Modified Potentiometric Sensors for the Separate Determination of Cationic Surfactants

The main electrochemical and performance characteristics of solid-contact sensors for cationic surfactants based on an organic cetylpyridinium–tetraphenylborate ion exchanger were determined. It was found that the surface of the sensors for cationic surfactants can be modified with poly(vinyl chloride) molecular sieves. The modified electrodes can separately determine alkylpyridinium homologues with different lengths of hydrocarbon radicals (C₁₀–C₁₈).     

Selective transport of ions and molecules across layer-by-layer assembled membranes of polyelectrolytes, p-sulfonato-calix[n]arenes and Prussian Blue-type complex salts

Our recent studies in the field of ultrathin membranes prepared upon layer-by-layer assembly of various polyionic compounds such as polyelectrolytes, calixarenes and polyelectrolytes, and metal hexacyanoferrate salts such as Prussian Blue are reviewed. It is demonstrated that polyelectrolyte multilayers can be used (a) as nanofiltration and reverse osmosis membranes suitable for water softening and seawater desalination and (b) as molecular sieves and ion sieves for size-selective separation of neutral and charged aromatic compounds. Furthermore, hybrid membranes of p-sulfonato-calixarenes and cationic polyelectrolytes showing specific host-guest interactions with permeating ions are described. The membranes exhibit high selectivities for distinct metal ions. Finally, it is demonstrated that purely inorganic membranes of Prussian Blue (PB) and analogues can be prepared upon multiple sequential adsorption of transition metal cations and hexacyanoferrate anions. Due to the porous lattice of PB, the membranes are useful as ion filters able to separate cesium from sodium ions, for example.     

Separate detection of homologous surfactants by means of solid-contact unmodified and modified with molecular sieves potentiometric sensors

To improve the selectivity of surfactant sensors, the surface of their membranes was modified with molecular sieves with predetermined pore sizes. Water-soluble anionic, cationic and non-ionic surfactants were used as pore generators in the molecular sieves and introduced into the source membrane at the stage of its formation. The modified sensors enable detection of alkylsulfates homologues and alkylpyridinium with different lengths of the hydrocarbon chain (C₁₀-C₁₈); homologous poly(oxyethylated nonylphenols) differing in the number of oxyethyl groups (m = 10-100).A novel approach to separate detection of surfactant homologues implies the usage of inselective sensors as a multisensor system. The software-supported multisensor approach allows information of both mixture composition and concentrations of separate components in multicomponent systems to be obtained with a certain accuracy. Inselective non-modified sensors with the highest cross-sensitivity were used to design multisensor systems like an “electronic tongue”.The cross-sensitivity parameters of both source and modified sensors were estimated and the possibility of their usage in multisensor systems like an “electronic tongue” for analysis of multicomponent solutions of homologous surfactant is shown. Analytical signals were processed by artificial neural networks.     

Surfactant fouling of nanofiltration membranes: measurements and mechanisms.

Fouling of nanofiltration membranes is studied during filtration of aqueous surfactant solutions under different conditions. To this purpose, four typical nanofiltration membranes (Desal51HL, NF270, NTR7450 and NFPES10) and three typical surfactants (nonionic neodol, anionic SDBS and cationic cetrimide) are selected. Fouling is studied as a function of the surfactant concentration, with and without addition of an electrolyte (NaCl), at different pH and when filtering a mixture of surfactants. Adsorption experiments and hydrophobicity measurements (to study the orientation of the surfactants on the membrane surface) are also performed under the different conditions. The least membrane fouling is found for the anionic surfactant SDBS, while for the cationic surfactant cetrimide very low relative fluxes are observed. Neodol shows an intermediate degree of fouling. Both hydrophobic and electrostatic interactions (in the case of ionic surfactants) between the membrane surface and the surfactant explain the degree of adsorption and hence fouling, as membrane fouling is correlated with the amount of adsorbed surfactant. The difference between cetrimide and SDBS becomes especially visible when changing the pH: increasing the pH leads not only to an opposite orientation of the adsorbed surfactants, but also to an opposite trend in adsorbed amount and membrane fouling. This study permits selection of an optimal nanofiltration membrane to recycle wastewater containing surfactants in the carwash industry. The optimal choice would be a hydrophilic membrane with a low molecular weight cut-off and a small negative surface charge at neutral pH. Cationic surfactants in the wastewater should also be avoided as much as possible.     

