Description of the Effects of Non‐ion‐exchange Extraction and Intra‐membrane Interactions on the Ion‐selective Electrodes Response within the Interface Equilibria‐triggered Model

The recently proposed interface equilibria‐triggered dynamic diffusion model of the boundary potential has proven its high predictive efficiency for quantification of the ion exchange and co‐extraction effects at the interface, as well as of the trans‐membrane transfer effect, on the electrode response. It is applicable for both ion exchanger‐based and neutral carrier‐based electrodes. In this communication, the adaptability of this model to more complex cases, when non‐ion‐exchange extraction processes at the interface (partition of organic acids’ and bases’ molecular forms and extraction of ionic associates) are coupled with protolytic equilibria in the aqueous phase and with self‐solvation process in the membrane phase, is demonstrated. By the example of electrodes reversible to ions of highly lipophilic physiologically active bases and acids (amiodarone, verapamil, vinpocetine, salicylic acid), it is shown that the peculiarities of their functioning, such as a very strong pH effect on the potential of cation‐selective electrodes, non‐monotonic pH dependence of the potential and super‐Nernstian response slope in certain pH region for a salicylate‐selective electrode, are well described within the model.     

Problems and prospects of solid contact ion-selective electrodes with ionophore-based membranes

The problems of stability and reproducibility of potentials of solid contact ion-selective electrodes with ionophore-based membranes were considered from the viewpoint of the existence of a sequence of equilibria providing the coupling of the ionic conductivity in the membrane and the electronic conductivity in the substrate. The both historical approaches to the stabilization of potentials of solid contact electrodes and the contemporary approaches based on the use of conducting polymers were briefly characterized. The role of water uptake by the membrane from the solution is also discussed.     

Comparative study between a CMS membrane and a CMS adsorbent: Part II. Water vapor adsorption and surface chemistry

The adsorption/desorption equilibria of water vapor in a carbon molecular sieve (CMS) membrane and a commercial CMS adsorbent were determined, exhibiting S-shaped, type V isotherms. The fits of several models found in the literature to the experimental data were evaluated. The results obtained led to the development of a new model accounting for both adsorption and desorption and essentially based on the work of Lagorsse et al. (2005) [15]. Furthermore, the adsorption kinetics was also assessed for both materials and well described by a linear driving force model. The existence of hydrophilic groups responsible for water vapor adsorption in such carbonaceous materials has been related to the surface chemistry by means of X-ray microanalysis and by thermogravimetry. It was concluded from X-ray microanalyses that the carbon membrane presents a lower C/O ratio and is thus more sensitive towards water vapor exposure, as evidenced by the measured water adsorption at lower relative pressures. It was also observed that the diffusion rates are higher for the CMS membrane than for the CMS adsorbent.     

因子效应函数的正交多项式展开优化离子电极的膜组成

本文应用因子效应函数的正交多项式展开数学方法,处理了两种电极的正交试验数据,导出电极斜率和电极膜的各组份含量的定量关系式,并用求导的方法求得电极的最佳膜组成。该法适用于探求电极响应参量与膜组成之间的定量关系并进行膜组成的优化。     

Superoxide dismutase biosensors working in non-aqueous solvent

Enzymatic electrodes based on superoxide dismutase enzyme were developed. Using the superoxide dismutase enzyme sensor assembled according to the classical model, poor results were obtained. Results were improved by adopting a new way of assembling the biosensor using a cellulose triacetate layer in which the SOD enzyme is entrapped and sandwiched between two gas-permeable membranes, or using a kappa-carrageenan gel layer entrapping the enzyme, sandwiched between an external gas permeable membrane and an internal cellulose acetate membrane, coupled in each case to the oxygen amperometric transducer. Results obtained by applying the newly developed biosensor to assaying hydrophobic compounds showing radical scavenging properties, operating in dimethylsulfoxide, were also satisfactory.     

