In this paper we review the use of supported BLMs (bilayer lipid membranes) as sensors for the detection of ions and molecular species in aqueous media. The inspiration for this exciting research, without question, comes from the biological world, where, for example, the lipid bilayer of the cell membrane is the most important self-assembling system. Although the first report on self-assembled bilayer lipid membranes (BLMs) in vitro was published in 1962, interface science including surface and colloid science has been dealing with these interfacial self-assemblies of amphiphilic molecules since Robert Hooke’s time (1672). Here, we present results of extensive study of physico chemical properties of two new membrane systems: metal and hydrogel supported bilayer lipid membranes (BLMs for short, also known as planar lipid bilayers). Several methods have been used to investigate the properties of these metal supported s-BLMs and hydrogel supported salt-bridge sb-BLMs, including cyclic voltammetry. Examples of a number of potential applications of s-BLMs for the construction of a glucose sensor for biomedical use as well as applications of agar supported BLMs for determining several toxins will be presented.

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The interface between two immiscible liquids with immobilized metalloporphyrins, enzymes and submitochondrial particles can serve as the simplest model of biological membrane convenient for the investigation of redox reaction accompanied by spatial separation of charges. In this mini review, we have described the vectorial charge transfer and molecular recognition at liquid/liquid interfaces. The structure of liquid interfaces, donor-acceptor interactions in the electrical double layer, hydrogen and specific bounding between molecules play the dominant role in molecular recognition and interfacial catalysis.

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A concerted 2:1:1-electron mechanism for cytochrome oxidase functioning is evaluated by thermodynamic and kinetic analysis. Possible ways of dioxygen reduction to water as well as the stoichiometry of the proton pump are analyzed thermodynamically. The conditions are formulated for the occurrence of multielectron oxygen reduction. Kharkats and Volkov first presented proofs that cytochrome c oxidase reduces molecular oxygen by synchronous multielectron mechanism without O₂^- intermediate formation (Yu. I. Kharkats and A. G. Volkov, Biochim. Biophys. Acta, 891, 56 (1987)). As the field progresses after this pioneering observations, it became clear that the first step of dioxygen reduction is two-electron concerted process. As follows from thermodynamics energy for the H⁺-pump functioning is liberated only at the last two steps of water formation on addition of third and fourth electrons independently of the reaction pathway. The media reorganization energy corresponding to simultaneous electrons and protons transfer will be minimal in the case then the directions of their transfers are close. In mitochondrial cytochrome c oxidase the donor of electrons is placed on the side C and the protons comes from the side M. In this case the minimal activation energy will be achieved at maximal possible at given geometry of the system angle between the directions of electrons and protons transfer.

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Second-harmonic generation and interfacial tension measurements have been used to study the adsorption behavior of a nonionic amphiphile, 1-(3′, 5′-di-t-butyl)phenyl-3-glycero-rac-glycerin at an air/water interface. The tensor components of the nonlinear surface susceptibility of the amphiphile have been determined by polarization rotation of the incident fundamental radiation. The comparison of the values of the several tensor components allows the description of the adsorption and orientation behavior of the molecules at an air/water interface.

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Voltammetric and impedance measurements were used to study the transfer of protonated forms of local anesthetics (procaine, tetracaine, cocaine, prilocaine, bupivacaine, lidocaine, dibucaine) between water (W) and o-nitrophenyl octyl ether (o-NPOE). The ion structure was shown to have little effect on the ion transport properties in the bulk aqueous phase, as characterized by the diffusion coefficient, D^w=(7 - 10) × 10^-6 cm² s^-1, as well as in the bulk o-NPOE phase, as characterized by the diffusion coefficient D⁰=(5 - 6) × 10^-7 cm² s^-1, and at the W ¦ o-NPOE interface, as characterized by the apparent standard rate constant, k₀^s=0.013 cm s^-1. While ion transport in the bulk of a phase is influenced mainly by the ion size and medium viscosity (Stokes law), the rate of interfacial ion transfer comprises the contributions of the ion distribution in the electrical double layer and of the activated transfer step, which compensate largely for each other. The pharmacological activity of local anesthetics correlates with the standard potential differences of their protonated forms at the W¦o-NPOE interface. This activity can be related to the ion partition between the body fluid and the membrane, or to the rate of membrane ion transfer at a constant potential difference.