纳滤膜对电解质溶液分离特性的理论研究(II): 混合电解质溶液

假定纳滤膜具有狭缝状孔, 使用纯水透过系数、膜孔径及膜表面电势来表征纳滤膜的分离特征, 用流体力学半径和无限稀释扩散系数表征了离子特性. 采用扩展Nernst-Planck方程、Donnan平衡模型和Poisson-Boltzmann理论描述了混合电解质溶液中离子在膜孔内的传递现象, 计算了三种商用纳滤膜(ESNA1-LF, ESNA1和LES90)对同阴离子、同阳离子和含四种离子的混合电解质体系中离子的截留率, 并与实验数据进行了比较. 计算结果表明, 电解质溶液中离子在纳滤膜孔内传递的主要机理是离子的扩散和电迁移, 纳滤膜对混合电解质溶液中离子的分离效果主要由空间位阻和静电效应决定. 该模型在低浓度时对含一价离子的混合电解质溶液通过纳滤膜的截留率计算结果比较准确, 但对高浓度或含高价离子的混合电解质溶液则偏差较大.     

合成高产率分子筛MCM-48

以十六烷基三甲基溴化铵（CTAB）和Triton X-100（TX-100）混合表面活性剂为模板剂,正硅酸乙酯（TEOS）为硅源,采用水热法合成了MCM-48分子筛.实验中发现,晶化3 d后用醋酸调节溶液pH值的方法可有效提高MCM-48分子筛的产率；同时采用混合表面活性剂使模板剂的利用效率达到了6.0 TEOS/1.0 Surf (摩尔比）,并通过XRD、N2-吸附/脱附和TG-DSC等测试手段对产物的结构性能进行了表征.     

Influence of membrane fouling by (pretreated) surface water on rejection of pharmaceutically active compounds (PhACs) by nanofiltration membranes

The effects of surface water pretreatment on membrane fouling and the influence of these different fouling types on the rejection of 21 neutral, positively and negatively charged pharmaceuticals were investigated for two nanofiltration membranes. Untreated surface water was compared with surface water, pretreated with a fluidized anionic ion exchange and surface water, pretreated with ultrafiltration. Fouling the nanofiltration membranes with anionic ion exchange resin effluent, resulted in the deposition of a mainly colloidal fouling layer, with a rough morphology. Fouling the nanofiltration membranes with ultrafiltration permeate, resulted in the deposition of a smooth fouling layer, containing mainly natural organic matter. The fouling layer on the nanofiltration membranes, caused by the filtration of untreated surface water, was a combination of both colloids and natural organic matter.Rejection of pharmaceuticals varied the most for the membranes, fouled with the anionic ion exchange effluent, and variations in rejection were caused by a combination of cake-enhanced concentration polarisation and electrostatic (charge) effects. For the membranes, fouled with the other two water types, variations in rejection were smaller and were caused by a combination of steric and electrostatic effects.Changes in membrane surface hydrophobicity due to fouling, changed the extent of partitioning and thus the rejection of hydrophobic, as well as hydrophilic pharmaceuticals.     

