechanisms for salt separation for the two investigated membranes. One membrane showed a salt retention that could be explained by a Donnan exclusion type of separation mechanism, whereas for the other membrane the salt rejection seemed to be a combination of size and Donnan excluion. Comparing the results obtained by the streaming potential measurements with those of the retention measurements, it could be concluded that the membrane with the highest kinetic surface charge density showed the Donnan exclusion type of separation, whereas the membrane with the lower surface charge density showed a separation mechanism that was not totally determined by Donnan exclusion, size effects seemed to play a role as well.     

Investigation of amphoteric polybenzimidazole (PBI) nanofiltration hollow fiber membrane for both cation and anions removal

The removal of both anions (phosphate, arsenate, arsenite and borate ions) and cations (copper ions) has been investigated by employing a lab-developed amphoteric polybenzimidazole (PBI) nanofiltration (NF) hollow fiber membrane. The amphoteric characteristics are due to the imidazole group within PBI molecules that makes the PBI NF membrane have an isoelectric point near pH 7.0 and show different charge signs based on the media pH. Investigations on the rejection capability of typical anions, e.g. phosphate, arsenate, arsenite, borate anions and typical heavy metal cations, e.g. copper ions, reveal that the PBI NF membrane exhibits impressive rejection performance for various ions removal. However, their rejections are strongly dependent on the chemical nature of electrolytes, solution pHs and the feed concentrations. The experimental results are analyzed by using the Spiegler–Kedem model with the transport parameters of the reflection coefficient (σ) and the solute permeability (P). The PBI NF membrane may have potential to be used in industrial removal of various environmentally unfriendly ion species.     

Relation between membrane characteristics and performance in nanofiltration

The performance of commercial membranes during nanofiltration of aqueous solutions containing dissolved uncharged or charged organic components, was studied on the basis of membrane characteristics by means of multiple linear regression.

The membrane characteristics studied were surface hydrophobicity, surface roughness, surface charge, molecular weight cut-off (MWCO), permeability and porosity of the top layer (expressed as the volume fraction of small and large pores, determined by Positron Annihilation Lifetime Spectroscopy). Filtration and adsorption experiments were performed in the presence of various components, which differ in molecular mass, hydrophobicity and (in the case of charged organic components) in charge.

It was concluded that in order to minimize fouling, the membrane should have a low volume fraction of small pores in the top layer. When the organic components are charged, a membrane with a large surface charge and a high hydrophilicity is also favourable. Not only the membrane, but also the feed characteristics have an influence on fouling: the best results during nanofiltration of dissolved uncharged or charged components were obtained with hydrophilic or negatively charged components, respectively. Dissolved organic components were the best retained by membranes with a low MWCO. In addition, uncharged organic components should be hydrophilic and small to obtain a high retention and minimal flux decline, while the interplay between membrane and component charge is crucial during filtration of dissolved charged organic components.     

Electrostatic interaction of ion-penetrable membranes. Effects of ionic solubility

An analytic, approximate expression for the electrostatic interaction between two membranes immersed in an electrolyte solution is derived on the basis of a simple membrane model. This model assumes that the membrane has a surface layer in which charged groups are uniformly distributed and that electrolyte ions can penetrate into the surface layer. The partition coefficients of cations and anions between the solution and the surface layer, which are related to their solubilities in the surface layer, may be different from unity.The electrostatic interaction depends on the ionic partition coefficients between the solution and the surface layer, and the relative permittivity of the surface layer, as well as on the membrane-fixed charges, the electrolyte concentration in the solution, and the surface layer thickness. It is shown, in particular, that even where the charge layer has no fixed charges, the electrostatic interaction force can be produced if the solubilities of cations and anions are different in the surface layer.     

Evaluating the charge of nanofiltration membranes

In this study, several methods were used to determine the charge of commercially available nanofiltration membranes, and were compared. First the ion-exchange capacity was determined by titration, this method is able to distinguish between positively and negatively charged functional groups on the membrane. Secondly, measurements of the streaming potential gave a value for the charge density at the exterior membrane surface; the effect of the pH of the solution on the membrane charge was studied. Finally, measurements of the membrane potential allowed to evaluate the total membrane charge density.The results of the three methods were used to compare the membrane charge of nanofiltration membranes mainly in a qualitative way. It is shown that measurements of the membrane potential are preferred for the evaluation of the membrane charge.     

