Anion, size

As shown above, increases in anion size lead to reductions in the melting points of salts by reducing the Coulombic attraction contributions to the lattice energy of the crystal and increasing the covalency of the ions. In general, for ionic liquids, increasing anion size leads to lower melting points, as can be seen for a selection of [EMIMjX salts in Table 3.1-2. 

Complex anions, formed when halide salts are combined with Lewis acids (e.g., Cl /AlCl3) lead to ionic liquids with reduced melting points through the formation 

The presence of several different anions in these ionic liquids has the effect of significantly decreasing the melting point. Considering that the formation of eutectic mixtures of molten salts is widely used to obtain lower melting points, it is surprising that little effort has been put into identifying the effects of mixtures of cations or anions on the physical properties of other ionic liquids [17]. 

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The overall trend in conductivity with respect to cation type follows the order imi-dazolium > sulfonium > ammonium > pyridinium. Interestingly, the correlation between the anion type or size and the ionic liquid conductivity is very limited. Other than the higher conductivities observed for ionic liquids with the [BF4] anion, there appears to be no clear relationship between anion size and conductivity. Ionic liquids with large anions such as [(CF3S02)2N] , for example, often exhibit higher conductivities than those with smaller anions, such as [CFF3C02] . 

Figlarz and his co-workers have suggested that the formula Ni(OH)2 nHzO is not the correct one for a - Ni(OH)2 [46, 47]. They studied -Ni(OH)2 materials made by precipitation of the hydroxide by the addition of NH4OH to solutions of various nickel salts. In addition to Ni(N03)2 and NiC03 they used nickel salts with carboxylic anions of various sizes. They found that the interlaminar distance in the a - Ni(OH)2 depended on the nickel salt anion size. For instance, when the nickel adipate was used the interlaminar distance was 13.2A. Infrared studies of a - Ni(OH)2 precipitated from Ni(N03)2 indicated that NO, was incorporated into the hydroxide and was bonded to Ni. They suggested a model based on hydroxide vacancies and proposed a formula Ni(OH)2 tA, B, H20... 

Careful comparison of Pt-P bond lengths for the series trany-Pt(Pcy3)2X2 (X = H, Cl, Br, I) with those for p-any-Pt(PR3)2X2 (PR3 = PMe3 or PEt3) shows a more definite increase in Pt-P with anion size for the cyclohexyl-phosphine complexes (Table 3.16) believed to be owing to intermolecular X- H and X- C non-bonded interactions arising from overcrowding [151]. 

Therefore, the pH values of these solutions are between 11 and 12. The speciation model used by 8chwarzenbach and Fischer is certainly too simple but these authors have been the first to demonstrate the strong dependence of the polysulfide anion distribution on the alkalinity. According to Eqs. (26)-(28) higher pH values in dilute solutions will favor smaller anion sizes. 

Tadic et al. studied the polymer poly-vynilidene fluoride/hexa-fluoropropylene ( PVdF/HFP ) containing lithium salt solution in Ethylene carbonate/diethylene carbonate ( EC/DEC )- In order to understand better the effect of anion size in the electrolyte, two Li salts were compared, namely LiN(CF3S02)2 (termed Liimide by the authors) and LiN(C2F5S02)2 (termed Libeti ). 

SbFg, CF3(CF2)nS03 (n - 0,3,7), it appears that the maximum accessible oxidation levels are largely dictated by packing limitations (anion size). Evidence is also presented for reversible reductive doping of [Si(Pc)0]n in THF/(n-Bu)4N+BF4. 

Figure 9. Spherical anion size effects on maximum possible [Si(Pc)0]Xy n doping levels as seen from a simple packing model.