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We report a series of molecular dynamics simulations on 18-crown-6 (18C6) uncomplexed, and on its K⁺ Pic^- complexes, at the water-chloroform interface, using an explicit representation of the solvents. The (18C6)₁₀ aggregate, initially at the interface, displays after 1 ns a complex equilibrium between molecules adsorbed at the interface, and others which have diffused to the organic phase. The (18C6·K⁺Pic^-)₆ inclusive complexes, initially at the interface, also display after 1 ns, an equilibrium between 18C6 molecules, 18C6·K⁺ complexes and the Pic^- counterions adsorbed at the interface, while some decomplexed K⁺ cations are captured by water. The spontaneous migration of a 18C6·K⁺ complex to the organic phase is also observed, facilitated by specific water molecules and the Pic^- counterions. Comparison with simulations on the (18C6·K⁺Pic^-)₁ and (18C6·Sr²⁺2Pic^-)₁ complexes highlights the effect of concentration and of the cationic charge on the interfacial behavior. We finally describe a “computer extraction experiment”, which starts with 18C6 molecules in the organic phase and K⁺Pic^- ion pairs in water. The results are discussed in relation with experimental data. We emphasize the surfactant-like behavior of free and complexed extractant molecules, the preorganization of ionophores induced by the interface, and the role of counterions. Surface active anions attract the cations at the interface and facilitate their capture by the adsorbed ionophores. Synergistic effects should result from the adsorption of co-extractant molecules or/and of counterions at the interface.

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The K⁺ complex of 2-(4-dipropylaminophenylazo)benzoic acid octadecyl ester (ETH 2418), a new lipophilic chromoionophore, is used to monitor the water uptake of dry solvent polymeric membranes based on 2-nitrophenyl octyl ether/poly(vinyl chloride) (2:1). Upon contact with an alkaline solution, its λ_max changes from 535 to 435 nm. This process can be reversed by drying the membranes. From the time-dependent recordings of absorbance changes, an apparent water diffusion coefficient, D_H2O^*, of 2×10^-8 cm² s^-1 is determined. In contrast to results obtained with hydrophilic water indicators, no light scattering due to water droplets is observed in the bulk of the membrane.

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The ion transfer of UO₂²⁺ ion across an aqueous(w)/nitrobenzene(NB) interface was investigated by ion-transfer polarography using an aqueous electrolyte dropping electrode. It was found that the transfer of UO₂²⁺ from w to NB was facilitated by bis(diphenylphosphoryl)methane (BDPPM), and that the transfer reaction had irreversible characteristics. A potential generated at the UO₂²⁺ ion-selective electrode, which was prepared while referring to ion transfer data, revealed a reversible Nernstian response against the concentration of the UO₂²⁺ ion in an aqueous sample solution. The irreversible nature of the UO₂²⁺ transfer observed in the polarogram resulted from such slow processes involved in the ion transfer reaction as the adsorption/desorption of BDPPM or the UO₂²⁺-BDPPM complex at the w/NB interface. The feasibility of the electrolytic separation of uranium from neptunium and plutonium on the basis of a selective ion-transfer reaction is discussed.

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A novel stripping voltammetric method for the determination of non-redox ionic species is presented. Using choline ion (Ch⁺) as an example, we demonstrate how a micro machined PVC-NPOE gel membrane can be used to accumulate Ch⁺ ion under potentiostatic steady-state conditions. The amperometric stripping of the target ion can be carried out by any voltammetric method but we presently illustrate the principle using linear sweep voltammetry. The effect of accumulation time, accumulation potential and choline ion concentration has been examined. The present work also includes the quantitative study of an enzyme reaction generating choline, namely the reaction of calcium phosphocholine with alkaline phosphatase. With this experimental approach, µg ml^-1 concentration level of alkaline phosphatase enzyme can be determined.