Potentiometric anion selectivity of polymer-membrane electrodes based on cobalt, chromium, and aluminum salens

Metallo-salens of cobalt(II) (Co-Sal), chromium(III) (Cr-Sal), and aluminum(III) (Al-Sal) are used as the active ionophores within plasticized poly(vinyl chloride) membranes. It is shown that central metal-ion plays a critical role in directing the ionophore selectivity. Polymer-membrane electrodes based on Co-Sal, Cr-Sal, and Al-Sal are demonstrated to exhibit enhanced responses and selectivity toward nitrite/thiocyanate, thiocyanate, and fluoride anions, respectively. The improved anion selectivity of the three ionophore systems is shown to deviate significantly from the classical Hofmeister pattern that is based only on ion lipophilicity. For example, optimized membrane electrodes for nitrite ion based on Co-Sal exhibit logK(Nitrite,Anion)(pot) values of -5.22, -4.66, -4.48, -2.5 towards bromide, perchlorate, nitrate, and iodide anions, respectively. Optimized membrane electrodes based on Co-Sal and Cr-Sal show near-Nernstian responses towards nitrite (-57.9+/-0.9 mV/decade) and thiocyanate (-56.9+/-0.8 mV/decade), respectively, with fast response and recovery times. In contrast, Al-Sal based membrane electrodes respond to fluoride ion in a super-Nernstian (-70+/-3 mV/decade) and nearly an irreversible mode. The operative response mechanism of Co-Sal, Cr-Sal, and Al-Sal membrane electrodes is examined using the effect of added ionic sites on the potentiometric response characteristics. It is demonstrated that addition of lipophilic anionic sites to membrane electrodes based on the utilized metallo-salens enhances the selectivity towards the primary ion, while addition of cationic sites resulted in Hofmeister selectivity patterns suggesting that the operative response mechanism is of the charged carrier type. Electron spin resonance (ESR) data indicates that Co(II) metal-ion center of Co-Sal ionophore undergoes oxidation to Co(III). This process leads to formation of a charged anion-carrier that is consistent with the response behavior obtained for Co-Sal based membrane electrodes.     

Membrane capacitive deionization

Membrane capacitive deionization (MCDI) is an ion-removal process based on applying an electrical potential difference across an aqueous solution which flows in between oppositely placed porous electrodes, in front of which ion-exchange membranes are positioned. Due to the applied potential, ions are adsorbed in the electrodes and a product stream with a reduced salt concentration is obtained. Including the membranes in the process has two advantages: first, they block co-ions from leaving the electrodes, thereby increasing the salt removal efficiency of the process, and second, when during ion release a reversed voltage is used, counterions can be more fully flushed from the electrode region, thereby increasing the driving force for ion removal in the next cycle. Here we present pilot-plant experimental data for salt removal in MCDI as function of inlet ionic strength and flow rate. In the subsequent stage of ion release the flow rate is temporarily reduced to zero and the voltage sign reversed. This “stop-flow” operation mode results in a small and concentrated product stream. We present a theoretical process model for MCDI which describes the time-dependent electric current and effluent ion concentration, both during the deionization stage and during the subsequent stage of ion release. The process model describes the MCDI cell as a number of stirred volumes placed in-series, and includes the transport resistance of the ion-exchange membrane and of the stagnant diffusion layer in front of the membrane. Ion storage in the electrodes is described according to the equilibrium Gouy–Chapman–Stern model for the electrostatic double layer.     

Molecular probes of solvation phenomena

The properties of the molecules present in any chemical or biological system are dependent on interactions with the environment, and a quantitative understanding of solvation phenomena remains a major challenge. Molecular recognition probes provide a new approach to quantitatively measure the properties of solvents. Traditionally, solvent polarity scales have been based on spectroscopic probes that provide insight into the nature of solvent-solute interactions. This review compares the solvent polarity parameters obtained from the wavelengths of UV/Visible absorption maxima with solute H-bond parameters obtained from the free energies of solution equilibria. The similarity of the solvent and solute H-bond scales leads to a general H-bond scale that uses the same parameters to describe both solvent and solute. The general H-bond scale provides a framework for understanding the relationship between local intermolecular interactions and the properties of the bulk medium. Intermolecular interactions are sensitive to solvation equilibria, so molecular recognition probes provide fundamentally different information from spectroscopic probes that are sensitive to the populations of different solvation states of the solute. Studies of mixed solvents demonstrate the potential of molecular recognition probes for providing new insights into solvation phenomena.     

