Solvent polymeric membranes

Polyelectrolyte complex membranes are phase-inversion membranes where polymeric anions and cations react during the gelation. The reaction is suppressed before gelation by incorporating low molecular weight electrolytes or counterions in the solvent system. Both neutral and charged membranes are formed in this manner (14,15). These membranes have not been exploited commercially because of then lack of resistance to chemicals. 

Tan S.S.S., Hauser P.C., Chaniotakis N.A., Suter G., Simon W., Anion-selective optical sensors based on a coextraction of anion-proton pairs into a solvent-polymeric membrane, Chimia 1989 43 257. 

Solvent polymeric membranes, conventionally prepared from a polymer that is highly plasticized with lipophilic organic esters or ethers, are the scope of the present chapter. Such membranes commonly contain various constituents such as an ionophore (or ion carrier), a highly selective complexing agent, and ionic additives (ion exchangers and lipophilic salts). The variety and chemical versatility of the available membrane components allow one to tune the membrane properties, ensuring the desired analytical characteristics. 

FIGURE 4.6 Schematic view of the equilibria between sample, ion-selective membrane, and inner filling solution for three important classes of solvent polymeric ion-selective membranes. Top electrically neutral carrier (L) and lipophilic cation exchanger (R ) center charged carrier (L-) and anion exchanger (R+) and bottom cation exchanger (R-). 

The selectivity here is directly proportional to complex formation constants and can be estimated, once the latter are known. Several methods are now available for determination of the complex formation constants and stoichiometry factors in solvent polymeric membranes, and probably the most elegant one is the so-called sandwich membrane method [31], Two membrane segments of different known compositions are placed into contact, which leads to a concentration polarized sensing membrane, which is measured by means of potentiometry. The power of this method is not limited to complex formation studies, but also allows one to quantify ion pairing, diffusion, and coextraction processes as well as estimation of ionic membrane impurity concentrations. 

The disadvantages described above in terms of the irreversibility of the polyion response stimulated further research efforts in the area of polyion-selective sensors. Recently, a new detection technique was proposed utilizing electrochemically controlled, reversible ion extraction into polymeric membranes in an alternating galvanostatic/potentiostatic mode [51]. The solvent polymeric membrane of this novel class of sensors contained a highly lipophilic electrolyte and, therefore, did not possess ion exchange properties in contrast to potentiometric polyion electrodes. Indeed, the process of ion extraction was here induced electrochemically by applying a constant current pulse. 

Ionophore-based solvent polymeric membranes were used as sensing layers for the development of LAPS selective for lithium [70], potassium and calcium ions [71]. Anion-selective LAPS for the determination of nitrate and sulfate ions were described [72],... 

Solvent polymeric membranes conventionally consist of ionophore, ion exchanger, plasticizer, and polymer. The majority of modem polymeric ISEs are based on neutral carriers, making the ionophore the most important membrane component. Substantial research efforts have focused on the development of highly selective ionophores for a variety of analytes [3], Some of the most successful ionophores relevant to biomedical applications are depicted in Fig. 4.1. 

Quaternary ammonium salts (QAS) are anion exchangers used in solvent polymeric membranes. Variation of substituents at the nitrogen atom is an option for tuning QAS... 

The second aspect of biocompatibility is a leaching problem. Ion-selective electrode materials, especially components of solvent polymeric membranes, are subject to leaching upon prolonged contact with physiological media. Membrane components such as plasticizers, ion exchangers and ionophores may activate the clotting cascade or stimulate an immune response. Moreover, they can be potentially toxic when released to the blood stream in significant concentrations. 

T. Sokalski, T. Zwickl, E. Bakker, and E. Pretsch, Lowering the detection limit of solvent polymeric ion-selective membrane electrodes. 1. Steady-state ion flux considerations. Anal. Chem. 71,1204—1209 (1999). 

T. Zwickl, T. Sokalski, and E. Pretsch, Steady-state model calculations predicting the influence of key parameters on the lower detection limit and ruggedness of solvent polymeric membrane ion-selective electrodes. Electroanalysis 11, 673-680 (1999). 

E. Bakker and E. Pretsch, Lipophilicity of tetraphenylborate derivatives as anionic sites in neutral carrier-based solvent polymeric membranes and lifetime of corresponding ion-selective electrochemical and optical sensors. Anal. Chim. Acta 309, 7-17 (1995). 

The above difficulties are removed in the new version of the liquid membrane, which employs a polymeric film with the ion-exchanger solution functioning as a plasticizer. Then it is much easier to prepare a membrane without leaks and using only a minute amount of the ion-exchanger solution. When the membrane ceases to function, it is simply replaced. For a survey of those electrodes see [109,111,112,113, 180] they are generally termed solvent-polymeric membranes [180] or polyvinyl chloride-matrix membranes [112]. 

Solvent polymeric membranes 1 (SPM) were prepared by dissolving polyvinyl chloride (approximately 30 wt.%), plasticiser (approx. 65 wt.%),... 

TABLE I. Properties of Solvent Polymeric and Black Lipid Membranes... 

Solvent polymeric membrane 35 wt.% polyvinyl chloride, Black lipid membrane... 

