Mixed solvate

Girault and Schiffrin [4] proposed an alternative model, which questioned the concept of the ion-free inner layer at the ITIES. They suggested that the interfacial region is not molecularly sharp, but consist of a mixed solvent region with a continuous change in the solvent properties [Fig. 1(b)]. Interfacial solvent mixing should lead to the mixed solvation of ions at the ITIES, which influences the surface excess of water [4]. Existence of the mixed solvent layer has been supported by theoretical calculations for the lattice-gas model of the liquid-liquid interface [23], which suggest that the thickness of this layer depends on the miscibility of the two solvents [23]. However, for solvents of experimental interest, the interfacial thickness approaches the sum of solvent radii, which is comparable with the inner-layer thickness in the MVN model. 

A fair number of mixed solvates, compounds containing molecules of crystallization of two different solvents, are also known. Generally, these are obtained either by recrystallizing halide hydrates from a nonaqueous solvent, or by crystallizing a halide from an appropriate solvent mixture, such as an alcohol intentionally or unintentionally containing significant amounts of water. Examples include... 

The form of the statistical term required in Equation 78 is complex in the general case. It should presumably take account of symmetry both of the mixed solvates formed and of the solvating molecules as ligands themselves. In a previous paper (4), an approximate general treatment was attempted by treating pairs of sites in the solvation sphere. Since its limitations were not made fully clear, the problems will be discussed further here. 

Figure 19. PMR spectra at 185°K for magnesium perchlorate in acetone-water mixtures at low water content (6) showing mixed solvate species.

Table V. Observed Chemical Shifts of Mixed Solvates of Mg2+ in Acetone—Water (6/ppm)...

The second basic assumption of the present treatment concerns the intrinsic chemical shifts or shielding constants of the mixed solvate species. These are proportional to the amounts of co-solvent P which they contain. Some further evidence for the validity of this comes from the studies of Al3+ solvates by Del-puech and co-workers (58), who found that when substituting a water molecule by an organic ligand shift changes were approximately additive (3.5 ppm per substitution) as shown in Table VI. 

Synergism by mixtures of different neutral donors was observed in the extraction of thorium from nitrate media101,207) and chloride media211) and that of Pu(IV) from nitric acid medium212), and the mixed solvates were assumed to be responsible for it. 

There has recently been increasing interest in mixed solvation using either the chemical shift (29, 36-43) or the relaxation (27, 44-49) as an experimental variable. Either method bears the potential to provide information on preferential solvation as a function of solvent composition. The chemical shift method is based on the assumptions (a) that the shielding of the solvated ionic nucleus varies linearly with the composition of the solvation sheath and (b) that outer-sphere... 

Mixed solvation was first studied by Canet et al. (124) who observed separate partly overlapping Al resonances from A1(C104)3 dissolved in a TMPA-H2O solvent mixture. The four peaks can be assigned to Al(H20)i +, A1(H20)5(TMPA)3 +, A1(H20)4(TMPA) -, and probably A1(H20)3(TMPA)3. It is noteworthy that proton, and PNMR have failed to detect the individual species as separate resonances. 

More recently the solvation of AP in binary mixtures of DMF and DMSO (126) and nitromethane and DMF (127) was investigated. For the former solvent system all possible mixed solvates Al(DMF) (DMSO)g were found to give rise to separate Al resonances. In the presence of nitromethane as a diluent the distribution of solvates follows the bulk solvent composition, and the coordination number is retained at all mixing ratios. In the undiluted binary system, however, AP is preferentially solvated by DMSO. 

NMR speetroseopie studies of Li- N labeled lithium hexamethyldisilazide in solvents ineluding THF, 2-methyltetrahydrofuran (2-MeTHF), 2,2-dimethyltetrahydrofuran (2,2-Me2THF), diethyl ether (Et20), t-butyl methyl ether (Bu OMe), n-butyl methyl ether (Bu OMe), tetra-hydropyran (THP), methyl i-propyl ether (Pr OMe), and trimethylene oxide (oxetane) have been used to eharaeterize the nature of the solvated speeies. Mono-, di-, and mixed-solvated dimers ean be identified in the limit of slow solvent exehange, but ligand exehange is too fast to observe... 

Ammonia adducts or solvates include a lanthanide series LnFa-l.S NH3 YF3 adds less ammonia, ScFs adds only O.4NH3 (4a). The other trihalides also form series of ammoniates, for example CeBra-SNHs and Cela-SNHa (134). There are numerous references to species LnCl3 xNH3 (135), and to ammoniates of ScCls and ScBra (136-140), and of YCI3 (141). Lanthanide chlorides also form mixed solvates with hydrazine (vide infra). 

