Many researchers have been trying to predict limiting activity coefficients of nonelectrolytes in water and found that they are complicated and include a large number of parameters which are not always available. This investigation presents a simple semiempirical model that combines the enthalpic contribution from regular solution theory, and the entropic contribution from Flory-Huggins. A single empirical parameter is introduced to correct for nonaccounted effects. The nature of this parameter indicates that it corrects the entropic description. The parameter is a linear function of the solute's molar volume. The model is extremely simple, since it only requires the molecular structure to obtain the solubility parameters and molar volumes from a group-contribution method (such as Fedors). The model has been tested with alcohols, aldehydes, amides, amines, aromatics, esters, halogenated hydrocarbons, ketones, nitro-compounds, and other miscellaneous nonelectrolytes in aqueous solutions at 25°C.
Aqueous solutions are found in almost every industrial process. Due to the wide applicability of these solutions, it is important to understand their phase equilibria behavior, so that effective separation processes can be designed. Activity coefficients (y) describe the nonideal behavior, and can be used to design separation systems directly. Infinite-dilution activity coefficients (7") provide a descriptor for dilute solutions. They are not only vital to solve specialty separations for dilute solutions, but the two limiting activity coefficients of a binary system are the only requirements to understand the complete phase equilibria diagram, provided that a twoparameter model is applied. They also provide an indication of the size and type of intermolecular forces between unlike molecules.
Although the scientific community has looked at the prediction of y's and y " ' ~ in numerous ways (such as ASOG, UNIFAC, MOSCED, SPACE), these models do not work well when water is the solvent. This is mainly due to the unique size and shape of the water molecule (entropic contributions), which differs from almost all other molecules. Also, the hydrogen bonding capacity of water (both as donor and acceptor) makes it a unique solvent.