(60c) Using the Solvation Model to Predict the Salt Effect on Vapor-Liquid Equilibrium

The vapor-liquid equilibrium (VLE) of a mixture containing dissolved salt behaves quite differently from a mixture composed of volatile components alone. When a salt is added to a volatile component system, the composition of the vapor phase will change, as well as the temperature or pressure. This change, called the salt effect on VLE, means that prediction requires a different approach than that for the no-salt system. This is because the structure of the salt-containing system, or electrolyte system, is entirely different from that of the no-salt system, or non-electrolyte system. A typical model of the configuration of a non-electrolyte system is the local composition model. In contrast, a typical model of the configuration of an electrolyte system is the solvation model. The formation of solvate in electrolyte solutions is well known. The solvation numbers for each ion of the salts have been reported by Yitzahk Marcus, who used Stokes' radii. We have developed a prediction method that can express such a system thermodynamically based on the solvation model. The solvation model can express the salt effect fully in terms of change of liquid phase composition and total vapor pressure. In our model, we assume that: 1. Both volatile components make solvates with each ion of a salt. 2. The salt is perfectly ionized to anion and cation. 3. Solvates cannot contribute to vapor liquid equilibria. 4. A free volatile component can contribute to the equilibria. 5. The free volatile component is not affected by the solvate. 6. The activity coefficient is divided into two parts: ? expression of non-ideality ? vapor pressure lowering Thus, the number of liquid molecules that determine vapor-liquid equilibria changes from that in the original composition. We call this resulting changed composition the ?effective liquid composition?. Using these assumptions, our model affords a more versatile method for predicting and correlating VLE for electrolyte solutions. We calculated for VLE using our model and found that the solvation numbers determined by our model, when extrapolated up to infinite dilution, coincide with Marcus's numbers. The solvation number we determined using our model shows significant tendencies. First, it closely approaches that of the Stokes radii when the salt concentration is 0. The values under that condition are closely similar to the ion solvation numbers reported by Marcus at infinite dilution. Second, as the salt concentration increases, the solvation number decreases ? as we would expect, when ionic activity is involved. Our model's ion solvation numbers derived from Stokes' radii are the values at infinite dilution. Thus we can predict two facts: the solvation numbers will be at maximum value; and, as the salt concentration increases, the solvation number can only decrease. Our examination of the observed data confirmed these facts, and our model shows these relations. This outcome qualitatively verified the consistency of our model's predictive capability. In our model we have two methods for determining solvation numbers. One uses data on the elevation of the boiling point of a pure solvent with a salt. The other works directly from the observed data of the salt effect on VLE. Our model can both predict the salt effect on vapor liquid equilibrium and correlate the observed data. We call the first method the predictive method and the second the correlative method. Using the predictive method with data on the boiling point elevation of a pure solvent with salt, we can predict and confirm the breaking of the azeotropic point (for example, in an ethanol, water, and NH4Cl system at 101.3 kPa). At present, no other method can attain this result. We applied the solvation model to predict the methanol, ethanol, water and calcium chloride system, with quite satisfactory results. The solvation numbers used for each volatile component were those determined from the constituent binary volatile components of the salt system. Our method has the following notable advantages: 1. The solvation number can be obtained from the vapor pressure lowering data of a pure solvent with salt, or from the salt effect data. Furthermore, as explained earlier, the solvation numbers of ions also can be used directly. 2. VLE can be predicted from the solvation numbers. 3. Our model expresses the salt effect more meaningfully. It can explain and demonstrate thermodynamically the mechanism of the salt effect on VLE. 4. Our model's visual interpretation of parameters reflects more realistically the actual chemical structure of the liquid solution, making it easier to comprehend. We have applied our method for almost all systems reported in the literature with satisfactory results. Our model is a true prediction method, because it can use the solvation number from a pure solvent with salt. In our model, the salt effect can be predicted from the solvation number alone. The Solvation Model can predict values that are authentic ?? not just correlational. The local composition model should not be extended for electrolyte solutions, since those solutions form complex solvates rather than locally distributed species.