The most commonly encountered solvates among pharmaceuticals are those of 1 1 stoichiometry, but occasionally mixed solvate species are encountered. For structures containing more than one solvent type, one generally finds nonpolar solvents crystallizing together on the one hand and polar solvents on the other. For example, the most common solvents found cocrystallizing with water are (in order of im-... 

In this scheme Kj and Kj are dimensionless solvation equilibrium constants, the concentrations of water and cosolvent being expressed in mole fractions. The symbols RW2, RWM, RMj are not meant to imply that exactly two solvent molecules are associated with each solute moleeule rather RWj represents the fully hydrated species, RM2 the fully eosolvated speeies, and RWM represents species ineluding both water and cosolvent in the solvation shell. This deseription obviously could be extended, but experience has shown that a 3-state model is usually adequate, probably beeause the mixed solvate RWM cannot be algebraically (that is, functionally) differentiated into sub-states with data of ordinary preeision. 

The process [9.45] takes place in mixed solvent A-B. Solvation energies of equilibrium members are the algebraic sum of those for each of the mixed solvent components (this sum also takes into account the energy of mixed solvates such as EA, By and FA BJ ... 

Strengthening or weakening interaetion (ion-dipole interaetion or dipole-dipole interaetion) of universal solvation leads to re-distribution of moleeules in the mixed solvate and to the ehange of the eomposition of solvate shell in eontrast to the eomposition of mixed solvent. 

When the eomponents of mixed solvate are solvate-inert in relation to the eomplex, equation [9.53a] takes the following form ... 

In the optimal case, even the compositions of mixed solvates formed in solvent mixtures can be determined by means of proton resonance measurements. As an example, measurements at — 75 "C on a solution of magnesium perchlorate in a 10 1 methanol-water solvent mixture revealed that statistically 5 methanol molecules and only 0.7 water molecules were coordinated to the magnesium [Am 66]. 

If solvent mixtures consisting of two solvents are used and the ratio of these components is varied, it is possible to demonstrate the differences between the Mossbauer spectra of the two parent solvate complexes and the various mixed solvates. 

This result indicates that after attaining equilibrium, in HMPA-water mixtures more than 90% of parent solvates containing either only water or only HMPA as ligands hence the stability constants of the mixed solvates are small compared with those of the parent solvates. 

A complete understanding of the solvation conditions in solvent mixtures is made extremely difficult by the fact that, in addition to the parent solvates formed with the two solvent components, mixed solvates simultaneously containing both solvents are also formed in fact, in systems with low relative permittivities the central metal ion may be capable of binding not only the two kinds of solvent molecule, but also the anion, in its first coordination sphere. 

In simpler cases, solubility measurements in solvent mixtures may also provide information on the solvating effects of the solvent components. On the basis of such solubility measurements in solutions of Hgl2 in a dimethylformamide-dimethyl sulphoxide solvent mixture, for instance, Gaizer and Beck [Ga 67] have shown that Hgl2 forms a mixed solvate with the two solvents. In an analogous study, they drew attention to the phenomenon that in a dimethylformamide-water system mixed solvates Hgl2 DMF-H20 are formed. 

The dependence of the molar absorptivity on the composition of the dioxane-water solvent mixture indicates that in these mixtures both solvents are capable of coordinating to the cobalt(III) parent complex thus, the system may contain different solvates (and possibly mixed solvates), the absorbances of which are not identical. The concentrations of these solvates depend on the composition of the solvent mixture. In this way the composition of the solvent mixture influences the value of the absorbance. On the other hand, the stability of the iodo complex examined is governed by the stabiUties of those solvates from which the mixed solvate is formed in the given system by means of substitution of the coordinated solvent molecules by iodide. Hence, it is understandable that the correlation... 

The chemical characterization of a solvent mixture is much more difficult because it is not the mixture, but its components, that take part in the solvation reactions. However, the solvating powers of the components are affected by their interactions with one another. Depending on the chemical properties of the system, the solvating powers of the individual components may differ widely. Hence, the effects of the components in forming a solvate sheath will not be proportional to their ratio in the mixture. An understanding of the system is made even more complicated by the fact that, besides solvates that contain only one or the other solvent, the solute can also form mixed solvates in the solvate spheres of which both solvents are present together, possibly in various ratios. This topic is treated in greater detail in Chapter 8. 

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