Aspen Physical Property System - Physical Property Models

to multicomponent systems. In this simple system, three different
arrangements exist:
In the case of a central solvent molecule with other solvent molecules,
cations, and anions in its immediate neighborhood, the principle of local
electroneutrality is followed: the surrounding cations and anions are such that
the neighborhood of the solvent is electrically neutral. In the case of a central
cation (anion) with solvent molecules and anions (cations) in its immediate
neighborhood, the principle of like-ion repulsion is followed: no ions of like
charge exist anywhere near each other, whereas opposite charged ions are
very close to each other.
The effective local mole fractions are related by the following expressions:
(central solvent cells)
(central cation cells)
(central anion cells)
102 2 Thermodynamic Property Models
(11)
(12)
(13)
Using equation 11 through 13 and the notation introduced in equations 9 and
10 above, expressions for the effective local mole fractions in terms of the
overall mole fractions can be derived.
i = c, a, or B
(14)
(15)
(16)
To obtain an expression for the excess Gibbs energy, let the residual Gibbs
energies, per mole of cells of central cation, anion, or solvent, respectively, be
, , and . These are then related to the
effective local mole fractions:
(17)
(18)
(19)