Asymmetric ion exchange mosaic membranes with unique selectivity

In this paper, we describe the investigation of membranes to concentrate aqueous low molecular weight (<500 Da) organics streams, while removing electrolytes including divalent salts such as sodium sulfate. Such membranes would be useful in many industrial applications as currently used pressure driven process such as nanofiltration (NF) or electrical processes such as electric dialysis (ED) cannot achieve such separations and concentrations. An analysis of ion/water transport in different membranes and, selectivity and flux requirements indicated that ion exchange mosaics in the form of integrally skinned asymmetric structures could achieve the required performance. The relationship between the internal structure of the mosaic membrane elements and the required separation properties was further analyzed as a development guide. It was found that such membranes could be made by casting a homogenous solution of two mutually incompatible polymers in a common solvent, containing non-solvents and additives, followed by a chemical modification. The process of forming such membranes involves phase separation between the two polymers and the phase inversion of each polymer. In this study the membrane consists of a cation exchange asymmetric membrane with a uniform distribution of anion exchange particles in the dense integrally skin layer. The choice of polymeric materials, their molecular weights, solvent combinations and surfactants determined the membranes’ surface morphology, mosaic dimensions and particle density. In this way membranes were formed with ∼1 μm sized anion exchange particles uniformly dispersed in a thin (∼1.0 μm) cation exchange selective layer of an asymmetric membrane. The best performance to date: Fluxes of 500+LMD, 10% rejection to sodium sulfate, 90% to sucrose and >98% rejection to 400 molecular weight organic ions. The membranes also show a mosaic effect of decreasing sulfate rejection with decreasing sulfate concentration. The membranes also show a musaic effect of decreasing sulfate rejection with decreasing sulfate concentration, which is desired to perform effectively the removal of mono and bivalent ions during diafiltration.     

Effect of Solubilization in Bitemplate Synthesis of a Homologous Series of Organosilica Mesophases and Mesoporous Molecular Sieves

Peculiarities of bitemplate synthesis of a homologous series of organosilica mesophases (precursors of mesoporous molecular sieves of MCM-41 type) were studied, and an influence of a nature of templating substances and a solubilization on a perfection of their spatial structure was found. n-alkyl(C₈–C₁₈)pyridinium halides as solubilizers (micelle-forming surfactants) and monoethanolamides of saturated n-(C₁₀–C₁₆)aliphatic acids as solubilizate were used. Some physicochemical factors promoting a spatial ordering of organosilica mesophases and mesoporous molecular sieves were considered.     

用双表面活性剂为共模板合成中孔分子筛MCM-48

利用水热法以非离子表面活性剂聚乙二醇辛基苯基醚和阳离子表面活性剂十六烷基三甲基溴化铵为共模板合成了中孔分子筛MCM-48.实验中发现利用较强的范德华力和氢键,聚乙二醇辛基苯基醚可在很大程度上降低合成MCM-48所需阳离子表面活性剂的用量,且利于制备有序性好、骨架聚合度高、稳定性好的MCM-48.通过调节聚乙二醇辛基苯基醚与十六烷基三甲基溴化铵的比例,可得到不同物相结构的分子筛.     

Physico-chemical characterization of nanofiltration membranes.

This study presents a methodology for an in-depth characterization of six representative commercial nanofiltration membranes. Laboratory-made polyethersulfone membranes are included for reference. Besides the physical characterization [molecular weight cut-off (MWCO), surface charge, roughness and hydrophobicity], the membranes are also studied for their chemical composition [attenuated total reflectance Fourier spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS)] and porosity [positron annihilation spectroscopy (PAS)]. The chemical characterization indicates that all membranes are composed of at least two different layers. The presence of an additional third layer is proved and studied for membranes with a polyamide top layer. PAS experiments, in combination with FIB (focused ion beam) images, show that these membranes also have a thinner and a less porous skin layer (upper part of the top layer). In the skin layer, two different pore sizes are observed for all commercial membranes: a pore size of 1.25-1.55 angstroms as well as a pore size of 3.20-3.95 angstroms (both depending on the membrane type). Thus, the pore size distribution in nanofiltration membranes is bimodal, in contrast to the generally accepted log-normal distribution. Although the pore sizes are rather similar for all commercial membranes, their pore volume fraction and hence their porosity differ significantly.     