革除或减少增塑剂的敏感膜离子选择电极

基于国内外最新研究工作,系统总结了离子选择电极膜中革除或减少外增塑剂的新膜基体,包括丙烯酸酯类聚合物、羟基功能化的乙烯基树脂、聚氨酯、硅橡胶以及导电聚合物,对其物理化学性能以及传感器检测等进行了全面归纳与讨论.指出该类革除外增塑剂的传感膜不仅避免了增塑剂的泄漏及其对生物样品的污染,而且较传统增塑聚氯乙烯(PVC)膜扩散系数降低了约3个数量级,有利于抑制过膜离子流,使其检测下限较传统增塑PVC下降了5个数量级,且选择系数也有不同程度的改善.另外,该类传感膜材料由于与固体支撑材料间优良的粘附性保证了电极的使用寿命,特别是在微型化固态电极中.以这类传感膜构建的电位型离子传感器将以其独特的优势在环境监测、食品卫生,尤其是在医疗诊断、生物物质检测中展示出不可替代的作用.     

On the use of the dual-process Langmuir model for correlating unary equilibria and predicting mixed-gas adsorption equilibria

A new model has been developed for predicting mixed-gas adsorption equilibria from multicomponent gas mixtures based on the dual-process Langmuir (DPL) formulation. It predicts ideal, nonideal, and azeotropic adsorbed solution behavior from a knowledge of only single-component adsorption isotherms and the assertion that each binary pair in the gas mixture correlates in either a perfect positive (PP) or perfect negative (PN) fashion on each of the two Langmuir sites. The strictly PP and strictly PN formulations thus provide a simple means for determining distinct and absolute bounds of the behavior of each binary pair, and the PP or PN behavior can be confirmed by comparing predictions to binary experimental adsorption equilibria or from intuitive knowledge of binary pairwise adsorbate-adsorbent interactions. The extension to ternary and higher-order systems is straightforward on the basis of the pairwise additivity of the binary adsorbent-adsorbate interactions and two rules that logically restrict the combinations of PP and PN behaviors between binary pairs in a multicomponent system. Many ideal and nonideal binary systems and two ternary systems were tested against the DPL model. Each binary adsorbate-adsorbent pair exhibited either PP or PN behavior but nothing in between. This binary information was used successfully to predict ternary adsorption equilibria based on binary pairwise additivity. Overall, predictions from the DPL model were comparable to or significantly better than those from other models in the literature, revealing that its correlative and predictive powers are universally applicable. Because it is loading-explicit, simple to use, and also accurate, the DPL model may be one of the best equilibrium models to use in gas-phase adsorption process simulation.     

Maltodextrins as new chiral selectors in the design of potentiometric, enantioselective membrane electrodes

Maltodextrins (dextrose equivalent (DE) 4.0-7.0, 13.0-17.0, and 16.5-19.5) are proposed as novel chiral selectors for the construction of potentiometric, enantioselective membrane electrodes. The potentiometric, enantioselective membrane electrodes can be used reliably for the assay of S-captopril as raw material and in pharmaceutical formulations such as Novocaptopril tablets, by use of direct potentiometry. The best response was obtained when maltodextrin with higher DE was used for construction of the electrode. The best enantioselectivity and time-stability was achieved for the lower DE maltodextrin. L-proline was found to be the main interferent for all the proposed electrodes. The surface of the electrodes can be regenerated by simply polishing; this furnishes a fresh surface ready for use in a new assay.     

New method for determining the concentration of ionic impurities in solvent polymeric membranes

A new method is proposed for ascertaining the amount of ionic impurities in solvent polymeric membranes of ion-selective electrodes. The method is based on determining the selectivity coefficient for ions of different valences as a function of the concentration of a lipophilized tetraphenylborate salt added to the membrane phase. Thus, the concentration of anionic impurities in commercially available poly(vinyl chloride) and of cationic ones in Tecoflex® (a polyurethane) was obtained as 0.063 ± 0.016 and 0.044 ± 0.006 mmol/kg, respectively.     