Combined electrochemical and radiotracer study of anion sorption from aqueous solutions into polypyrrole films

The sorption of SO²⁻₄ and Cl⁻ ions into polypyrrole films has been studied by the radiotracer method under potential cycling and steady state conditions using labelled H₂SO₄ and HCl. Although a potential dependent migration and penetration of anions in the film can be detected, no strong correlation was found between the amount of charge consumed in the oxidation and reduction processes and the number of sorbed anions. The number of positively charged sites attracting anions into the film seems to be significantly lower than that expected from the amount of charge involved in the electrochemical transformations.     

Influence of steric, electric, and dielectric effects on membrane potential

The membrane potential arising through nanofiltration membranes separating two aqueous solutions of the same electrolyte at identical hydrostatic pressures but different concentrations is investigated within the scope of the steric, electric, and dielectric exclusion model. The influence of the ion size and the so-called dielectric exclusion on the membrane potential arising through both neutral and electrically charged membranes is investigated. Dielectric phenomena have no influence on the membrane potential through neutral membranes, unlike ion size effects which increase the membrane potential value. For charged membranes, both steric and dielectric effects increase the membrane potential at a given concentration but the diffusion potential (that is the high-concentration limit of the membrane potential) is affected only by steric effects. It is therefore proposed that membrane potential measurements carried out at high salt concentrations could be used to determine the mean pore size of nanofiltration membranes. In practical cases, the membrane volume charge density and the dielectric constant inside pores depend on the physicochemical properties of both the membrane and the surrounding solutions (pH, concentration, and chemical nature of ions). It is shown that the Donnan and dielectric exclusions affect the membrane potential of charged membranes similarly; namely, a higher salt concentration is needed to screen the membrane fixed charge. The membrane volume charge density and the pore dielectric constant cannot then be determined unambiguously by means of membrane potential experiments, and additional independent measurements are in need. It is suggested to carry out rejection rate measurements (together with membrane potential measurements).     

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.     

Theory of pressure-driven transport of neutral solutes and ions in porous ceramic nanofiltration membranes

Hindered transport theory and homogeneous electro-transport theory are used to calculate the limiting, high volume flux, rejection of, respectively, neutral solutes and binary electrolytes by granular porous nanofiltration membranes. For ceramic membranes prepared from metal oxides it is proposed that the membrane structural and charge parameters entering into the theory, namely the effective pore size and membrane charge density, can be estimated from independent measurements: the pore radius from the measured hydraulic radius using a model of sintered granular membranes and the effective membrane charge density from the hydraulic radius and the electrophoretic mobility measurements on the ceramic powder used to prepare the membrane. The electro-transport theory adopted here is valid when the membrane surface charge density is low enough and the pore radius is small enough for there to be strong electrical double layer overlap in the pores. Within this approximation the filtration streaming potential is also derived for binary electrolytes.     

用流动电位法表征纳滤膜的结构及性能

利用测量流动电位的方法考察了纳滤膜的表面电学性能对纳滤膜的截留性能的影响.首先,采用不同功能层材料制备了复合纳滤(NF)膜,考察功能层的交联时间、单体结构等对表面电性能的影响,研究纳滤膜对不同无机盐的选择截留性能与表面电性能的关系.通过流动电位法测定纳滤膜的表面电学参数,如流动电位(ΔE)、zeta电位(ζ)和表面电荷密度(σd).实验表明,这些电学参数的变化与功能层交联时间和纳滤膜截留率的变化一致,在交联时间为45 s时,3种电学参数的绝对值均最大,而纳滤膜对无机盐的截留率也最大.复合纳滤膜zeta电位的绝对值(|ζ|)按照Na2SO4>MgSO4>MgCl2变化,同截留率的变化相同.带侧基单体交联后得到的纳滤膜的表面电性能参数的绝对值小于不带侧基单体的.因此,流动电位法可用于研究复合纳滤膜的截留机理和功能层结构.     

