Effect of Ion Solvation

Besides those introduced into FAIMS deliberately, vapors may come from solvated ions entering from ESI or similar sources. Such ions may desolvate inside 

FIGURE 3.41 Measured c at (, = 20 kV/cm for ions in N2 at 50 °C depending on the composition of (H20/acetonitrile) solvent in ESI. (Adapted from Kolakowski, B.M., McCooeye, M.A., Mester, Z., Rapid Commun. Mass Spectrom., 20, 3319, 2006.) The ions are cations of methylphosphonic acid (MPA, 96 Da), ethylphosphonic acid (EPA, 110 Da), thiodiglycol (TDG, 122 Da), triethyl phosphate (TEP, 182 Da), and trihutyl phosphate (TBP, 266 Da). The trends with H20/methanol are similar though less pronounced.  

Large Ec shifts appeared despite the conditions thought to provide good desolvation in ESI (source T = 400 °C, maximum source gas flow of 800 L/min, and FAIMS gas T=50 °C) and actually providing it according to the MS spectra. While no variation of source or FAIMS parameters could remove the dependence of Ec on the solvent at high ESI flow rates, it disappeared at the rates of 20 0 p.L/min, presumably because of complete ion desolvation. Hence, for a stable and reproducible ESI/FAIMS operation, one should preferably use the lowest ESI flow possible, best in nano-ESI. That regime also offers maximum sensitivity and minimum ionization suppression for most accurate analytical quantification.  

---

Bjerrum s theory includes approximations that are not fully justified the ions are considered to be spheres, the dielectric constant in the vicinity of the ion is considered to be equal to that in the pure solvent, the possibility of interactions between ions other than pair formation (e.g. the formation of hydrogen bonds) is neglected and the effect of ion solvation during formation of ion pairs is not considered (the effect of the solvation on ion-pair structure is illustrated in Fig. 1.7). 

These theories based on simple electrostatic models, however, do not always predict well the experimental data, because they have some problems regarding the effects of ion solvation on the value a and of the dielectric saturation on e. See also -> association constant. 

A radical solution to all of the above-mentioned difficulties is to eliminate the solvent medium entirely and to measure structural effects on heteroaromatic reactivity in the gas phase. During the last decade, a revolution has occurred in the experimental and theoretical approaches to understanding gas-phase ion chemistry. This has occurred as the result of the simultaneous development of several experimental methods for studying organic ion-molecule kinetics and equilibria in the gas phase with precision and range of effects equivalent to or even better than that normally obtained in solution and by very sophisticated molecular orbital calculations. The importance of reactivity studies in the gas phase is twofold. Direct comparison of rates and equilibria in gaseous and condensed media reveals previously inaccessible effects of ion solvation. In addition, reactivity data in the gas phase provide a direct evaluation of the fundamental, intrinsic properties of molecules and represent a unique yardstick against which the validity of theoretical estimates of such properties can be adequately assayed. 

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

The conductivities of melts, in contrast to those of aqueous solutions, increase with decreasing crystal radius of the anions and cations, since the leveling effect of the solvation sheaths is absent and ion jumps are easier when the radius is small. In melts constituting mixtures of two salts, positive or negative deviations from additivity are often observed for the values of conductivity (and also for many other properties). These deviations arise for two reasons a change in hole size and the formation of new types of mixed ionic aggregates. 

In chapter 2, Profs. Contreras, Perez and Aizman present the density functional (DF) theory in the framework of the reaction field (RF) approach to solvent effects. In spite of the fact that the electrostatic potentials for cations and anions display quite a different functional dependence with the radial variable, they show that it is possible in both cases to build up an unified procedure consistent with the Bom model of ion solvation. The proposed procedure avoids the introduction of arbitrary ionic radii in the calculation of insertion energy. Especially interesting is the introduction of local indices in the solvation energy expression, the effect of the polarizable medium is directly expressed in terms of the natural reactivity indices of DF theory. The paper provides the theoretical basis for the treatment of chemical reactivity in solution. 

The mechanism of the reaction of hydrochloric acid and 2-methyloxetane has been investigated by a kinetic study in different solvents. The principal product is 4-chloro-2-butanol (equation 19), with a relatively small amount of 3-chloro-l-butanol formed as the by-product. This product distribution and the relatively small effect of the solvating power of the solvent indicates the mechanism involves an 5n2 attack of the chloride ion on the a-carbon atom of the protonated oxetane (67MI51301). 

Solvation is a process in which solute particles (molecules or ions) in a solution interact with the solvent molecules surrounding them. Solvation in an aqueous solution is called hydration. The solvation energy is defined as the standard chemical potential of a solute in the solution referred to that in the gaseous state.11 The solvation of a solute has a significant influence on its dissolution and on the chemical reactions in which it participates. Conversely, the solvent effect on dissolution or on a chemical reaction can be predicted quantitatively from knowledge of the solvation energies of the relevant solutes. In this chapter, we mainly deal with the energetic aspects of ion solvation and its effects on the behavior of ions and electrolytes in solutions. 