An approach to the mathematical description of the permeation of hydrocarbons through liquid membranes

A mathematical model describing transport of hydrocarbons through liquid membranes is proposed. This model describes both the diffusion of hydrocarbons through the surfactant layer and through the aqueous core of the membrane, and also takes into account the equilibria in the oilwater-micelle system. Diffusion of hydrocarbons through surface layers is described with the help of Eyring's theory. The equilibria in the system studied were calculated on the basis of the Scatchard-Hildebrand theory of solubility using the available values of the solubility parameters. Diffusion through the aqueous layer, treated as facilitated transport, was described with the help of Cussler's model of solubilization. The model was verified by comparison of calculated and experimental data (obtained by the author or available in the literature). The verification was positive and the proposed model allows one to predict the course of the process as a function of feed composition, type of surfactant used and membrane thickness.     

Kinetic considerations of ion selectivity coefficients of ion-selective electrodes based on neutral carriers

Ion selectivity coefficients of ion-selective electrodes based on neutral carriers are described by means of a mixed potential model of ion transport reactions at the aqueous solution/ion-sensitive membrane interface. The decrease in ion selectivity can be explained by the deviations from the equilibrium conditions, which arise from the ionic partial current across the interface, but the proposed correspondence of the exchange current density of ion transfer reactions with the ion selectivity coefficients is rationalized only for certain conditions of the kinetic parameters. The ion selectivity for liquid membrane transport is discussed starting from three different rate-determining steps. It is shown that the potentiometric selectivities of ion-selective electrodes and the transport selectivities are correlated when the ionic transfer across the aqueous solution/ membrane interface is fast compared with the complex ion transport through the membrane. The significance of a kinetic approach for the design of neutral carriers for ion-selective electrodes is stressed.     

Polypyrrole Nanoparticles Based Disposable Potentiometric Sensors

In this work polypyrrole nanoparticles of high electrochemical activity were used to prepare disposable, potentiometric sensors with a paper support. The paper support modified with polypyrrole nanoparticles served as electrical lead and ion‐to‐electron transducer and it was covered by a typical poly(vinyl chloride) based ion‐selective membrane. The properties of this arrangement were tested on example of potassium‐selective electrodes. The sensors prepared benefited from the properties of conducting polymer nanostructures: high electrical conductivity and electroactivity as well as absence of suspension stabilizing agent. The obtained electrodes were characterized with analytical parameters well comparable with those of classical ion‐selective electrodes.     

Fabrication of catalyst-coated membrane by modified decal transfer technique

A decal transfer method based on colloidal ink was developed for the fabrication of membrane electrode assemblies (MEAs). The new method requires fewer steps and utilizes H⁺ form of membrane compared to conventional decal method based on solution ink utilizing Na⁺ form of membrane. The structural features of the electrodes made by the modified decal method were investigated by scanning electron microscopy (SEM). The performance of fabricated electrode was evaluated for oxygen reduction reaction in a proton exchange membrane fuel cell. The results indicate that the modified decal method has the potential to be a facile method of fabricating electrodes with high performance.     

Sulfonated poly(ether ether ketone) as an alternative charged material to poly(vinyl chloride) in the design of ion-selective electrodes

To date, poly(vinyl chloride) (PVC) is the most used polymer in the design of ion selective electrode (ISE) membranes. This paper is focused on the use of sulfonated poly(ether ether ketone) (SPEEK) as an alternative material to PVC for the design of ISEs. SPEEK of the desired degree of sulfonation is synthesized from poly(ether ether ketone) (PEEK). An NH₄⁺-ISE has been chosen as a model electrode to study the efficiency of SPEEK as polymer matrix of the membrane. The material was evaluated in ionophore free ion exchanger membranes as well as in ion-selective electrodes membranes containing nonactine as ionophore. Analytical performance parameters of the prepared electrodes were evaluated. The electrodes show a slope between 50 and 60 mV dec⁻¹ depending on both the calibration medium and the membrane composition. A linear range of response between 10⁻⁴ and 1.0 M and a lifetime of 1-2 months were obtained. The interferences of cations such us Ca²⁺, Na⁺, Li⁺ and K⁺ over the prepared ISEs are studied as well. Although the plasticizer in the SPEEK based membrane matrix is not necessary, its presence improves the sensibility. This makes SPEEK a good potential choice over alternative membrane matrices reported in the literature and a promising platform for the establishment of membrane components.