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The potentiometric anion responses of ion-selective electrodes prepared with polymeric membranes doped with gallium(III), indium(III) and thallium(III) metalloporphyrins are examined. These group XIII metals, when inserted into either octaethyl- or tetraphenyl-porphyrin derivatives and subsequently incorporated into plasticized poly(vinyl chloride) membranes, serve as anion ionophores with selectivity patterns that deviate significantly from the classical Hofmeister series for anions. The gallium(III) porphyrin-based electrodes exhibit significantly enhanced response toward fluoride, whereas the indium(III) and thallium(III) porphyrins display some preference for chloride and also effectively discriminate less hydrated anions such as perchlorate and nitrate. All of the metalloporphyrins investigated are determined to function via a charged carrier response mechanism. This mechanism is elucidated by correlating the effect of adding lipophilic ionic sites, either cationic (quaternary ammonium) or anionic (tetraphenylborate) salts, to the observed anion selectivity and response patterns of the metalloporphyrin-based liquid /polymer membrane electrodes.

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When an aqueous solution (W) containing flavin mononucleotide, pyruvic acid and O₂ was put into contact with a nitrobenzene solution (NB) containing Na⁺, K⁺, valinomycin and a ferrocene derivative, CO₂ was evolved in W and Na⁺ was transferred from NB to W, though K⁺ remained in NB. The above-mentioned respiration mimetic reaction process, accompanied by the selective ion transfer, was elucidated by referring to the ion and electron transfer voltammograms at the W/NB interface and by taking into account the energetics of the coupling between the ion and electron transfers.

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A variety of lipophilic amines incorporated in PVC matrix liquid membranes exhibited anionic potentiometric responses to phenolic compounds at the pH conditions under which the phenols exist mainly or exclusively in their undissociated, neutral forms. The examined lipophilic amines include a macrocyclic pentaamine, tri(decyl)amine, 4,7-diphenyl-1,10-phenanthroline (bathophenanthroline), 4-octadecylpyridine, and sapphyrin. The potentiometric selectivities of the membranes based on lipophilic aliphatic amines (B) reflected the acidity (hydrogen bond donor activity) and lipophilicity (extractability) of the phenols (ArOH), similarly as membranes based on lipophilic quaternary ammonium salts (Q⁺X^-). The anionic responses were explained on the basis of a decrease in the charge separation of protonated amines (BH⁺) and their counteranions (X^-) across the membrane interface. Possible processes leading to a decrease in the charge separation between BH⁺ and X^- are (i) complexation between ArOH and BH⁺X^-, followed by proton dissociation and ejection of HX into the aqueous phase, as well as (ii) complexation between ArOH and B. The membrane based on sapphyrin showed a high potentiometric selectivity to catechol, possibly due to geometrical discrimination of the ortho dihydroxy structure of catechol by the nitrogen(s) on the rigid macrocyclic structure of sapphyrin.

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The potentiometric responses of liquid membrane electrodes based on expanded porphyrins such as sapphyrin, rubyrin and triphenylrosarin, toward a series of organic carboxylates and inorganic anions were investigated. pH titration of these expanded porphyrin based electrodes with and without a guest anion in the sample solution showed that protonation of the expanded porphyrins at the surface of the electrode membrane is a prerequisite for accommodating the guest anions and yielding their potential responses. The electrodes strongly responded to benzoates, but rather less to inorganic anions and saturated aliphatic organic carboxylates. The selectivity sequences for the sapphyrin based electrode were found in some cases to deviate from the Hofmeister series; fluoride preference over chloride and bromide is particularly noted. Interesting responses were observed by the electrodes for discriminating geometrical isomers: hydrogenmaleate over hydrogenfumarate, and positional isomers: phthalate over iso- and terephthalate and trichloroacetate over di- and mono-chloroacetates.

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The potentialities of reverse micelles of AOT (bis(2-ethylhexyl) sodium sulfosuccinate) for the selective transport of tryptophane (Trp) and p-iodophenylalanine (pIPhe) are investigated using two different experimental devices. Winsor II systems were obtained from mixtures of AOT/isooctane/water (0.1 M phosphate buffer). The microemulsion phase was used as a liquid membrane in which the reverse micelles play the part of mobile carriers. In the first device there was a direct liquid-liquid contact between the different phases whereas in the second one semi-permeable membranes were introduced at the interfaces. The transfer rates measured for Trp and pIPhe at different AOT concentrations show very different behaviors, which are only moderately altered by the presence of the semi-permeable membranes. Single uptake and release experiments, which were independently undertaken show that the rate-determining step is the release of the amino acids. A transport mechanism assuming a competitive release by two processes (droplet opening at the interface and ion pair association/dissociation) is proposed. The best separation between the two amino acids is obtained at very low AOT concentrations.

