Membrane solvent

The caprolactam obtained must meet die specifications of permanganate number, volatile bases, hazen color, UV transmittance, solidification point, and turbidity in order to be used for repolymerization alone or in combination witii virgin CL.5 Reported CL purification methods include recrystallization, solvent extraction, and fractional distillation. One solvent extraction technique involves membrane solvent extraction. Ion exchange resins have been shown to be effective in the purification of aqueous caprolactam solutions. In one such process,... 

However, there seems to be some drawback in the solubility or dispersibility of ion-sensing material in silicone rubber. This is mainly because silicone rubber does not contain a large quantity of plasticizer as the membrane solvent, in which neutral carriers can be dissolved easily, unlike in plasticized-PVC ion-sensing membranes. This issue is serious, especially with silicone-rubber membranes containing neutral carriers that show high crystallinity. Valinomycin, a typical ionophore, seems applicable to silicone-rubber-based K" -selec-tive electrodes [7,8,12-14]. Conventional crown-ether-based neutral carriers are also quite soluble in silicone rubber. 

The formation of ion-pairs between complex JX and anion A in the membrane, provided that the membrane solvent is not very polar, does not affect the Nemstian response of the ISE and appears only in the constant term in (3.3.2). If most of the complex is in the form of ion-pair JXA but the concentration of ion-pair JA is much less than the concentration of JXA, then for the activity of the free complex ions... 

The ISE life-time is closely connected with the drift and is at least one year for good electrodes. With some systems, e.g. enzyme electrodes (see chapter 8), the life-time is only a few weeks. It follows from the results of Oesch and Simon [119] that the life-time of electrodes based on ionophores in solventpolymeric membranes depends on the kinetics of dissolution of the ionophore and the plasticiser in the analyte. If both the ionophore and the membrane solvent have distribution coefficients between water and the membrane greater than 10 , then the ISE life-time is at least one year. 

Table 7.1 surveys ionophores of practical importance for ISEs. Table 7.2 gives a survey of membrane solvents. Further sections in this chapter describe selected applications of ISEs with liquid membranes, arranged according to the determinand. 

Of the K ISEs, the best properties are those of the electrode based on the ionophore valinomycin XXII [51,196], marked by high selectivity for potassium with respect to sodium. Esters of phthalic acid (9,10) are used as membrane solvents and it is preferable for the reasons given in section 3.3 if the membrane contains the potassium salt of hydrophobic anion XV or XVI [119, 166]. The ISE containing cyclic polyether XXV is useful for only some applications [183] because its selectivity for potassium with respect to sodium is much smaller than with the valinomycin ISE. 

The hydrolysis of lipids rarely occurs in a single homogeneous phase, and the behavior of lipases at membrane-solvent and micelle-solvent interfaces has been discussed in detail by Verger and Jain et aP See Micellar Catalysis... 

EMF-measurements are, therefore, a method for directly assessing the relative stability of the complexes of the valinomycin group antibiotics in water and water-like solvents (cf. Fig. 11 and Ref. (87)). The selectivity constants of the liquid membranes are in this case independent of the ion-selective behavior of the membrane solvents used. 

EMF-measurements may also be used to estimate relative complex formation constants in membrane solvents. To this end, the selectivity constant Ryot of the pure membrane solvent must be measured, in addition to Kyot, the selectivity constant for the membrane plus ligand. Using Table 6 and Eq. (9) one finally obtains ... 

This shows that the parameter derived from EMF-measurements, Kyot, used to describe the monovalent-divalent ion selectivity, depends not only on the ligand and the membrane solvent but is also a function of the activities in the outside solution (26)1). Therefore, any numerical values pertaining to a so-called selectivity constant are meaningful only as long as the exact composition of the outside solution is cited. 

By similar arguments, the behavior of the membrane solvent containing no ligand comes out to be ... 

This is formally but not numerically identical to the factor in Eq. (22). Therefore, a comparison of the Kyot-values from Eqs. (22) and (24) does not yield more than a qualitative check on the relative complex formation constants in the membrane solvent ... 

The electrostatic interaction between a positively charged complex and the membrane solvent can be approximated by the Bom free energy, AGs, which becomes less negative as the dielectric constant (the polariza-... 

Fig. 22. Selectivity of the cell (37) for Na+ relative to Ca2+ as function of the dielectric constant of the membrane solvent. Membrane solvents (1) dibutyl-sebacate, (2) tris-(2-ethylhexyl)-phosphate, (3) 1-decanol, (4) acetophenone, (5) 2-nitrocymene, (6) p-nitroethylbenzene, (7) nitrobenzene...

Electrodes based on 9 but no nucleobase derivative [3.0 wt% 9 bis(2-ethylhexyl) phthalate ( dioctyl phthalate , DOP) as the membrane solvent] showed similar potentiometric responses to 5 -GMP and 5 -AMP (Figure 8a), which is not surprising because cation 9 cannot interact specifically with the base pairing site of nucleotides. The EMF slope (-29 mV decade" 0.1 M HEPES-NaOH buffer solution, pH 6.8) was much greater than in case of the electrode based on the macrocyclic pentaamine 1 (-15 mV decade" ) and corresponds to the slope as expected for a dianion according to the Nemstian equation. Extraction experiments confirmed that at this pH it is indeed the dianion that enters the organic phase. 