At the surface of non-aqueous solutions of anionic surfactants

We have determined the concentration–depth profiles of sodium dodecyl sulfate (SDS) and cesium dodecyl sulfate (CDS) in their pure solutions, by which the surface structure of those solutions are characterized. With the identical bulk concentration, more Cs ions than sodium ions are present at the topmost layer and they penetrate deeper than sodium ions into the layer formed by the heads of the anions, shielding the electrostatic repulsion among those negatively charged anions more efficiently. The distributions of the charge at the surface of each studied solution were determined from those concentration–depth profiles of surfactant ions. The charge density varies more drastically in SDS solutions than in CDS solutions when their bulk concentrations are identical. These charge density profiles exhibit a visible and direct insight into the electric charge structure of the surface of ionic surfactant solutions. The experimental findings might be helpful to the investigations on the surface structures of aqueous solutions of ionic surfactants.     

Electronic structure and optical properties of charged oligofluorenes studied by VIS/NIR spectroscopy and time-dependent density functional theory

The electronic structure and optical properties of charged oligofluorenes were studied experimentally and theoretically. Measurements of the optical absorption spectra of charged oligofluorenes in dilute solutions have been performed by using the pulse radiolysis technique. In addition, optical absorption spectra of radical cations and anions in a solid matrix were measured after gamma-irradiation at 77 K. The optical absorption spectra were measured in the range of 440-2100 nm (0.6-2.8 eV) and compared with results from time-dependent density functional theory (TDDFT) calculations. The calculated charge induced deformations and charge distribution do not indicate the occurrence of polaronic effects. The potential energy profiles for rotation around the inter-unit bond show that oligofluorenes are nonplanar in their neutral state, while they tend to more planar structures in their charged state. The optical absorption spectra of charged oligofluorenes are dependent on the angle between neighboring units. TDDFT absorption energies shift to lower values with increasing chain length, which suggests that the charge delocalizes along the oligomer chain.     

Nanofiltration membranes synthesized from hyperbranched polyethyleneimine

Four nanofiltration membranes, two negatively and two positively charged, were fabricated by interfacial polymerization. Three different amines, ethylenediamine (EDA), diethylenetriamine (DETA), and hyperbranched polyethyleneimine (PEI) were selected to react with two acyl chlorides, trimesoyl chloride (TMC) and terephthaloyl chloride (TPC). The two membranes containing hyperbranched PEI, PEI/TPC and PEI/TMC, are positively charged at the operational pH. But the other two membranes, EDA/TMC and DETA/TMC, are negatively charged. It is found that the two PEI membranes own special rejection characters during nanofiltration. The PEI/TPC membrane has a similar pore size to the EDA/TMC membrane but owns simultaneously the higher salt rejection and permeation flux. The PEI/TMC has a pore size as large as 1.5 nm and still has a higher NaCl rejection than the EDA/TMC membrane of which the pore size as small as 0.43 nm. We consider that the special rejection characters are derived from the special structure of PEI. The hyperbranched structure allows some of the charged amine groups drifting inside the pores and interacting with the ions in the pathway. The drifting amines increase salt rejection but have little effect on water permeation. It implies that a high flux and high rejection membrane for desalting can be obtained by attaching freely rotating charged groups.     

In situ study of diffusion and interaction of water and mono- or divalent anions in a positively charged membrane using two-dimensional correlation FT-IR/attenuated total reflection spectroscopy

Two-dimensional (2D) correlation analysis based on time-resolved FT-IR/attenuated total reflection (ATR) spectroscopy has been used to study the diffusion behavior of water and mono- or divalent anions in the positively charged membranes of different charge density. In 2D FT-IR/ATR spectra, the splitting of the water delta(OH) bending band in the spectral range 1700-1500 cm-1 indicates that there are three different states of water in the positively charged membrane, that is, the water molecules forming strong or weak hydrogen bonds with hydrophilic groups of the membrane and water molecularly dispersed with weaker hydrogen bonds. The wavenumber difference of the delta(OH) band in the low- and high-charge-density membrane indicates that water molecules form much stronger hydrogen bonds with hydrophilic groups in the high-charge-density membrane. The sequential order of the three water bands intensity changes shows that, in the process of water diffusion into the high-charge-density membrane, the hydrogen-bonding interaction between hydrophilic groups of the membrane and water molecules takes place gradually due to the highly cross-linked network structure of the membrane; in the process of water diffusion into the low-charge-density membrane, the strong hydrogen-bonding interaction between hydrophilic groups of the membrane and water molecules takes place instantaneously and this type of water easily diffuses due to the weak interactions between the water molecules and the membrane polymer. Furthermore, the diffusion processes of the electrolyte solution such as NaAc and Na2SO4 aqueous solutions in the positively charged membrane have also been examined.     

