Selective solvation of ions

Selective Solvation of Ions and Competition Between Solvation and Ion Association. 

The treatment of all those properties of electrolyte solutions, where selective solvation of ions in mixed solvents may play a major role, would result in an accumulation of data hard to follow up. Therefore, only those theoretical treatments of ion solvation have been mentioned in the following whose results have been used to ... 

Fig. 2-10. Schematic model for the selective solvation of ions by one component of a binary 1 1 mixture of the solvents A and B [119].

A comprehensive tabulation of selective solvation of ions in a number of binary solvent systems is given by Gordon [96] (p. 256). 

NMR is a useful tool for studying selective solvation of ions. Most of the work to date has been concentrated on cations, with water as one component of the solvent mixture. For the time being we will exclude the situation where the solvation shell of the ion includes another ion (ion pairing). 

Selective solvation of the cations and anions takes place if the molar fraction xs of the solvent S differs from that in the vicinity of the ions ... 

The solvent can also affect regioselectivity. Consider O- vs C-alkylation of phenoxide ion with allyl chloride or bromide. In water, with allyl chloride the O- to C-alkylation ratio is 49 41 with phenol as a solvent it is 22 78 with methanol, dimethylformamide, and dioxane 100% O-alkylation is achieved. The selective solvation of the more electronegative O by the more protic solvents perhaps leads to some C-alkylations. 

The main classes of plasticizers for polymeric ISEs are defined by now and comprise lipophilic esters and ethers [90], The regular plasticizer content in polymeric membranes is up to 66% and its influence on the membrane properties cannot be neglected. Compatibility with the membrane polymer is an obvious prerequisite, but other plasticizer parameters must be taken into account, with polarity and lipophilicity as the most important ones. The nature of the plasticizer influences sensor selectivity and detection limits, but often the reasons are not straightforward. The specific solvation of ions by the plasticizer may influence the apparent ion-ionophore complex formation constants, as these may vary in different matrices. Ion-pair formation constants also depend on the solvent polarity, but in polymeric membranes such correlations are rather qualitative. Insufficient plasticizer lipophilicity may cause its leaching, which is especially undesired for in-vivo measurements, for microelectrodes and sensors working under flow conditions. Extension of plasticizer alkyl chains in order to enhance lipophilicity is only a partial problem solution, as it may lead to membrane component incompatibility. The concept of plasticizer-free membranes with active compounds, covalently attached to the polymer, has been intensively studied in recent years [91]. 

This selective solvation of lithium ions by high-e solvent molecules would exclude the solvents of low-rj from the solvation sheath and leave the latter as free, noncoordinating solvent molecules. As a result, the media in which the solvated ions migrate are mainly composed of these free solvent molecules, which impart their low- to benefit the movement of the solvated ions. In this way, a synergistic participation from both high-e and low- solvents contributes to the optimization of ion conduction. 

The range of accessible dielectric constant is somewhat limited by the fact that in low-dielectric-constant media, ion pairing begins to become important, and the contribution of ion triplets and quadruplets may also become important. In addition, selective solvation of the ions by the higher-dielectric-constant solvent will here assume its most important aspects. Little information is available on this point, but the kinetic data do show significant deviations from straight lines in the In kn vs. 1/D plots at low D. 

Nucleophilic fluorination using [18F]fluoride ion is a typical displacement reaction that is generally performed in a polar aprotic solvent such as acetonitrile, dimethyl sulfoxide, or dimethylformamide [100], In polar aprotic solvents, the nucleophilicity of anions including [18F]fluoride is enhanced by the selective solvation of the cations. Conversely, solvation in protic solvents likely reduces an anion s nucleophilicity by interaction with the partial... 

The titration of proteins with weak acids is not considered in this review, since in the only detailed study of the subject (Steinhardt, Fugitt, and Harris (1943) on wool keratin) the effect of selective solvation of the fiber by undissociated weak acid was added to the effect of combination with hydrogen ion. 

A measure of selective solvation of single ions is the isosolvation point, that is, the solvent composition at which the mean numbers of molecules of the two solvent components bound to an ion are the same. Rahimi and Popov (1976) found by Ag+ NMR that the isosolvation point for Ag+ in water-acetonitrile was at 0.075 mol fraction of acetonitrile. This means that solvation by this component is preferred by about 6.3 kJ mol over water. 

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