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A paint-freeze method for preparing self-assembled alkanethiol/phospholipid bilayers on a gold surface has been described (by cyclic voltammetry, a.c impedance, polarized FTIR-ATR) to be well-ordered and packed, stable, solvent-free bilayers. The lipid order parameter was 0.67, calculated from the dichroic ratio, consistent with a well-ordered lipid film in which the methylene groups have segmental flexibility and are disordered to a degree which is typical for a lipid bilayer in the liquid-crystalline phase. Such a supported membrane provides a useful way for studies in biophysics, physiology and electrochemistry.

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The rate of the interfacial nucleophilic reaction of 1-p-toluenesulfonyl-2,4-dinitronaphthalene (TSDNN) with OH^- ions at the polarized nitrobenzene (NB)-water (W) interface was studied using a polarographic technique. The TSDNN partitioned into the aqueous phase underwent a displacement of the p-toluenesulfonyl moiety with OH^-. The transfer of one of the products, 2,4-dinitronaphthalate (DNN^-), from the W phase to the NB phase was monitored as the current across the interface. The rate constant of the reaction of TSDNN with OH^- in the aqueous phase was estimated to be 1.03±0.23 dm³ mol^-1 s^-1 at 25°C, which is ten-times higher than the rate constant of the displacement of 2,4-dinitrofluorobenzene with OH^- in the aqueous phase, 1.01±0.13×10^-1 dm³ mol^-1 s^-1. The contribution from the reactions in the NB phase and at the interface to the formation of DNN^- seems to be negligible, suggesting the absence of an interfacial facilitation of the reaction.

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A sodium ion-sensing plate was prepared as an easily-handled ion-sensing optical chemical sensor (optode) probe. The sensing plate consisted of an ion-sensing liquid thin layer containing a Na⁺-selective neutral ionophore (16-crown-5 derivatives) and a lipophilic anionic dye (KD-A3), which was placed on a chemically modified (octadecyl silanized) frosted glass plate that was covered with a cellulose dialysis membrane. This ion-sensing plate can easily determine Na⁺ ranging from 10^-4 to 10^-1 M by simply dipping the plate into the sample solution. The response time was ca. 80 s for 10^-1 M NaCl with good reproducibility (relative standard deviation (RSD)=±1.33% for ten sequential measurements).

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A method based on statistical thermodynamics has been used to describe concentration distributions of neutral and charged molecules in the vicinity of liquid-liquid or polymer membrane-liquid interfaces. The model is outlined and various applications are shown. One of these is a study of the transition region at the interface of two immiscible liquids. The results show that this transition is not sharp. The width of the interfacial region (where the concentration varies gradually) depends on the mutual solubility of the two liquids and on the size of the molecules. In another application the potential drop across the interfacial region has been calculated as a function of composition of the bulk phases. This type of calculation allowed the numerical simulation of the potentiometric behavior of a recently developed ion-selective electrode which employs negatively charged ion carrier.

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We report on the development and characterization of supramolecular assemblies of amphiphiles which are able to transduce changes of pH into optical signals. A series of assemblies have been investigated, including vesicular bilayer lipid membranes, monolayer films of fatty acids or phospholipids at the air-water interface, and covalently immobilized alkylsilane monolayers. In all cases, these membrane assemblies were formed from mixtures of species having zwitterionic and ionizable headgroups. In addition, a small amount of the fluorescently labeled phospholipid nitrobenzoxadiazole phosphatidylethanolamine (1 - 3%) was incorporated into the membranes. This resulted in systems which were able to transduce alterations of pH into changes in fluorescence intensity. Investigations into the physical mechanism responsible for alterations of fluorescence intensity from such membranes suggested that the nitro-benzoxadiazole phosphatidylethanolamine (NBD-PE) signal was based on changes of self-quenching of the probe which occurred when the physical and electrostatic characteristics of the membranes were altered. Useful signals were obtained only when microscopic phase separation existed within the membranes, indicating that careful design of the membrane was required to optimize the signal generated.