UV辐照接枝聚合制备亲水性纳滤膜

用紫外光引发自由基共聚接枝的方法对酚酞基聚芳醚酮(PEK-C)超滤膜表面进行改性制备了亲水性荷电纳滤膜. 研究了用不同单体接枝改性膜对盐溶液的截留性能, 证明了Donnan电荷效应对纳滤膜分离性能的影响. 在此基础上, 通过丙烯酸(AA)与对苯乙烯磺酸钠(SSS)的共聚接枝, 并改变它们在接枝液中的相对含量, 成功地制备出膜的表观截留率和渗透通量都较高的纳滤膜.     

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).     

New modified electrodes for the separate determination of anionic surfactants

Nylon, chitin, polyvinyl chloride, and polymethyl methacrylate molecular sieves with pores of various size are studied as membrane surface modifiers in electrodes reversible to anionic surfactants. A procedure for the synthesis of molecular sieves with pores of the specified size from water-insoluble polymeric matrix (polyvinyl chloride, polymethyl methacrylate) was developed. Different methods were proposed for modifying the electrode surface. It was demonstrated that modified electrodes provide the separate determination of alky 1 sulfates bearing alkyl radicals of different lengths (C₁₂-C₁₄).     

介孔硅基分子筛研究新进展

M41S、HMS、MSU 和SBA 等具有规则孔道结构的介孔硅基分子筛及其杂原子衍生物在多相催化、吸附分离及材料科学领域有重要的学术研究与工程应用价值。此类物质可以通过长链季铵盐、伯胺、双子胺或聚氧乙烯类表面活性剂胶束的模板作用, 在多种不同的条件下合成, 并可通过SEM、TEM、XRD、N₂ (A r) 等温吸附-脱附技术和FT-IR、激光Ram an、固体MA S NMR、UV-V is 漫反射、XPS、EXA FS、XAN ES 等谱学手段加以表征。本文对介孔硅基分子筛近年来所取得的进展进行了综述。     

Synthesis and characterization of mesoporous molecular sieves

In the synthesis of mesoporous molecular sieves of the type MCM-41, different cationactive surfactants and sources of silicon were used. Moreover, Al-MCM-41 samples with different content of aluminium were synthesized. MCM-41 and Al-MCM-41 were synthesized at elevated temperature in stainless-steel autoclaves. Prepared mesoporous molecular sieves were characterized by powder X-ray diffraction (XRD), physical adsorption of nitrogen at the temperature of −197°C, sorption capacity of benzene, and by infrared spectroscopy (FTIR). Acidity was measured for Al-MCM-41 by temperature programmed desorption of ammonia (TPDA) and by FTIR of adsorbed pyridine. Acid catalytic activity of Al-MCM-41 was tested by isomerization of o-xylene. Influence of the synthesis reproducibility, surfactant used, source of silicon, synthesis time, source of aluminium, and Si to Al mole ratio on the properties of mesoporous molecular sieves were evaluated.     

Polymeric Membrane Nanofiltration and Its Application to Separations in the Chemical Industries

The application of membrane technology, particularly water-based nanofiltration, as a separation process in the chemical industries has increased tremendously in recent years. However, the use of membranes capable of molecular separation in non-aqueous systems (e.g. nanofiltration) is a relatively new and growing application of membrane technology. The main challenge in applying polymeric nanofiltration membranes to non-aqueous systems is that the polymers developed for water-based applications are not suitable. Polyimide is a particularly interesting polymer as it has excellent chemical resistance, and membranes produced from it provide desirable separation properties – i.e. economically viable flux and good separation of nanoscale molecules. Various research works have shown that commercial polyimide organic solvent nanofiltration (OSN) membranes, trademark STARMEM™, 1 are robust and suitable for performing molecular separations. This work will discuss in detail the use of STARMEM™ in a pharmaceutical application. The EIC-OSN process was developed for separating the enantiomers of chiral compounds in pharmaceutical applications. High optical purity (94.9%) of (S)-phenylethanol from rac-phenylethanol was achieved through the use of STARMEM™122. Process simulation of the ideal eutomer-distomer system predicted that the highest theoretical resolvability from this process would be 99.2%. Other application areas of OSN are varied, including purification and fractionation in the natural products industry, homogeneous catalyst recovery, monomer separation from oligomers, etc. Currently, OSN is used in a small number of processes including a very large petrochemical application, but it has the potential to be applied to a wide range of separations across the full spectrum of the chemical industries.     

