Solvation numbers results

Three types of methods are used to study solvation in molecular solvents. These are primarily the methods commonly used in studying the structures of molecules. However, optical spectroscopy (IR and Raman) yields results that are difficult to interpret from the point of view of solvation and are thus not often used to measure solvation numbers. NMR is more successful, as the chemical shifts are chiefly affected by solvation. Measurement of solvation-dependent kinetic quantities is often used (<electrolytic mobility, diffusion coefficients, etc). These methods supply data on the region in the immediate vicinity of the ion, i.e. the primary solvation sphere, closely connected to the ion and moving together with it. By means of the third type of methods some static quantities entropy and compressibility as well as some non-thermodynamic quantities such as the dielectric constant) are measured. These methods also pertain to the secondary solvation-sphere, in which the solvent structure is affected by the presence of ions, but the... 

The existence of critical solvation numbers for a given process to happen is an important concept. Quantum chemical calculations using ancillary solvent molecules usually produce drastic changes on the electronic nature of saddle points of index one (SPi-1) when comparisons are made with those that have been determined in absence of such solvent molecules. Such results can not be used to show the lack of invariance of a given quantum transition structure without further ado. Solvent cluster calculations must be carefully matched with experimental information on such species, they cannot be used to represent solvation effects in condensed phases. 

Gas-phase solvation has so far given only very indirect evidence concerning the structure and details of molecular interactions in solvation complexes. Complex geometries and force constants, which are frequently subjects of theoretical calculations, must therefore be compared with solution properties, however, the relevant results are obscured by influences arising from changes in the bulk liquid or by the dynamic nature of the solvation shells. With few exceptions, structural information from solutions cannot be adequately resolved to yield more than a semiquantitative picture of individual molecular interactions. The concepts used to convert the complex experimental results to information for structural models are often those of solvation numbers 33>, and of structure-making or structure-... 

X-ray and neutron diffraction methods and EXAFS spectroscopy are very useful in getting structural information of solvated ions. These methods, combined with molecular dynamics and Monte Carlo simulations, have been used extensively to study the structures of hydrated ions in water. Detailed results can be found in the review by Ohtaki and Radnai [17]. The structural study of solvated ions in lion-aqueous solvents has not been as extensive, partly because the low solubility of electrolytes in 11011-aqueous solvents limits the use of X-ray and neutron diffraction methods that need electrolyte of -1 M. However, this situation has been improved by EXAFS (applicable at -0.1 M), at least for ions of the elements with large atomic numbers, and the amount of data on ion-coordinating atom distances and solvation numbers for ions in non-aqueous solvents are growing [15 a, 18]. For example, according to the X-ray diffraction method, the lithium ion in for-mamide (FA) has, on average, 5.4 FA molecules as nearest neighbors with an... 

A method of prediction of the salt effect of vapor-liquid equilibrium relationships in the methanol-ethyl acetate-calcium chloride system at atmospheric pressure is described. From the determined solubilities it is assumed that methanol forms a preferential solvate of CaCl296CH OH. The preferential solvation number was calculated from the observed values of the salt effect in 14 systems, as a result of which the solvation number showed a linear relationship with respect to the concentration of solvent. With the use of the linear relation the salt effect can be determined from the solvation number of pure solvent and the vapor-liquid equilibrium relations obtained without adding a salt. 

The establishment of the method of prediction has been attempted by the reverse calculation of the preferential solvation number from measured values, using Equations 4 and 7 which are based on the assumption that the salt effect in the vapor-liquid equilibrium is caused by the preferential solvation formed between a volatile component and a salt. The observed values were selected from Ciparis s data book (4), Hashitani s data (5-8), and the author s data (9-15). S was calculated by Equation 7 when the relative volatility as in the vapor-liquid equilibrium with salt is increased with respect to the relative volatility a in the vapor-liquid equilibrium with salt, but by Equation 4 when as is decreased. The results are shown in Figures 5-12. From these figures, it will be seen that the following three relations exist ... 

Table III. Primary Solvation Numbers. Comparison of Results from the Variation of B (vise.) with the Most Probable Values by Other Methods...