As described above, the role of ion solvation is crucial in the dissolution of electrolytes. Ion solvation also has significant effects on chemical reactions and equilibria. Ion-solvent interactions that may participate in ion solvation are shown in Table 2.3 [8],... 

In studies of ion solvation, electrode potentials in different solvents must be compared. However, if there is a reliable method for it, data on ion solvation can easily be obtained and it becomes possible to estimate solvent effects on various chemical reactions. Thus, establishing a reliable method to compare the potentials in different solvents is of vital importance in solution chemistry. 

Toteva and Richard210 showed that AG for F expulsion is about 3 kcalmol 1 higher than for Cl- expulsion. Since solvation of the fluoride ion is much stronger than that of the chloride ion, the difference in AGj must arise from the PNS effect of late solvation. 

Studies in nonaqueous dipolar aprotic solvents allowed the elucidation of the complicated role of the solvent nature in determining the - double layer structure and kinetics of electrochemical reactions. Special attention was paid to the phenomenon of ion - solvation and its effect on -> standard electrode potentials. Experimental studies of the various electrochemical systems in nonaqueous media greatly contributed to the advancement of the theory of elemental electron-transfer reactions across charged interfaces via the so-called energy of solvent reorganization. 

Polarizable solvents induce a red shift which balances the effects of ion pairing and of permanent-dipole solvation (132). ... 

Ions are formed by the dissociation of salts and heteropolar splitting of covalent bonds. The rules of ion formation and behaviour have been studied in detail, and for aqueous solutions they are fairly well known. Descriptions of ions, of their immediate vicinity, and of their reactions in less polar systems (e.g. in MeOH) are less clear. The available information on ion behaviour in non polar or weakly polar media (of relative permittivity 2-10) is even more limited. In non-polar systems, ions are much more reactive than even the most reactive radicals. Their electric charge is the cause of mutual ion associations, of ion solvation by the molecules of various compounds, and of many other effects. 

Nightingale, E.R., Jr. Phenomenological theory of ion solvation effective radii of hydrated ions. J. Phys. Chem. 1959, 63, 1381-1387. 

In cluster calculations, an element essential in solution calculations is missing. Thus, intrinsically, gas-phase cluster calculations cannot allow for ionic movement. Such calculations can give rise to average coordination numbers and radial distribution functions, but cannot account for the effect of ions jumping from place to place. Since one important aspect of solvation phenomena is the solvation number (which is intrinsically dependent on ions moving), this is a serious weakness. 

Two aspects of the theory of salting out are considered below. First, the effects of the primary solvation sheath have to be taken into account how the requisition of water by the ions causes the nonelectrolyte s solubihty to decrease. Second, the effects of secondary solvation (interactions outside the solvation sheath) are calculated. The... 

It has been stressed that solvation is a far-reaching phenomenon, although only the coordination number and the primary solvation number can be determined. However, there are effects of ions on the properties of solutions that lie outside the radius of the primary hydration sheath. These effects must now be accounted for, insofar as they relate to the solubility of a nonelectrolyte. Let the problem be tackled as though no primary solvation had withdrawn water from the solution. One can write... 

There a been a number of interesting applications of the framework developed in the studies of the simple ions were MD simulations of the quadrupolar relaxation has been performed on counterions in heterogeneous systems. Studies of a droplet of aqueous Na embedded in a membrane of carboxyl groups [54], showed that the EFG was strongly effected by the local solvent structure and that continuum models are not sufficient to describe the quadrupolar relaxation. The Stemheimer approximation was employed, which had been shown to be a good approximation for the Na ion. Again, the division into molecular contributions could be employed to rationalize the complex behavior in the EFG tensor. Similar conclusions has been drawn from MD simulation studies of ions solvating DNA... 

In this chapter some aspects of the present state of the concept of ion association in the theory of electrolyte solutions will be reviewed. For simplification our consideration will be restricted to a symmetrical electrolyte. It will be demonstrated that the concept of ion association is useful not only to describe such properties as osmotic and activity coefficients, electroconductivity and dielectric constant of nonaqueous electrolyte solutions, which traditionally are explained using the ion association ideas, but also for the treatment of electrolyte contributions to the intramolecular electron transfer in weakly polar solvents [21, 22] and for the interpretation of specific anomalous properties of electrical double layer in low temperature region [23, 24], The majority of these properties can be described within the McMillan-Mayer or ion approach when the solvent is considered as a dielectric continuum and only ions are treated explicitly. However, the description of dielectric properties also requires the solvent molecules being explicitly taken into account which can be done at the Born-Oppenheimer or ion-molecular approach. This approach also leads to the correct description of different solvation effects. We should also note that effects of ion association require a different treatment of the thermodynamic and electrical properties. For the thermodynamic properties such as the osmotic and activity coefficients or the adsorption coefficient of electrical double layer, the ion pairs give a direct contribution and these properties are described correctly in the framework of AMSA theory. Since the ion pairs have no free electric charges, they give polarization effects only for such electrical properties as electroconductivity, dielectric constant or capacitance of electrical double layer. Hence, to describe the electrical properties, it is more convenient to modify MSA-MAL approach by including the ion pairs as new polar entities. 

---