Models for water activity of solutions in general were recently reviewed by Rahman
(1995) and Gonzalez et al (1996). A number of the models used for the prediction of
water activities are shown in Table 8. Some of the models make use of the concept of
the activity coefficient, such as the Norrish, the Margules and the Crapiste ones.
Others, such as Chen and Schwarzberg provide a direct correction of Raoult's law
which is valid for ideal solutions. The nonideality of the solutions are due to: solute
size, intermolecular forces, solvation effects, solute-solute interaction, solute-solvent
interaction, dissociation effects of ionic solutes, order of mixing (Rahman, 1995).
Flory and Huggins (1941), as cited by Rahman (1995), were the first to express
non-ideality due to size differences between solute and solvent. Lilley and Sutton
(1991) combined the effects of size, solvation and solute-solute interaction in one
equation. In the absence of heterotactic interactions their model reduce to the well
known multicomponent Ross equation (Rahman, 1995). The order of mixing was
found to play a negligible role as mentioned by Bonne and Shannon (1991).