Conductance ion pairs

An ion pair is electrically neutral, and when an external electric field is applied, it does not contribute to the electric current (conductivity). Ion pairs have a short lifetime however, since there is a continuous interchange between ions in the solution, due to random thermal agitation. 

Conductance relaxation Is also shown to be critically dependent upon aggregation equilibria affecting non-conducting (ion-pairs) as well as Ionic species. The relaxation behavior In the presence of quadrupoles (ion-pair dimers) and triple Ions Is thoroughly analyzed. The experimental results show the potential of the field modulation techniques as a method for the Investigation of ionization processes, independent of conductance measurements. 

The conductance of lithium salt solutions in aprotic solvents generally shows a maximum as the concentration of electrolyte is increased, as illustrated in Fig. 4.5. Such maxima can be interpreted on the basis of the opposing influence of an increasing number of charge carriers on the one hand, and increasing viscosity and increasing ion association with the formation of non-conducting ion pairs, on the other. It has been shown that the conductance can be increased by the addition of crown ethers, such as 12-cr own-4 ... 

Table 7 Molar conductivities, ion pair dissociation constants, and thermodynamic parameters for the dissociation of carbenium ion salts in dichloromethane...

Table limiting molar conductivity, ion-pair formation constant and ion size parameters. 

In general, electrolytes can be classified as weak electrolytes, strong electrolytes, and ion-pair electrolytes. Weak electrolytes only dissociate to their component ions to a limited extent, and the degree of the dissociation is temperature dependent. However, strong electrolytes dissociate completely, and Equation 5.3 is applicable to evaluate its equivalent conductance. Ion-pair electrolytes can by characterized by their tendency to form ion pairs. The dissociation of ion pairs is similar to that of a weak electrolyte and is affected by ionic activities. The conductivity of ion-pair electrolytes is often nonlinear related to its concentration. 

EXAFS) studies [52-58]. Extrapolation of conductivity results from those obtained for salts dissolved in oligomeric PEO has been used to suggest that about 80% of the ions are present as non-conducting ion pairs [59]. Spectroscopic studies in PEO NaBX4, X = F or [60, 61], PPO-... 

For electrolyte conductivity, ion pairs of symmetric electrolytes are considered to be nonconducting, electrically neutral species. The appropriate expression of the thermodynamic equilibrium constant of ion-pair formation. 

One factor that can limit the free ion concentration in a strong electrolyte solntion is ion pairing, whereby free ions associate with ions of opposite charge to make electrically neutral ion pairs that do not contribute to the conductivity. Ion pairing differs from the process described above for weak electrolytes because the bonding in an ion pair is entirely electrostatic, whereas that in most weak electrolytes involves formation of covalent molecular bonds. Ion pairing may be understood as a competition between the energy of electrostatic... 

Gronert, S., and Streitwieser, A., Jr. "Carbon Acidity. 74. The Effects of Hetero-Substituted Pendant Groups on Carbanion Reactivity. Solvent-Separated-Conduct Ion Pair Equilibria and Relative pK Li / THF s for 9-Substitued Fluorenyllithiums in Tetrahydrofuran. The Importance of Internal Chelation." J. Am. Chem. Soc., 110,2836 (1988). 

The thermodynamic analysis of the dissolution process in a polymer is identical to that for all solvents. The salt can simply dissolve as a non-conducting ion pair, according to equation (23), or as separate solvated ions as in equation (24). In concentrated solution, multiple ion agglomerates can... 

Cryoscopic investigations suggest that in sulpholan nitronium tetrafluoroborate exists predominantly as ion pairs. - The specific conductivity of these solutions increases linearly with the concentration of the salt (up to 0-4 mol 1 ), and is attributed to the existence of ion-triplets rather than free ions. ... 