Modeling of the adsorption of organic compounds on polymeric nanofiltration membranes in solutions containing two compounds.

The adsorption of organic compounds in aqueous solution on polymeric nanofiltration membranes is studied; this process is one of the most important fouling mechanisms influencing the flux and retention behavior of nanofiltration membranes. It is shown that the adsorption of dissolved organic compounds on polymeric nanofiltration membranes is comparable to that on activated carbon. Freundlich and Langmuir isotherms are used to describe the relation between the adsorbed mass on the membrane and the equilibrium concentration of the organic compound in a single-compound solution. Based on these results, three models for the adsorption of solutions containing several compounds [i.e., the simple competitive adsorption model (SCAM), the model of Jain-Snoeyinck, and the model of Butler-Ockrent] were used to predict the adsorption behavior of an organic compound in an aqueous solution containing two compounds. The results of the three models were compared to experimental observations. It is shown that the SCAM allows a good prediction of the adsorption behavior.     

Influence of molecular size,polarity and charge on the retention of organic molecules by nanofiltration

Because of the growing interest in nanofiltration for industrial use, a better insight in the retention mechanisms in nanofiltration is needed, which will make it possible to understand membrane performances for specific applications. In this paper, the retention of a series of organic molecules by four nanofiltration membranes was studied. The membranes that were used are NF70 (Dow/FilmTec), NTR 7450 (Nitto-Denko), UTC-20 (Toray Ind.) and Zirfon^® (VITO). In order to correlate the retention with the size of the molecule, which is the main factor that determines the retention, use was made of different parameters for the molecular size: molecular weight, the Stokes diameter, the equivalent molar diameter, and a diameter obtained with energy minimisation calculations. For each size parameter, the correlation with retention in nanofiltration experiments was calculated. For the Zirfon^® membrane, retentions were too low to obtain a good correlation. For the three other membranes, a good correlation with retention was found for each of the size parameters. Two other factors were found to have an influence on retention of organic molecules: the polarity of the molecule, and the charge of the molecule. The importance of these factors depends on the molecules as well as on the type of membrane.     

Ion separations with membranes

Ion separations are important for resource recovery, water treatment, and energy production and storage. Techniques such as chemical precipitation, selective adsorption, and solvent extraction are effective, but membranes may separate ions continuously with less waste and lower energy costs. Separation of monovalent and multivalent ions with nanofiltration or electrodialysis membranes already enables water softening and edible salt purification. Similar membranes are attractive as separators in vanadium redox flow batteries. Selective partitioning of divalent counter-ions into ion-exchange membranes even allows transport of these ions against their concentration gradients in salt mixtures. However, separations of ions with the same charge is more challenging. Recent research demonstrated highly selective ion “sieving” at small scales. Separations using electrical potentials and differences in ion electrophoretic mobilities are promising, but relatively unexplored. Carrier-mediated transport affords high selectivity in liquid membranes, but these systems are not very stable, and selective transport via hopping between anchored carriers has proven elusive. Finally, this paper discusses how concentration polarization decreases selectivities in many membrane processes. Although development of selective, inexpensive ion-separation membranes is a work in progress, successes in water softening and edible salt purification suggests that future selective membranes will serve as complementary methods to traditional purification techniques.