FBAs can also be estimated quantitatively by fluorescence spectroscopy, which is much more sensitive than the ultraviolet method but tends to be prone to error and is less convenient to use. Small quantities of impurities may lead to serious distortions of both emission and excitation spectra. Indeed, a comparison of ultraviolet absorption and fluorescence excitation spectra can yield useful information on the purity of an FBA. Different samples of an analytically pure FBA will show identical absorption and excitation spectra. Nevertheless, an on-line fluorescence spectroscopic method of analysis has been developed for the quantitative estimation of FBAs and other fluorescent additives present on a textile substrate. The procedure was demonstrated by measuring the fluorescence intensity at various excitation wavelengths of moving nylon woven fabrics treated with various concentrations of an FBA and an anionic sizing agent. It is possible to detect remarkably small differences in concentrations of the absorbed materials present [67]. 

We have described above the evolution of the magnetic properties of the [Cp2M (dmit)]AsFg salts upon isomorphous Mo/W substitution. Another possibility offered by this attractive series is the isomorphous substitution of the counter ion, that is PFg- vs AsF6 vs Sbl- fi. Electrocrystallization experiments conducted with [Cp2Mo(dmit)] and the three different electrolytes afforded an isomorphous series, with a smooth evolution of the unit cell parameters with the anion size [32], This cell expansion with the anion size leads to decreased intermolecular interactions between the [Cp2Mo(dmit)]+ radical cation, as clearly seen in Table 2 from the decreased Curie-Weiss temperatures and Neel temperatures (associated with the transition they all exhibit to an AF ground state). 

Table 2 Evolution of the structural and magnetic characteristics of the [Cp2Mo(dmit)]X salts (X = PF6 , Asly,, Shly, ) with the anion size (BSf stands for the spin-flop field in AF state)...

Pseudorotaxanes are precursors of both rotaxanes and catenanes they consist of a guest molecule threaded through a macrocyclic host. Stoppering both ends of the threaded molecule gives a rotaxane, cycliza-tion of the thread gives a catenane. Pseudorotaxane formation may occur by spontaneous self-assembly, or may be template-controlled. Anion size can be of paramount importance for such templates - Cl- is effective, Br, I- less good, and PFe ineffective when the recognition motif demands a small template (454). 

Ionics studies by Webber and Ue revealed that this salt dissociates very well even in low dielectric solvents, although its large anion size usually results in a higher solution viscosity than those of other salts in a given solvent system. Thus, its good ion conductivity should be the result of a compromise between a high degree of dissociation and low mobility. 

Large anion size was also found to favor A1 stabilization, since the 1710 cm signature did not appear till a potential higher than 5.0 V was reached for the new, asymmetric imide. Unlike the interpretation of Krause et al., Kanamura et al. attributed its inertness to the well-distributed negative charge and the lower probability of these anions forming an ion pair with AP+. ... 

The electrostatic cation-anion interactions will depend on the properties of the anion its charge, its size, its shape, its polarizability. Large anions lead to weaker interactions because of larger cation-anion distances ion-paired complexes of divalent AEC s will be much more destabilized by an increase in anionic size than AC complexes ... 

However, since only values of rexpti are obtained, it is necessary to assume a value for the ionic radius of either r+ or r- in order to derive the ionic radius of the other. It is usual to assume a value of 1.40 A for the radius of the and 1.94 A for the radius of CP (Pauling, 1948) because these are half the minimum anion-anion distances found in crystal structures. Values for ionic radii (Shannon and Prewitt, 1969 Shannon, 1976 Brown, 1988) are listed in Table V for a coordination number of 6 around the metal atoms. Thus, values of radii are hypothetical, based on the idea of an additivity rule and a few initial assumptions on anion size. 

Nakano Y, Yamochi H, Saito G, Uruichi M, Yakushi K (2009) Anion size and isotope effects in (EDO-TTF)2XF6. J Phys Conf Ser 148 012007/1-4... 

The equations of state across the series are similar to Fig. 11, the principal change being that the f-contribution is progressively reduced. The f-pressure for USb is a httle more than half the value for UN and the d-pressure is relatively more important. The sp repulsive pressure is similarly reduced as the anion size is increased. The bulk modulus also decreases approximately inversely proportional to the lattice parameter. 

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