Solvent cation-transport properties

As has already been pointed out, the solubility properties of the anion, its lipophilicity, are extremely important for the dissolution of the complex in solvents of low polarity. Large and soft inorganic and, much more so, organic anions very strongly increase the solubility. Anion activation and cation transport processes both depend on such anion effects. 

Reaction between C in methanol and RTCNQ in acetonitrile yielded three kinds of ionic solids (1) insulators composed of methoxy substituted RTCNQ anions such as (CHC )[F4TCNQ-0Me ](H20) (Fig. 6) [136], (2) semiconducting CT solids with fully ionic RTCNQ radical anions such as (CHC )(TCNQ ) [137, 138], and (3) conducting CT solids of partially ionic or mixed valent RTCNQ radical anions such as (CHC"XMeTCNQ° >2 [138], where CHC" is the hemiprotonated cytosine pair (Fig. 6b). Cation units in aU products were found to be protonated cytosine species, most commonly CHC, where comes from methanol. This result suggests that the intrinsic transport properties of DNA should be studied not in protic solvents but under strictly dried conditions. 

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 CFS hollow fiber suppressor (see Section 3.4.3) that was developed for cation exchange chromatography can also be applied to cation analysis via ion-pair chromatography. It features good solvent stability and sufficient membrane transport properties for the anionic ion-pair reagent. This suppressor is regenerated with tetramethylam-monium hydroxide using a concentration of c = 0.04 mol/L. 

In the development of the theory of ionic conductance it has been shown that the viscosity of the solvent is an important parameter determining ionic mobility. Initially, conductivity data were only available in water so that attention was focused on the effects of ionic size, structure, and charge in determining mobility and its concentration dependence. More recently, data have become available in a wide variety of non-aqueous solvents [11, 12], that is, in media with a wide range of permittivities and viscosities. On the basis of these data one may examine in more detail the role of solvent viscosity in determining the transport properties of single ions. Values of the limiting ionic molar conductance for selected monovalent cations and anions are summarized in tables 6.4 and 6.5, respectively. 

Proton or sometimes alkali metal cations are used for countertransport of cationic or cotransport of anionic solutes because of their good transport properties. It is not the case with the coupling anions. In fact, for K+ transport by 18-crown-6 in a BLM, the anion effect differs by more than 100 [96]. Many studies of the anion effect on transport efficiency have been conducted [97-100]. The effects of anion hydration free energy, anion lipophilicity, and anion interactions with solvents have been mentioned, although anion hydration free energy seems to be the major determinant of transport efficiency. For example, transport of K+ with dibenzo-18-crown-6 as a carrier, decreased in the order picrate > PFr, > CIO > IO >... 

In an aqueous medium, cations are solvated by some number of water molecules, the number of water molecules being determined primarily by the charge and size of the cation. The size of the solvent sheath carried by the cation and its complexes is clearly of significance in predicting the relative mobility of lanthanide ions as they traverse an analytical column, as transport properties are proportional to the fit of the analyte into the normal solvent structure. In the following paragraphs, we will explore a few of the more interesting aspects of these phenomena. 

Molecular dynamics simulations were used to study a number of electrolytes with potential interest to lithium battery applications EC DMC/LiPFe[35], EC/LiTFSI [36, 37], DMC/LiTFSI [38], GBL/LiTFSI [38], oligoethers/Li salts [39 1], acet-amide/LiTFSI [42], EC/LiBF4 [43], PC/LiBF4 [43, 44], PC/LiPFg [44], DMC/ LiBF4 [43], oligoethers/LiPFs [45 7], and PC/LiTFSI [37]. The lithium cation coordination by solvent molecules, cation-anion aggregation, and transport properties were derived from MD simulations. It is important to pay attention to the reported simulation time because some of the earlier simulations by Li et al. [48]... 

Ionic mobilities or diffusivities are measured experimentally by combining a number of independent dynamic experiments to isolate the transport properties of interest. Auxiliary experiments must also be performed to establish the thermodynamic properties of the solution. Eor the simplest case of a binary electrolytic solution (comprising an electrolyte and a neutral solvent) of a binary electrolyte (comprising one anion and one cation), there are three species (solvent 0, cations -I-, and anions -). This case requires that a single thermodynamic characterization be implemented to quantify the electrolyte activity as a function of composition. Subsequently, three independent dynamic measurements must be implemented to quantify the three independent relative diffusivities Z)+o, D q, and D+. ... 

Investigations of membrane-transport properties of amino phosphor-ylic carriers in relation to metals ions of I-IV groups that have been previously conducted, allows determining that factors defining the efficiency of metals ions transfer can be different except the molecular structure of membrane-extracting agents. First of all, this is the concentration of a carrier in membrane phase, concentration of metal cations and anions in donating phase, the nature of added anion and, as pointed above, the nature of used solvent and others The influence of some of these factors to the values of membrane permeability has been studied on the example of membrane transport of ion Nd . 

The limiting ionic conductivity is a transport property of an ion, which does not depend on concentration but depends on tanperature and pressure (or density of the solvent). Contrary to any tliamodynamic property of an individual ion, the limiting ionic conductivities of individual ions can experimentally be obtained and are tabulated in [Chapter 10, Table 10.12] for a number of anions and cations. Therefore, the molar conductivity of an electrolyte at infinite dilution. A , is an additive value and can be calculated using the limiting ionic conductivities. This possibility is particularly useful for the weak electrolytes, whai A cannot be experimentally obtained using Kohlrausch s law by an extrapolation to the infinite dilution. For example, the limiting molar conductivity of acetic add, CHjCOOHCaq), can be calculated as follows ... 

The cyclohexane-containing system (203) was also prepared in an attempt to obtain metal complexes with clam type geometries (Owen, 1983). As for the previous system, it was considered that such complexes might show enhanced shielding of the cation, from both the solvent and the counter ion present, but still allow the bound metal to be readily released on demand. As is evident from our earlier discussion, both these are desirable properties for metal-ion transport systems. 

The unique ability of crown ethers to form stable complexes with various cations has been used to advantage in such diverse processes as isotope separations (Jepson and De Witt, 1976), the transport of ions through artificial and natural membranes (Tosteson, 1968) and the construction of ion-selective electrodes (Ryba and Petranek, 1973). On account of their lipophilic exterior, crown ether complexes are often soluble even in apolar solvents. This property has been successfully exploited in liquid-liquid and solid-liquid phase-transfer reactions. Extensive reviews deal with the synthetic aspects of the use of crown ethers as phase-transfer catalysts (Gokel and Dupont Durst, 1976 Liotta, 1978 Weber and Gokel, 1977 Starks and Liotta, 1978). Several studies have been devoted to the identification of the factors affecting the formation and stability of crown-ether complexes, and many aspects of this subject have been discussed in reviews (Christensen et al., 1971, 1974 Pedersen and Frensdorf, 1972 Izatt et al., 1973 Kappenstein, 1974). 

Cation, anion, and water transport in ion-exchange membranes have been described by several phenomenological solution-diffusion models and electrokinetic pore-flow theories. Phenomenological models based on irreversible thermodynamics have been applied to cation-exchange membranes, including DuPont s Nafion perfluorosulfonic acid membranes [147, 148]. These models view the membrane as a black box and membrane properties such as ionic fluxes, water transport, and electric potential are related to one another without specifying the membrane structure and molecular-level mechanism for ion and solvent permeation. For a four-component system (one mobile cation, one mobile anion, water, and membrane fixed-charge sites), there are three independent flux equations (for cations, anions, and solvent species) of the form... 

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