Solvated species, concentrations

In experiments done by Baxendale [24] and Baxendale and Sharpe [25] in mixed n-propanol-hexane systems, it was found that at low concentrations of alcohol, a solvated species was formed however, it was not the solvated electron. The existence of the solvated species was determined by the decrease in mobility of the electron in the solvent. The mobility never decreased to the level that would be expected for the solvated electron and no evidence of the solvated electron spectrum was detected. 

Whatever the precursor, the formation of an intermediate solid phase was always observed. It can be inferred from X-ray diffraction (Fig. 9.2.7) and infrared spectroscopy that this intermediate phase shows a lamellar, incompletely ordered structure (turbostratic structure) built up with parallel and equidistant sheets like those involved in the lamellar structure of the well-crystallized hydroxides Ni(OH)2 or Co(OH)2, these sheets are disoriented with intercalation of polyol molecules and partial substitution of hydroxide ions by alkoxy ions (29). The dissolution of this solid phase, which acts as a reservoir for the M(I1) solvated species, controls the concentration of these species and then plays a significant role in the control of the nucleation of the metal particles and therefore of their final morphological characteristics. For instance, starting from cobalt or nickel hydroxide as precursor in ethylene glycol, the reaction proceeds according to the following scheme (8) ... 

Experiments in the presence of tetrahydrofuran show that the polymerization rates are always proportional to the square root of added initiator concentration (0.1% to 15% THF). 34% of the initiator is found to lead to active chains. The dependence on monomer concentration is strange. The monomer reacts according to a first order law but the rates are usually dependent on initial monomer concentration. The square root dependence on initiator concentration can be explained if reaction proceeds via free (solvated) ions in labile equilibrium with the undissociated solvated species XLi wTHF, if the latter is still the major species present. The mechanisms suggested formally explain the observed... 

The difficulties of considering solvated hydrogen ions increase when a weak acid is dissolved in a mixed solvent of two or more components, several of which form solvated species with the hydrogen ion. If, or when, the individual species can be identified and their concentrations determined, the normal thermodynamic methods in terms of all of the independent equilibria involved could be used without any assumptions. Without this information, the assumption of unsolvated acid species is justified and most convenient. 

Concentrative refractivity — This is the first derivative of the refractive index of a solution over the concentration of a given species (dn/dC). The variations of the refractive index due to the inhomogeneous spatial concentration of chemical species can be decoupled as (dn/dC) = (dn/dp)c(dp/dC) + (dn/dC)p where the first term is related to the molecular (or ionic) volume of solvated species and the second one is associated to the electronic structure of such species [i]. The value of (dn/dCj) can be considered a constant for rather diluted aqueous solutions and therefore, it is characteristic for each solvated species [ii]. 

In a binary mixture of solvents Si and S2, a cation M with a coordination number k and charge z forms k+ ) cations of the type [M(Si)i(S2)a - ]" with i = 0.. .k, differently solvated in the first solvation shell. These differently solvated species have been called solvatomers [254]. For example, with octahedrally coordinated cations k = 6), k + + 2 = solvatomers are to be expected (including three cisitrans isomeric solvatomers with i = 2, 3, or 4). In favourable cases, the concentrations of all solvatomers have been obtained as a function of the solvent mole fraction by NMR measurements [254]. 

With the concentrations of individual species determined, it is possible to evaluate the equilibrium constants for the formation of each differently solvated species, and this can be done as a function of the solvent composition. 

In the literature there are studies dealing with the relationship between changes of the free energy of transfer and the concentration of different solvated species. For the transfer of a cation from a single solvent Sj to the mixture Sj +S2, one has [254]... 

From the dispersed fluorescence, excitation spectra and their dependence on solvent -concentration we identified the different solvated species and obtained the frequency of the new vibrational modes that result from complexation (typically 170 cm and below). For isoquinoline (IQ) three solvents were used (water, methanol and acetone) to deduce some particular effects regarding the nature of hydrogen bonding in the species ... 

In solution, organomagnesium-halide reagents comprise several molecular species in dynamic equilibrium. Over a wide concentration range in tetrahydrofuran (THF), and also in Et3N, hexamethylphosphoramide (HMPA) and related polar solvents, monomeric solvated species are involved in the equilibrium (X = Cl, Br, I) ... 

Construct a cell using a standard hydrogen electrode and an electrode designed around the redox couple of interest. The cel potential E is measured with a high impedance voltmeter under zero current conditions. When using SHE as a reference electrode, E is the desired half-reaction potential [7.13], Should the redox couple have one or more electroaclive species (i) that are solvated with concentration b, E must be measured over a range of b values. 

These references suffice to indicate the power of spectroscopy to identify complex ions in non-aqueous solvents. In general the species extracted is not the predominant species in the aqueous phase (but compare ref. H54). Frequently a species of high co-ordination number exists in the organic phase. Chemical analysis reveals that it is often associated with a solvated cation, for example H3O +, M TBP, etc. It is surprising that no manifestation of H30 has been reported in the vibrational spectrum, especially in the infrared spectrum. The explanation, in terms of electrolyte theory, for the nature and stability of the species in the non-aqueous phase is lacking. To date no quantitative intensity studies of the species concentrations in the extracts, similar to those performed for species in aqueous media, have come to our attention. This field offers considerable scope to the spectroscopist interested in non-aqueous solutions of electrolytes. 

Figure 4.5 Concentrations of solvated species versus overall salt concentration in PEG-CoBr2 system.

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