Partition Estimation using Unifac

Baner, A.L., Partition coefficients of aroma compounds between polyethylene and aqueous ethanol and their estimation using UNIFAC and GCFEOS. Ph.D. Dissertation, Michigan State University, E. Lansing, 1992. 

A test set of 6 to 13 aroma compound partition coefficients between different food contact polymers (low density polyethylene (LDPE), high density polyethylene (HDPE) polypropylene (PP), polyethylene terephthalate (PET), polyamide (PA)) and different food simulant phases (water, ethanol, aqueous ethanol/water mixtures, methanol, 1-propanol) were taken from the literature (Koszinowski and Piringer, 1989, Baner, 1992, Franz, 1990, Koszinowski, 1986, Franz, 1991, Baner, 1993, Piringer, 1992). Table 4-2 shows the test set of 13 different aroma compounds, with their properties and their structures. The experimental data were compared to estimations using different estimation methods of UNIFAC-FV, GCFLORY (1990), GCFLORY (1994) and ELBRO-FV. 

Li, A. Doucette, W. J. Andren, A. W. Estimation of aqueous solubility, octanol/water partition-coefficient, and henrys law constant for polychlorinated biphenyls using UNIFAC. Chemo-sphere 1994, 29, 657-669. 

We recommended two of these methods for general use, estimation from octanol-water partition coefficient and a group contribution method named AQUAFAC. Three other methods are also valuable under certain circumstances, the connectivity, UNIFAC, and sol-vatochromic approaches. 

A modern set of methods which can be used to estimate partition coefficients is the group contribution method. These methods were developed to allow chemical engineers to estimate activity coefficients in liquid and polymeric systems. Of the numerous methods developed, UNIFAC, the oldest and most thoroughly tested method, is probably the most universally applicable to a wide variety of substances and sytems despite its known weaknesses (Baner, 1999). The use of UNIFAC and example calculations will be described later in this chapter. 

The liquid phase and polymer phase activity coefficients were combined from different methods to see if better estimation accuracy could be obtained, since some estimation methods were developed for estimation of activity coefficients in polymers (e.g. GCFLORY, ELBRO-FV) and others have their origins in liquid phase activity coefficient estimation (e.g. UNIFAC). The UNIFAC liquid phase activity coefficient combined with GCFLORY (1990 and 1994 versions) and ELBRO-FV polymer activity coefficients were shown to be the combinations giving the best estimations out of all possible combinations of the different methods. Also included in Table 4-3 are estimations of partition coefficients made using the semi-empirical group contribution method referred to as the Retention Indices Method covered in the next section. 

Estimate the polymer/liquid partition coefficient for cis-3-hexenol between a package with a LDPE material food contact layer. The package contains a food system whose partitioning characteristics can be simulated using 100 % ethanol at 25 °C. Use the UNIFAC activity coefficient estimation method and assume a dilute solution (x, = w, 0.00001). 

Because of the lack of quantitativeness of the Regular Solution Theory and large amount of effort and computing power required for the UNIFAC method, yet another way will be taken here. This way leads to values using simple means which can adequately estimate values for the most important practical cases. The method described in this section is based on the potential already recognised in gas chromatography that the partition of a substance between a gas and a polymer liquid can be estimated based on its structural increments and these can be used as characteristic quantities for identification. 

In the case of polar liquids and polymers an estimation of the activity coefficients for z and z + 1 in L or P can be tried using another method, e.g. the UNIFAC method. If this is not possible then both the partition coefficient for z and z + 1 in the gas/liquid or gas/polymer system must be experimentally determined. For the relatively volatile alkanes this is possible without a great amount of work. 

---