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An investigation by using ion-selective electrodes on the discrimination of Pb²⁺ and R₃Pb⁺, where R is an alkyl substituent, is the subject of the present paper. Selective macrocyclic thiacrown ethers are applied as active ligands. The response and selectivity of electrodes based on thiacrown ethers and ionophore-free electrodes towards inorgnaic lead ions and alkyllead substituents in model solutions and mazine cell sap are reported. The presented electrodes could be recommended as a relatively cheap and simple method for lead speciation.

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A new liquid/liquid-type reference electrode for ion-transfer voltammetry with the nitrobenzene (NB)/water (W) interface has been developed. This reference electrode is expressed as Pt/1.8 mM α-[SiMo₁₂O₄₀]^4- + 0.2 mM α-[SiMo^V₂Mo₁₂O₄₀]^4- + 0.1 M H₂SO₄ (W)/0.3 mM α-[SiMo₁₂O₄₀]^4- + 0.02 M tetrahexylammonium tetraphenylborate (NB). The Galvani potential difference between the reference NB/W interface is determined virtually by the distribution equilibrium of the heteropolyanion (α-[SiMo₁₂O₄₀]^4-), whereas the Pt electrode potential is determined by the redox equilibrium of the heteropolyanion couple. With this reference electrode, a potential window of about 0.4 V is available in a fairly negative potential region, where the transfer of hydrophilic inorganic anions (Br^-, I^-, SCN^-, NO₃^-, and ClO₄^-) can be observed voltammetrically.

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A highly selective solid state multilayer sensor is presented which enables fluorescence optical determination of potassium ions. It is based on co-extraction using valinomycin along with a fluorescent anion, both contained in different layers. The sensor membrane is composed of two layers of highly different lipophilicity. The lipophilic phase contains the carrier valinomycin whilst the hydrophilic phase contains the anionic fluorophore sulforhodamin B. On exposure to potassium ions, the lipophilic phase becomes fluorescent after coextraction of both potassium ions and the anionic fluorophore. To prevent leaching of the dye, an additional blocking layer (nafion) was spread on the sensor. The dynamic range is from 0.1 to 50 mmol/l K⁺ with very low cross-sensitivity to pH and ionic strength. The response times vary from 1 to 2 h.

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A new bis(crown ether) ionophore containing two benzo-15-crown-5 moieties connected with each other via a conformationally constrained bridge attached to a C₁₈-lipophilic side chain was designed and synthesized. The bridge consisted of an isophthalic acid derivative, which was selected in order to enhance potassium over sodium binding. Liquid-polymeric membrane ion-selective electrodes (ISEs) based on this ionophore were prepared using different plasticizers and mole ratios of lipophilic ionic additives. Membrane electrodes with optimum composition (using o-nitrophenyloctyl ether or bis(1-butylpentyl)adipate as plasticizer and 60 mol% lipophilic borate additive) show Nernstian responses toward potassium (57 and 58 mV/decade, respectively) over a wide concentration range with a micromolar detection limit. These ISEs exhibit enhanced potassium over sodium selectivity (K^pot_K+,Na+=6×10^-4). In addition, the electrodes show selectivities for potassium over Cs⁺ and NH₄⁺ that are better than those of valinomycin-based ISEs.