Since experimental results were available for the high-frequency (" -2080 cm-1) diatomic CN in water (as opposed to CH3C1) (17), with an estimated T value of some 25 ps, an MD study was undertaken by Rey and Hynes (29) to clarify the role of Coulomb forces for VET in this accesible case. The charge distribution of CN in the solvent was modeled by a negative charge on N and a finite dipole located on the C site (30). The equilibrium solvent structure about this ion involved greater solvation number on the N end compared to the C end, a result consistent with some small cluster calculations (31). Since the frequency shift from the vacuum and the anharmonicity in the CN bond are both relatively small (29), the static vibrational aspects of the ion are evidently fairly clean. ... 

The interaction of ions with solvent molecules suggests a more detailed picture in which during electrolysis the cations are transporting nj+ solvent molecules into the cathode compartment and the anions nj solvent molecules out of that region into the opposite direction. The residual molecules of the solvent, which remain unaffected by the ion movement, are regarded as free , n = ni+, ni are total solvation numbers of the ions which differ from those in Chapter III. The transference numbers t[ referred to the free solvent (index ) are called true transference numbers The diffusion current density referred to the velocity v j of the free solvent results from Eq. (51) ... 

TT Then salt is added to a volatile solvent mixture, there is a salt effect—a change in the vapor-liquid equilibrium relation. This salt effect occurs because salt forms a preferential solvate with a particular component of the solvent mixture, causing a drop in partial pressure of the particular component which forms the preferential solvate. Results of the studies conducted based on this idea are reported by the author in References 1 and 2. In the past studies, the vapor-liquid equilbrium relation of the system for which formation of preferential solvate had been expected was observed, preferential solvation number was calculated based on the actually observed values, and further, salt effect was predicted based on the preferential solvate number. The author has... 

Figures 4, 5, and 6 indicate caluculated results of the preferential solvation numbers for the three systems. As shown by each figure, preferential solvation numbers are almost constant against compositions of the solvent. On the other hand, the concentration of salt increases linearly against an increase in the concentration of alcohol in the solvent as indicated in Figures 1, 2, and 3. This fact denotes that for an increase of solvent which forms a preferential solvate in a solvent mixture, the salt required to form a certain solvation number with that solvent is dissolved. For essential concentration x1SL in Equations 3 and 4, which are required in calculating solvation numbers, the data observed by the author et al. (I) were used for the methanol-ethyl acetate system ...

The values of AS , of models A and C as well as their comparisons with experimental values are shown in Table 2.20 and in Figs. 2.43 and 2.44. It is necessary to reject model 1C because X-ray determinations of the CN indicate not 4, but numbers that vary from 6 to 8 as a function of the ion. The experimental results on solvation numbers have similar inference a sharp distinction between the solvation numbers of... 

These sophisticated calculations are impressive but they err in representing their results as solvation numbers. They are, rather, coordination numbers and grow larger with an increase in the size of the ion (in contrast to the behavior of the hydration numbers, which deaease as the ion size increases). 

Suppose the results from Exercise 7 are true and calculate the solvation number for NaCl. Comment on the reliability of the result. (Xu)... 

Table P.4 lists measured dielectric constants at 25 °C for 1.0 M LiCl, NaCl, and KCl solutions, respectively. Calculate the percentage of water in the primary sheath and the total solvation number. Compare the results with those of the compressibility method (see problem 21) and comment on their reliability. 

H2O molecules in the hydration spheres with the bnlk H2O solvent varies by many orders of magnitude depending on the metal ion. This, combined with the use of many different experimental techniqnes, has resulted in discrepancies in the literatnre, particnlarly for metal ions with a weakly bonnd, rapidly exchanging hydration sphere. The Na+ cation, for example, has had qnoted values for the primary solvation number ranging from 2 to 13, although a value close to six is generally accepted." ... 

A more quantitative check of the validity of equation (30) shows that the predicted values of AC I AS agree fairly well with those observed in the four acetone-water mixtures for which data are available, provided that the entropy and heat capacity lost by water on solvation is approximately one-third the loss suffered on freezing pure water (Kohnstam, 1962) this is not unlikely (see Frost and Pearson, 1961). The solvation numbers, Ui, resulting from such calculations are also not unreasonable and vary only slightly as the solvent composition is altered. However, these conclusions could easily be fortuituous since they are based on an... 

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