Solvation of Alkali-metal Ions

The theory of hydrogen bonding in water and ion hydration has been the topic for a conference in which the structures and energies of M(H20) (M = Li, Na, or K) and X(H20) (X = F or Cl) were discussed. Electric permittivity, dipole moments, and structure in solutions of ions and ion pairs have also been reviewed.  

The hydration energies of lithium chloride, sodium chloride, and potassium chloride in their saturated aqueous solutions at 298 K oxt reported to be 12.1, 

The proceedings of a conference on metal-ammonia solutions have been published, featuring reviews of the physical properties of dilute and concentrated solutions, electrical, n.m.r., i.r., and Raman spectroscopic studies of diffusion, the solvated electron, kinetics, and solution structure. Electron spin resonance in metallic Li-NHa systems has been investigated from 12 to 296 K. In the liquid solutions and in the cubic phase of Li(NH3)4 the conduction e.s.r. lineshapes are in agreement with theory. To a good approximation the solvated ions are the only spin scatterers in the liquid state. The paramagnetic susceptibility of liquid Li(NH3)4 indicates that the concentration of localized moments is low and they order antiferromagnetically below 20 K.  

The vapour pressures of solutions of lithium in mcthylamine have been determined from 218 to 278 K, The activity of the amine showed positive deviations from Raoult s law at low metal concentrations and negative deviation with increasing concentration as in metal-ammonia solutions. The system was considered to contain solvated metal atoms coexisting with free MeNH molecules.  

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Furthermore, the applicability of the donicity rule may be unexpected for the solvation of alkali metal ions, where a complete explanation of the observations may be provided by considering electrostatic interactions between ion and dipolar solvent molecules. 

Although PEO is an excellent solvent for the solvation of alkali metal ions, polymer electrolytes derived from pure PEO-metal salt complexes do not show high ionic conductivities at ambient temperatures, due to the partial crystalline nature of PEO [27,29,37,59,79] (vide supra). 

Interactions between alkali metal ions and solvent molecules are considered to be essentially electrostatic in nature. Although covalent contributions to the solvate bonds cannot be completely neglected (see Sect. 5d and 6), one woidd not expect the donicities to be generally applicable to the solvation of alkali metal ions. This problem will be discussed in detail in the following section. 

The following elementary electrostatic (and consequently oversimplified) model for the solvation of alkali metal ions may account for these results. 

Relatively little is known about competitive solvation in mixtures of nonaqueous solvents. Complex formation between Na+ and THF in solutions of Na+[AlBu4] in hexane at molar ratios 1 1 (solvated contact ion pair) and 1 4 (solvent-separated ion pair) was reported by Schaschel and Day with proton NMR as well as IR and conductivity measurements (83, 84). Preferential solvation of alkali metal ions by DMSO in 1-pentanole and by acetone in nitromethane was observed by Popov et al. 69, 85). 

PRINCIPLES OF SOLVENT EXTRACTION OF ALKALI METAL IONS UNDERSTANDING FACTORS LEADING TO CESIUM SELECTIVITY IN EXTRACTION BY SOLVATION Bruce A. Moyer and Yunfu Sun... 

A useful probe of the immediate chemical environment of solute ions is the NMR chemical shift of alkali metal ions obtained in binary solvent mixtures [111, 126, 295]. These measurements are based on the assumption that the chemical shift of the solute cation is determined in an additive fashion by the solvent molecules comprising the first solvation shell. For example (cf. Fig. 2-11), the iso-solvation point of Na in dimethyl sulfoxide/acetone mixtures occurs at xxO.21 cmol/mol dimethyl sulfoxide, indicating the higher solvating ability of this solvent relative to acetone. As shown schematically in Fig. 2-11, the preferential solvation of Na by dimethyl sulfoxide displaces its chemical shift towards <5dmso and a deviation from the straight line is observed. 

Ill] A. I. Popov The Use of Alkali Metal Nuclear Magnetic Resonance in the Study of Solvation and Complexation of Alkali Metal Ions, in J. F. Coetzee and C. D. Ritchie (eds.) Solute-Solvent Interactions, Dekker, New York, London, 1976, Vol. 2, p. 271ff. A. I. Popov Alkali Metal, Mag-nesium-25, and Silver-109 NMR Studies of Complex Compounds in Nonaqueous Solvents, in G. Mamantov (ed.) Characterization of Solutes in Non-Aqueous Solvents, Plenum Publ. Corp., New York, 1978. [111a] P. Laszlo Kernresonanzspektroskopie mit Natrium-23, Angew. Chem. 90, 271 (1978) Angew. Chem. Int. Ed. Engl. 17, 254 (1978). [112] N. A. Matwiyoff, P. E. Darley, and W. 

The effect of anions on the selectivity of alkali metal ion extraction by crown ethers has been investigated by Olsher et al. [15]. They have shown that the overall extraction efficiency decreases in the order C1O4 >I, SCN >NO3 >Br . It was also shown that this efficiency is dependent upon the solvation energy of the anion in both phases, but not upon the anion softness parameter. On the other hand, the extraction selectivity (for K /Rb by cis, syn, cw-dicyclohexano-18-crown-6) decreased in the order NOj" >SCN >C1O4 > 1 > Br . They concluded that higher selectivity was observed with nonspherical counterions. 

Table 3 Free energies of solvation/kca mol of the alkali-metal chlorides, AG°, (MCI), in IM-AICI3-PC, and of alkali-metal ions, AG°oiv(M), in IM-AICI3-PC, PC, and H2O at 298 K...

Table 19 Solvation Factors Promoting the Partitioning of Alkali Metal Ions from Water to Organic Solvents...

In an extensive series of papers Popov and his collaborators have studied the solvation of alkali metal cations in various nonaqueous media, using Na magnetic resonance shifts (66), and far-infrared spectroscopy. The ion-solvent vibrational bands... 

The solubilities of fluorides in bromine (III) fluoride are similar to those in liquid hydrogen fluoride, but few observations have been made in liquid chlo-rine(III) fluoride. Alkali fluorides show good solubilities with the formation of solvated anions, fluorides of the alkaline earth metals have limited solubilities with the exception of barium fluoride. Most of the other ionic fluorides are sparingly soluble or insoluble, since solvation of the metal ions would require a... 

A separation of alkali metal ions was first attempted in water alone using a lightly sulfonated macroporous cation exchanger with aqueous 3 mM methane-sulfonic acid as the eluent Under these conditions the sample cations exhibited very similar retention times. When the macroporous resin column was used with the same acidic eluent in 100% methanol, the chromatographic separation was improved considerably, as the alkaU metal ions were solvated with methanol and the resin matrix was probably coated with a thin layer of methanol, which made the ions and the resin surface more compatible with one another. 

The most significant structural feature of crown ethers is that the diameter of fhe cavity created by the repeating oxygen atoms of fhe ring is comparable to the diameter of alkali metal ions. The diameter of the cavity in 18-crown-6, for example, is approximately the diameter of a potassium ion. When a potassium ion is inserted into the cavity of 18-crown-6, the unshared electron pairs on the six oxygens of the crown ether are dose enough to the potassium ion to provide very effective solvation for K. ... 

The cavity of a crown efher is a polar region, and fhe unshared pairs of electrons on the oxygen atoms lining the cavity provide effective solvation for alkali metal ions. The outer surface of the crown is nonpolar and hydrocarbon-like thus, crown ethers and their alkali metal ion complexes dissolve readily in nonpolar organic solvents. 

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