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The association constants (K) of complexes of chiral neutral amines with optically active crown ethers ((S,S)-1, (S,S)-2, (S,S)-4 and (S,S)-5) which contain the phenyl chiral barriers and the phenol moiety bearing an additional para-substituent were determined at various temperatures by the ¹H n.m.r. spectroscopic method in CDCl₃. The thermodynamic parameters for complexation, which were determined from the van’t Hoff plots of the K values, suggested that the acidity of the phenolic crown ethers markedly affected the enthalpy and entropy changes upon complexation with neutral amines. The crown ethers ((S,S)-2, (S,S)-4 and (S,S)-5) having higher acidity showed more negative ΔH and ΔS values and a larger slope in the enthalpy-entropy compensation plot than crown ether (S,S)-1 having a lower acidity. The temperature-dependent enantioselectivity in complexation of chiral neutral amines with (S,S,S,S)-6 containing phenyl substituents and the cyclohexane moieties as chiral barriers together with the p-(2,4-dinitrophenylazo)phenol moiety was also examined, demonstrating that steric interactions between the bulky steric barriers and the amine induced larger conformational changes of the host and guest molecules upon complexation. The data of the temperature-dependent enantioselectivity demonstrate a reversal of the relative merits of the enantioselectivity depending upon the temperature. A linear relatioship between the TΔ_R,SΔS values and the Δ_R,SΔH values (TΔ_R,SΔS=0.812Δ_R,SΔH-0.342, R=0.983) shows that the Δ_R,SΔH and Δ_R,SΔS values compensate each other. Therefore, complexation having a larger Δ_R,SΔH value does not always show a higher degree of the enantioselectivity at any temperature than that having a smaller Δ_R,SΔH value.

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This article is concerned with our recent achievement in developing synthetic chromogenic receptors for optical active amines and amino acid derivatives. Pre-organized (S)-1,1′-bi-2-naphthyl- and bis(indophenol)-derived calix[4]crown 1a shows naked-eye detectable chiral recognition between those enantiomers. The spectroscopic properties are reported, as well as an estimation of the enantioselectivity using an HPLC equipped with a chiral packing column.

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The analytical characterization of a novel chromogenic calix[4]arene derivative based sodium selective, direct optode membranes has been investigated in detail. The dynamic concentration range, ion-selectivity, response time and reproducibility are discussed. The effect of the nature of the membrane matrix on the dynamic concentration range and the operation pH of the relevant optode membranes are explained. The sodium ion response is interpreted on the basis of the ion-exchange theory, which is underlined by the results of calixarene mediated ion-transport experiments.

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Intramolecular charge transfer (ICT) processes of 2,6-p-toluidinonaphthalene sulfonate (TNS) in the water pool of sodium dioctyl sulfosuccinate (AOT) reverse micelle in heptane, is studied using picosecond emission spectroscopy. It is observed that while in aqueous solutions the lifetime (τ_f) of TNS is 80 ps, τ_f increases to 4 ns in the microemulsion with water to AOT ratio, w₀=4. With increase in w₀ as the water pool swells in size, the lifetime and quantum yield of emission decrease and the rate of the ICT process increases. However, the magnitude of the change (at most ≈70 times at w₀=4) in the rate of the ICT process of TNS compared to ordinary water is smaller than the several thousand fold retardation in the solvation dynamics of water in such a water pool relative to bulk water. This is attributed to the fact that while the solvation dynamics in the water pool is governed by the dielectric relaxation, dynamics of the ICT process is controlled by the static polarity of the medium.

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An enhancement of the chemiluminescence (CL) emission, observed when the iron(III) complex of 8-quinolinol (oxine), Fe(oxine)₃, was mixed with a reversed micellar solution of cetyltrimethylammonium chloride in chloroform-cyclohexane (6:5 v/v)-water (1.0 M NaOH) containing luminol and hydrogen peroxide, was investigated in order to develop a method for iron(III) determination based on the direct coupling of on-line solvent extraction with a reversed micellar-mediated CL reaction in a reverse-flow injection system using a microporous Teflon membrane filter for phase separation. In the CL process, uptake of the complex by reverse micelles and its subsequent decomposition occurs easily, followed by an iron(III)-catalyzed luminol reaction. In the on-line process, iron(III) was extracted from an aqueous solution into chloroform via complex formation with oxine. Upon mixing the reversed micellar luminol solution with the extract stream in a flow cell of a CL monitor, the produced CL signal was measured. A detection limit of 5 ng cm^-3 Fe(III) and a linear calibration graph was obtained in the concentration range of 10 - 500 ng cm^-3 Fe(III). In a sample solution containing iron(II) and iron(III), total iron, Fe(II)+Fe(III), was measured as the peak height in the presence of the hydrogen peroxide used to oxidize iron(II) to iron(III) prior to solvent extraction, while only iron(III) could be determined in the absence of hydrogen peroxide.