Influence of operating conditions on the retention of phosphate in water by nanofiltration

The objective of this study was to investigate the retention of phosphate anions, H₂PO₄⁻ and HPO₄²⁻, by nanofiltration. The first part of this study deals with the characterisation of the NF200 membrane used in permeation experiments with aqueous solutions of neutral organic and charged inorganic solutes. In the second part the effects of feed pressure, ionic strength, concentration and pH on the retention of phosphate anions were investigated. Results show that the membrane is negatively charged, its pore radius is around 0.5 nm and the retention order for the salts tested was R(Na₂SO₄) > R(NaCl) > R(CaCl₂). The retentions of phosphate anions are in the order of 85% for H₂PO₄⁻ and 96% for HPO₄²⁻. They are relatively high when compared to retentions of other anions with the same charge. The retentions of phosphate anions, particularly the monovalent species, depend on the chemical parameters (feed concentration, ionic strength, and pH) and applied pressure. The experimental data were analysed using the Speigler–Kedem model and the transport parameters, i.e., the reflection coefficient (σ) and solute permeability (P_s) have been determined.     

Fabrication and characterization of γ-Al2O3–clay composite ultrafiltration membrane for the separation of electrolytes from its aqueous solution

In this paper, we have reported the preparation of low cost γ-Al₂O₃ membrane on a macroporous clay support by dip-coating method. For the synthesis of γ-Al₂O₃ top layer on the support, a stable boehmite sol is prepared using aluminium chloride salt as a starting material by sol–gel route. The structural properties of the composite membrane as well as γ-Al₂O₃ powder is carried out using scanning electron microscopy (SEM), X-ray diffraction (XRD), nitrogen adsorption–desorption isotherm data, Fourier transform infrared analysis (FTIR) and dynamic light scattering (DLS) analysis. The mean particle size of the boehmite sol used for coating is found to be 30.9 nm. The pore size distribution of the γ-Al₂O₃–clay composite membrane is found to be in the range of 5.4–13.6 nm. Separation performance of the membrane in terms of flux and rejection of single salts solution such as MgCl₂ and AlCl₃ as a function of pH, salt concentration and applied pressure is also studied. The rejection and flux behavior are found to be strongly dependent on electrostatic interaction between the charged molecules and γ-Al₂O₃–clay composite membrane. The intrinsic rejection has been determined by calculating the concentration at membrane surface (C_m) using Speigler–Kedem model. It is found that the observed rejection shows anomalous trend with increase in applied pressure and the intrinsic rejection increases with increase in applied pressure, a trend typical of the separation of electrolyte through charged membranes. At acidic pH, both the salt solution shows higher rejection. With increase in the salt concentration, observed rejection of salt decreases due to the enhanced concentration polarization. The maximum rejection of MgCl₂ and AlCl₃ is found to be 72% and 88%, respectively for salt concentration of 3000 ppm.     

Specific anion and cation binding to lipid membranes investigated on a solid supported membrane

Ion binding to a lipid membrane is studied by application of a rapid solution exchange on a solid supported membrane. The resulting charge displacement is analyzed in terms of the affinity of the applied ions to the lipid surface. We find that chaotropic anions and kosmotropic cations are attracted to the membrane independent of the membrane composition. In particular, the same behavior is found for lipid headgroups bearing no charge, like monoolein. This general trend is modulated by electrostatic interaction of the ions with the lipid headgroup charge. These results cannot be explained with the current models of specific ion interactions.