Possible measures that are expected to improve the potentiometric selectivity are (1) use of hosts that form stronger complexes, (2) modification of the host to avoid ionophore self-association, and (3) an improved choice of the membrane solvent to avoid strong solvation of the hosts in the membrane. Evidence for the importance of (2) and (3) has been obtained from C NMR spectra of 12. While the properties of 1 1 host-guest complexes are very often of primary interest in supramolecular chemistry, the above results show that use of receptors for sensing purposes must be based on a receptor design that goes beyond this viewpoint. 

Photoinduced Changes in Phase Boundary Potentials. The photoinduced membrane potentials were measured by using PVC matrix liquid membranes in contact with a polypyrrole-coated Pt electrode [dibutyl phthalate (DBP) as the membrane solvent]. The polypyrrole layer allows to obtain a stable and sample-in-dependent potential drop between Pt and the PVC membrane. The phase boundary potential at the interface of a membrane containing ionophore and an aqueous RbCl or KCl solution could be reversibly altered by UV and visible light irradiation, as shown for ionophore 89 in Figure 23a,b. The values of the photoinduced potential... 

If product inhibition occurs, either a stirred-tank reactor in batch or a plug-flow reactor should be used. In these two reactors, the product concentration increases with time. Alternatively a reactor with integrated product separation (membrane, solvent, etc.) is preferable. 

M chloride solutions. Membranes consist of 66 wt.% membrane solvent (here DOA,... 

Extensive studies have been carried out concerning ion transfers, electron transfers and combinations of ion and electron transfers at liquid-liquid interfaces. Po-larography and voltammetry at liquid-liquid interfaces are of analytical importance, because they are applicable to ionic species that are neither reducible nor oxidizable at conventional electrodes. They are also usefid in studying charge-transfer processes at liquid-liquid interfaces or at membranes solvent extractions, phase transfer catalyses, ion transport at biological membranes, etc. are included among such processes. 

Certain materials, including those that make up the membranes around living cells, are semipermeable. That is, they allow water or other small molecules to pass through, but they block the passage of large solute molecules or ions. When a solution and a pure solvent (or two solutions of different concentration) are separated by the right kind of semipermeable membrane, solvent molecules pass... 

Danesi, P.R. Chiarizia, R. Rickert, P. Horwitz, E.P. Separation of actinides and lanthanides from acidic nuclear wastes by supported liquid membranes, Solvent Extr. Ion Exch. 3 (1985) 111-147. 

Two types of Li+ sensing optode based on DCA were developed one was constructed from TTD14C4, KD-M11, KD-S1, TFPB, and NPOE (film 1), where the abbreviations refer to the Li+ ionophore, color-changeable dye, screening dye, lipophilic anionic additive, and membrane solvent, respectively (Fig. 5), in a PVC membrane and the other optode was made from TTD14C4, KD-Mll, KD-C4, TFPB, and NPOE (Film 2) [1,2,16,17]. KD-C4 (Fig. 5) was a color-changeable cationic dye, the pKa values of which were different from... 

The NH4+ sensing film optode was fabricated from TD19C6, KD-M13, K-TCPB, and NPOE under optimum conditions, which correspond to the NH4+ ionophore, the color-changeable dye of pfCa 7.9, the lipophilic anionic additive, and a membrane solvent, respectively, included in a PVC membrane [23,24], These chemical structures are shown in Fig. 13. KD-M13 becomes yellow in its protonated form and turns blue in the deprotonated form. When the quantity of the protonated form of the dye equals that of the deprotonated form, the mixture becomes green. 

Figure 5.28 Simplified flow schemes of (a) a conventional and (b) Mobil Oil s membrane solvent dewaxing processes. Refrigeration economizers are not shown. The first 3 million gallon/day commercial unit was installed at Mobil s Beaumont refinery in 1998. Polyimide membranes in spiral-wound modules were used [41-43]...

As described above, the initial cause of membrane fouling is concentration polarization, which results in deposition of a layer of material on the membrane surface. The phenomenon of concentration polarization is described in detail in Chapter 4. In ultrafiltration, solvent and macromolecular or colloidal solutes are carried towards the membrane surface by the solution permeating the membrane. Solvent molecules permeate the membrane, but the larger solutes accumulate at the membrane surface. Because of their size, the rate at which the rejected solute molecules can diffuse from the membrane surface back to the bulk solution is relatively low. Thus their concentration at the membrane surface is typically 20-50 times higher than the feed solution concentration. These solutes become so concentrated at the membrane surface that a gel layer is formed and becomes a secondary barrier to flow through the membrane. The formation of this gel layer on the membrane surface is illustrated in Figure 6.6. The gel layer model was developed at the Amicon Corporation in the 1960s [8],... 

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