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A new method is proposed for recognizing organic molecules dissolved in water using a chemiluminescence-based sensor made with a γ-Al₂O₃ catalyst. When a mixture of air and organic molecules, e.g. ethanol and acetone vaporized from a solution, flows around the sensor, chemiluminescence (CL) is emitted during its catalytic oxidation. The CL spectra consist of subbands peaking at the same wavelengths independent of the type of vapors. The peak wavelengths of these subbands are the same as those for CL with a CaCO₃ catalyst. The relative CL intensity of each band, however, depends on the type of the vapor and the temperature of the sensor. This implies that CL originates in the same kinds of luminous species produced during the course of catalytic oxidation. By keeping a sample gas around the sensor in a state of laminar flow and by keeping the catalytic oxidation on the sensor under the diffusion-controlled condition, the CL intensity becomes stable and reproducible. The CL intensity is proportional to the concentration of these organic vapors in the gas phase within the concentration range of from 1 - 500 ppm.

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The actual constants of the overall extraction equilibrium, the distribution for various organic solvents having low dielectric constants, and the aqueous ion-pair formation (K_MLA) of 18-crown-6 (18C6)-alkali metal (Li-Cs) picrate 1:1:1 complexes were determined at 25°C; the partition constants of 18C6 were also measured at 25°C. The log K_MLA values are 2.53±0.21 for Li, 3.29±0.23 for Na, 4.76±0.27 for K, 4.62±0.36 for Rb, and 4.49±0.36 for Cs. The relatively large difference in the log K_MLA value between light (Li, Na) and heavy alkali metals (K, Rb, Cs) reflects the difference in the structure of the 18C6-alkali metal ion 1:1 complex between light and heavy alkali metals. Almost all of the partition behavior of 18C6 and its 1:1:1 complexes with alkali metal picrates can be explained by regular solution theory. The molar volumes and solubility parameters of 18C6 and the complexes were determined. For every diluent, the extraction-selectivity order of 18C6 is K > Rb > Cs > Na > Li. The extraction-ability and -selectivity of 18C6 for the alkali metal ion were completely elucidated in terms of the four fundamental equilibria.

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Data for the kinetics of Cu(II) extraction by P50 (5-nonyl-2-hydroxybenzaldoxime) obtained from the total internal reflectance (TIR) static transfer cell is presented. An interfacial mechanism involving two pararell initial addition steps is proposed and forward rate constants determined. These are discussed in relation to other data available for this system and clearly indicate further work is required in the identification of more general reaction mechanisms.

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The alkali cations (M⁺) extraction process from water to 1,2-dichloroethane (DCE) with dibenzo 18-crown-6 (DB18C6) using either picrate (Pi^-) or 2,4-dinitrophenolate (DNP^-) as counter ions (A^-) is analyzed. Picrates are extracted in a higher extent as a consequence of its higher hydrophobicity. Extraction constants (K_ext) were determined analyzing the organic phase resulting of the extraction process, either spectrophotometrically or voltammetrically against an aqueous phase containing LiCl. A very good agreement between both methods was obtained except for Cs⁺. The extraction using DNP^- can only be performed provided it is initially present in the aqueous phase. On the other hand, if HDNP is present in the organic phase the extraction process does not occur even at high pH values of the aqueous phase due to the strong interaction with DB18C6. Ion pair constants (K_IP) were determined from K_ext and Gibbs energy of M⁺ and A^- transfer.

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Interfacial tension isotherms were determined for individual palladium(II) extractants and interpreted. The following extractants were considered: 4-alkylphenylamines containing from 6 to 14 carbon atoms in the alkyl group, dihexylsulfide, 2-hydroxyethyldecyl sulfide and its partly fluorinated derivative, decylnicotiniate and N,N-dihexylpyridine-3-carboxamide. 4-Alkylphenylamines exhibited significantly stronger interfacial activity than did the other extractants. They adsorbed at the hydrocarbon/water interfaces already at concentrations 10^-5 M and permit to decrease the interfacial tension to 5 mN m^-1. As a result, they can be used as a phase transfer catalyst to increase the rate of palladium(II) extraction with other extractants. When used alone, the initial rate of extraction increases with the extractant increasing hydrophobicity, thus in the order of increased interfacial activity.

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