Solvation number, preferential

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. 

Prediction of Salt Effect from Preferential Solvation Number... 

Method of Prediction. Preferential SOLVATION NUMBER. The formation of preferential solvation is caused by the ionization of salt. The stability of the ion in a solution depends on the magnitude of the dielectric constant of a solvent. Debye (2) explained salting out by the formation of preferential solvation and found that the following relation exists between salting out and the dielectric constant of a solvent. 

Then, we can obtain the preferential solvation number from the observed values of the salt effect. As the concentration of solvent is decreased by the number of solvated molecules, the actual solvent composition participating in the vapor-liquid equilibrium is changed. Assuming that a salt forms the solvate with the first component, the actual composition X a is given by... 

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 ... 

Prediction of salt effect. The procedure for calculation of the preferential solvation number S has been described above. By reversing this procedure, that is, by determining xia from S, we can estimate the salt effect using the vapor-liquid equilibrium without a salt. When the salt concentration is below saturation, the preferential solvation number S can be expressed as follows in cases where the solvation is formed with the first component. 

The values of So obtained from the observed values shown in Figures 4-12 were used as the preferential solvation number for prediction. As So is the solvation number between a pure solvent and a salt, it should not be obtained from... 

The preferential solvation formed between salt and solvent molecules causes a salt effect on vapor-liquid equilibria. A method of prediction of salt effect based on the preferential solvation number was reported previously for the case in which salt was solved below the saturation level. The idea introduced in this chapter applies for salt solved in saturation. The alcohol-ester-calcium chloride system for which the preferential solvation was thought to be formed was examined. Specifically, calcium chloride dissolves readily in alcohol but only sparingly in ester. Thus, when calcium chloride is solved into alcohol-ester mixed solvent, the calcium chloride will form a preferential solvation with alcohol only. Methanol-methyl acetate, butanolr-butyl acetate, and methanol-ethyl acetate systems were selected for the mixed-solvent systems. 

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... 

When a preferential solvate is formed across salt and a particular component in a solvent mixture, the preferentially solvated component is assumed to be nonvolatile. Hence, the essential concentration of the preferentially solvated component in the solvent mixture is reduced as much as the solvated component. The vapor-liquid equilibrium relation obtained under the addition of a salt may well be considered to be the same as the vapor-liquid equilibrium without the salt for liquid-phase composition from which the solvents forming solvates are excluded. Based on this idea, the essential concentration at the time when salt forms a preferential solvate with the primary component is given by Equation 1. Then we can obtain the preferential solvation number from the observed values of the salt effect. As the concentration of solvent is decreased by the number of solvated molecules, the actual solvent composition participating in the vapor-liquid equilibrium is changed. Assuming that a salt forms the solvate with the first component, the actual composition Xia is given by... 

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 ...

Figure 4. Preferential solvation number in the methanol-ethyl acetate system at 1 atm (O), CaCl2 5 wt % (A), CaCl2 10 wt % (V), CaCl2 20 wt % (D)y CaCl2 25 wt % (%), CaCl2 saturated (1).

A method to predict salt effect on vapor-liquid equilibrium in which salt is dissolved in a saturated state is introduced. In this method, salt effect is predicted by using preferential solvation numbers, the concentration of the salt, and the vapor-liquid equilibrium data for which salt is not involved. It is possible to predict salt effect completely without using actually measured data if the preferential solvation number can be predicted. Presently, however, it is impossible to completely predict preferential solvation number. Hence, the preferential solvation numbers are obtained through actual measurements, and these numbers are used for the prediction. If preferential solvation number can be predicted independently in the future, this method will be an extremely hopeful one. The salt effect prediction method is entirely in reverse sequence of that used to obtain preferential solvation number. Specifically, it is carried out in the following sequence. 

The preferential solvation number across salt and solvent is determined. 

As a cause of salt effect, the existence of a preferential solvate formed across salt and particular component in a solvent mixture is considered. Preferential solvation number was calculated for the alcohol-ester-cal-cium chloride system in which formation of preferential solvate was believed to exist. The preferential solvation number was found to be constant regardless of compositions of a solvent mixture. On the other hand, the solubility of salt into mixed solvent increased linearly against an increase of composition of the component which dissolved salt. This fact indicates that the entire dissolved salt contributes to the formation of solvation with the particular component in the mixed solvent. The author feels that the study should be continued in order to make the salt effect clear so that it will be possible to apply the above described idea to other systems also. 

S = preferential solvation number ( —) x = liquid-phase composition (mole fraction) y = vapor-phase composition (mole fraction)... 

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