The behavior of ionic liquids as electrolytes is strongly influenced by the transport properties of their ionic constituents. These transport properties relate to the rate of ion movement and to the manner in which the ions move (as individual ions, ion-pairs, or ion aggregates). Conductivity, for example, depends on the number and mobility of charge carriers. If an ionic liquid is dominated by highly mobile but neutral ion-pairs it will have a small number of available charge carriers and thus a low conductivity. The two quantities often used to evaluate the transport properties of electrolytes are the ion-diffusion coefficients and the ion-transport numbers. The diffusion coefficient is a measure of the rate of movement of an ion in a solution, and the transport number is a measure of the fraction of charge carried by that ion in the presence of an electric field. 

Ion pair Solvent Temp., °C Viscosity, jj Limiting conductance, A i A... 

Conductivity measurements yield molar conductivities A (Scm2 mol-1) at salt concentration c (mol L-1). A set of data pairs (Af, c,), is evaluated with the help of non linear fits of equations [89,93,94] consisting of the conductivity equation, Eq. (7), the expression for the association constant, Eq. (3), and an equation for the activity coefficient of the free ions in the solution, Eq.(8) the activity coefficient of the ion pair is neglected at low concentrations. 

Ion-pair association constants K A determined with the set of conductivity equations (7)—(15) agree with those obtained from Eq. (18) and (19) [100]. Salomon and Uchiyama have shown that it is also possible to extend the directly Fuoss-Hsia equation to include triple-ion formation [104],... 

Conductivity curves (A versus c ) of salts in solvents of low-permittivity commonly show a weakly temperature-dependent minimum around 0.02 molL-1 followed by a strongly temperature-dependent maximum at about 1 mol L 1. According to Fuoss and Kraus [101,102] the increase of conductivity behind the minimum is due to the formation of new charge carriers from the ion pairs. They assume that coulombic forces suffice to form bilateral cationic [C+A-C+] and anionic [A C+A ] triple ions in solvents of low-permittivity ( <15) if the ions have approximately equal radii. 

The conductivity functions of such electrolytes can be evaluated at the level of limiting laws with the help of Eq. (18), permitting the determination of the tripleion-constant KT and the ion-pair association constant KA. 

Table 5 contains a selection of ion-pair association constants, triple ion formation constants, and limiting conductivities for various electrolytes which have been studied in connection with the optimization of battery electrolytes. It shows... 

Figure 5. (a) The ( A, SO,) anion symmetric streching mode of polypropylene glycol)- LiCF,SO, for 0 M ratios of 2000 1 and 6 1. Solid symbols represent experimental data after subtraction of the spectrum corre-ponding to the pure polymer. Solid curves represent a three-component fit. Broken curves represent the individual fitted components, (b) Relative Raman intensities of the fitted profiles for the ( Aj, SO,) anion mode for this system, plotted against square root of the salt concentration, solvated ions ion pairs , triple ions, (c) The molar conductivity of the same system at 293 K. Adapted from A. Ferry, P. Jacobsson, L. M. Torell, Electrnchim. Acta 1995, 40, 2369 and F. M. Gray, Solid State Ionics 1990, 40/41, 637. 

At salt concentration below those shown in Fig. 5, molar conductivity behavior has been identified with the formation of electrically neutral ion pairs [8]. Between concentration of 0.01 and 0.1 mol L 1 (up to an 0 M ratio of -50 1) the molar conductivity rises and this can be explained by the formation of mobile... 

The ion pair mechanism initially suggested by Darwish and McLaren28 (equation 2) has received further support from related studies conducted by several other investigators38-42. For example, Fava and coworkers38 have reported that during isomerization in acetic acid, optically active benzhydryl p-toluenesulfmate loses optical activity at a rate which is about two and a half times faster than the rate of sulfone formation, thus indicating that return from an ion-pair species is occurring (equation 3). 

Polyphosphazenes sulfonates XIX with the anion covalently attached to the polymer are a new class of cation conductors that have been synthesized by Shriver [625]. They were obtained by reaction of Na0C2H4S03Na with an excess of polydichlorophosphazene in the presence of 15-crown-5, followed by the reaction of the partially substituted product with the sodium salt of poly(ethylene glycol methyl ether). The conductivity at 80 °C of the polymer with x=1.8, m=7.22 is 1.7x10 S cm This low conductivity can be attributed to an extensive ion pair formation between the sodium and sulfonate ions. 

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