For similar solvent polymeric membranes (78 wt.% dicresyl butyl phosphate in polyvinyl chloride) self-diffusion coefficients of the order of 10-7 cm2s 1 have been reported.12 These diffusion coefficients, as well as measurements of rotational mobilities,14 indicate that the solvent polymeric membranes studied here are indeed liquid membranes. This liquid phase is so viscous, however, that convective flow is virtually absent. This contrasts with pure solvent membranes where an organic solvent is interposed between two aqueous solutions either by sandwiching it between two cellophane sheets or by fixing it in a hole of a Teflon sheet separating the aqueous solutions.15 The extremely high convective flow is one of the reasons why the term membrane for extraction systems... 

Independent of the assumptions A to C the cation selectivity of the membranes in the equilibrium domain is therefore controlled by the ratio of the complex formation constants (6) and should therefore be identical for different types of neutral carrier membranes.18 Figure 2 indicates that there is indeed a close parallelism between the selectivities of solvent polymeric membranes (SPM) and bilayer lipid membranes (BLM) modified with valinomycin 1, nonactin 2, trinactin 5, and tetranac-tin 6 (see also Ref. 18). This is in good agreement with findings from Eisenman s45 and Lev s15 research groups. 

Fig. 2. Comparison of the selectivities of neutral-carrier-modified solvent polymeric- [43] and bilayer membranes. The permeability ratios PJP (at equilibrium" (Ref. 18) as far as available) fulfilled for the glyceryl dioleate BLM s are taken from Figs. 10 and 11 in Ref. 18. Values on the SPM s were obtained using 0.1 M solutions of the aqueous chlorides and membranes of the composition 33.1 wt.% polyvinyl chloride, 66.2 wt.% dioctyl adipate, 0.7 wt.% carrier. For the macrotetrolides I2 NH( for valinomycin IZ K. ...

Fig. 8. Experimental current-voltage curves of a solvent polymeric membrane containing the neutral carrier 10 (3wt.% carrier 10, 65wt.% o-nitrophenyl-octyl ether, 32wt.% polyvinyl chloride, thickness of membrane approximately 100/urn, 25°C). The membrane was in contact with aqueous CaCI2-soIutions of the molarity indicated.55...

A series of ion-selective membrane electrodes based on neutral carrier solvent polymeric membranes has been designed for the potentiometric determination of ion activities (for reviews see Refs. 52, 65). Systems with analytically relevant selectivities for Li+, Na+, K+, NHJ, Ca2+, and Ba2+, are available. In agreement with the treatment given in Sections III and IV, the ions preferred in potentiometric studies may be transported preferentially through the same membranes in electrodialytic experiments. So far, selective carrier transports have been realized for Li+, Na+, K+, and Ca2+. 

A more detailed study of transport processes in solvent polymeric membranes was initiated recently.72 One aim was to get information on the distribution within the membrane of the carrier and the cation transported after a steady state has built up during an electrodialysis experiment. A further objective was the demonstration of a relaxation of the concentration gradients of both carrier and cation. To this end the transport properties of solvent polymeric membranes containing the carrier l4C-valinomycin (66 wt.% dioctyladipate, 33 wt.% polyvinyl chloride, 1 wt.%, JC-valinomycin) in contact with aqueous solutions of -,H-a-phenylethylammonium chloride were studied. 

The question hits the big unknown in explaining the cation permselectivity of the neutral carrier systems mentioned in my report. In the solvent polymeric membranes we studied, the contribution of anions to the electrical current (anion transport number) is usually negligible if hydrophilic anions (e.g., Cl ) are involved. In the presence of lipophilic anions (e.g., SCN ) there exists some contribution of anions to the total ion flux across the membrane [see Anal. Chem., 48, 1031 (1976)]. The reasons for such a behavior may be ... 

We have to expect such selectivity changes and it would be very interesting to measure them. Unfortunately, the solvent polymeric membranes mentioned can usually not be contacted with nonaqueous solvent systems because of the expected increased solubility of the membrane components involved. 

Although attractive isotope effects may be expected especially in the Ca2H selective solvent polymeric membranes mentioned [see B. E. Jepson and R. DeWitt, J. Inorg. Nucl. Chem., 38, 1175 (1976)], we have not so far studied such effects. As compared to the flux of ions in the elec-trodialytic transport experiments we carried out on solvent polymeric membranes, the flux of ions is negligible in the absence of an electric field (other parameters kept constant). 

A superficial examination of experimental results obtained by using labeling techniques (electrodialytic transport through solvent polymeric membranes) indicates that there might be a substantial transport of water coupled with the carrier-mediated ion transport. This would be rather surprising because the cation in the carrier-cation complex is not hydrated. 

J. Bobacka. T. Lindfors, A. Lewenstam. and A. Ivaska, All-Solid-State Ion Sensors Using Conducting Polymers as Ion-to-Electron Transducers, Am. Lab., February 2004, 13 A. Konopka, T. Sokalski, A. Michalska, A. Lewenstam, and M. Maj-Zurawska, Factors Affecting the Potentiometric Response of All-Solid-State Solvent Polymeric Membrane Calcium-Selective Electrode for Low-Level Measurement, Anal. Chem. 2004, 76, 6410 M. Fouskaki and... 

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