Abstract
The activity coefficient model (ACM) was originally developed from a local composition model for use with non-electrolyte liquid solutions with adjustable parameters. This ACM is often applied also to electrolyte liquid solutions to determine the salt effect on vapor-liquid equilibria (VLE). This is achieved by extending the original ACM, by the addition of "electrolyte terms," to include electrolytes. Our purpose here is to show 1) that both the original ACM and the extended ACM offer versatility in the correlation or fitting of activity coefficients of volatile components in liquid solutions containing dissolved salt, but 2) if the salt is treated as the third component, that the original ACM has enough flexibility to represent the activity coefficients, because of its capability for curve fitting. An ACM based on the local composition model for a non-electrolyte solution should not be applied for an electrolyte solution, in which electrolytes and solvent molecules are strongly associated. In our approach, we simply assume that there are two types of molecules in the solution containing dissolved salt. The first is a solvate - a combination of salt and volatile components; the second is a non-solvated free volatile component. The second can contribute to VLE whose liquid compositions differ as to the numbers of their molecules that are solvated with salt, while the first cannot contribute to VLE at all, because of salt's non-volatility. Using this simple assumption, our model affords, compared to the extended ACM, a simpler, more easily visualized method for correlating and predicting VLE for electrolyte solutions, yet with superior accuracy. Once solvation numbers between each of the volatile components and salt is determined, the salt effect on VLE can be determined for both binary and multi-component solvents with salt systems. The solvation numbers can be obtained from data on either the lowering of vapor pressure or raising the boiling point of either pure or binary solvents with salt systems. The effect of salt on VLE can be predicted from the solvation numbers using the data of a pure solvent and salt system.
