UNIFAC

In statistical thermodynamics, the UNIFAC method (UNIQUAC Functional-group Activity Coefficients)[1] is a semi-empirical system for the prediction of non-electrolyte activity in non-ideal mixtures.

UNIFAC uses the functional groups present on the molecules that make up the liquid mixture to calculate activity coefficients.

The UNIFAC model was first published in 1975 by Fredenslund, Jones and John Prausnitz, a group of chemical engineering researchers from the University of California.

Subsequently they and other authors have published a wide range of UNIFAC papers, extending the capabilities of the model; this has been by the development of new or revision of existing UNIFAC model parameters.

UNIFAC is an attempt by these researchers to provide a flexible liquid equilibria model for wider use in chemistry, the chemical and process engineering disciplines.

Obtaining this free energy data is not a trivial problem, and requires careful experiments, such as calorimetry, to successfully measure the energy of the system.

To alleviate this problem, free energy prediction models, such as UNIFAC, are employed to predict the system's energy based on a few previously measured constants.

Similarly, UNIFAC can be off, and for both methods it is advisable to validate the energies obtained from these calculations experimentally.

The UNIFAC correlation attempts to break down the problem of predicting interactions between molecules by describing molecular interactions based upon the functional groups attached to the molecule.

This is done in order to reduce the sheer number of binary interactions that would be needed to be measured to predict the state of the system.

The activity coefficient of the components in a system is a correction factor that accounts for deviations of real systems from that of an Ideal solution, which can either be measured via experiment or estimated from chemical models (such as UNIFAC).

The activity of a real chemical is a function of the thermodynamic state of the system, i.e. temperature and pressure.

Equipped with the activity coefficients and a knowledge of the constituents and their relative amounts, phenomena such as phase separation and vapour-liquid equilibria can be calculated.

UNIFAC attempts to be a general model for the successful prediction of activity coefficients.

The UNIFAC model splits up the activity coefficient for each species in the system into two components; a combinatorial

-th molecule, the activity coefficients are broken down as per the following equation: In the UNIFAC model, there are three main parameters required to determine the activity for each molecule in the system.

obtained from the Van der Waals surface area and volumes.

These parameters depend purely upon the individual functional groups on the host molecules.

These parameters must be obtained either through experiments, via data fitting or molecular simulation.

The combinatorial component of the activity is contributed to by several terms in its equation (below), and is the same as for the UNIQUAC model.

are the molar weighted segment and area fractional components for the

are calculated from the group surface area and volume contributions

(Usually obtained via tabulated values) as well as the number of occurrences of the functional group on each molecule

is due to interactions between groups present in the system, with the original paper referring to the concept of a "solution-of-groups".

is the activity of an isolated group in a solution consisting only of molecules of type

The formulation of the residual activity ensures that the condition for the limiting case of a single molecule in a pure component solution, the activity is equal to 1; as by the definition of

This is calculated using an Arrhenius equation (albeit with a pseudo-constant of value 1).

is the energy of interaction between groups m and n, with SI units of joules per mole and R is the ideal gas constant.

, but has the somewhat unusual units of absolute temperature (SI kelvins).

These interaction energy values are obtained from experimental data, and are usually tabulated.