Partition coefficient estimation

Tomlinson, E., Hafkenscheid, T. L. (1986) Aqueous solution and partition coefficient estimation from HPLC data. In Partition Coefficient, Determination and Estimation. Dunn, III, W. J., Block, J. H., Pearlman, R. S., Eds., pp. 101-141, Pergamon Press, New York. 

The physical properties of -hexane (see Table 3-2) that affect its transport and partitioning in the environment are water solubility of 9.5 mg/L log Kow (octanol/water partition coefficient), estimated as 3.29 Henry s law constant, 1.69 atm-m3 mol vapor pressure, 150 mm Hg at 25 °C and log Koc in the range of 2.90 to 3.61. As with many alkanes, experimental methods for the estimation of the Koc parameter are lacking, so that estimates must be made based on theoretical considerations (Montgomery 1991). 

For the linear isotherm model, the parameter (Kd) that relates both sorbate and solute is called the partition coefficient. A number of studies have developed empirical relationships for partition coefficients in natural solid phases and several of these studies are summarized in Table 1.Various theoretical-based methods of partition coefficient estimations also exist (Table 1, Eqs. a- f). 

Table 1. Various theoretical methods for partition coefficient estimations... 

BJ Herbert, JG Dorsey. A-Octanol—water partition coefficient estimation by micellar electrokinetic capillary chromatography. Anal. Chem. 67 744—749... 

Table 4-3, with partition coefficient estimation results for 13 aroma compounds partitioned between polyethylene (PE) and ethanol, shows an example of the estimation accuracy one can expect comparing UNIFAC to experimental data and the other partition coefficient estimation methods (Baner, 1999). In order to compare the different estimation methods, average absolute ratios of calculated to experimental values were calculated partitioned substances. When the calculated values are greater than experimental values the calculated value is divided by the experimental value. For calculated values less than the experimental values the inverse ratio is taken. Calculating absolute ratios gives a multiplicative factor indicating the relative differences between values of the experimental and estimated data. A ratio of one means the experimental value is equal to the estimated value. 

Table 4-3 Accuracy of partition coefficient estimations for 13 aroma compounds between 100 % ethanol and LDPE. 

Octanol-water partition coefficient estimation. PCs under DOS and Windows. 

With the widespread application of lipophi-licity and partitioning to biophysical processes, a wide variety of tools is currently available to estimate partition coefficient. Several recent reviews of programs and methods that are commercially available have been published (25-28). Predictive methods may be broken down into the following basic approaches to partition coefficient estimation (i) group contribution methods, by use of molecular fragments (2) group contribution, by use of atom-based contributions (5) conformation-dependent or molecular methods (4) combined fragment and atom-based methods and (5)other physicochemical methods. 

It is possible to modulate the likelihood levels by relationships with experimental or calculated physicochemical (or other) parameters such as octanol-water partition coefficients. Such calculators may be external to the main program. For example, within the skin sensitisation endpoint, a very low skin permeability coefficient (Log Kp),as calculated from the molecular weight and the octanol-water partition coefficient estimation using the method of Potts and Guy, reduces the default skin sensitisation alert likelihood from plausible to equivocal (Figure 11.4). 

Hattis et al. (1993) reviewed a number of experimental studies of humans exposed to tetrachloroethylene and calculated ratios of arm blood to alveolar air tetrachloroethylene concentrations following inhalation exposure. These ratios suggest that the blood/air partition coefficient should be 15-18. Among the human blood/air partition coefficients estimated by in vitro methods (Table 2-4), the in vivo values are closest to the human blood/air partition coefficient of 19.8 estimated by Byczkowski and Fisher (1994) using the smear method. 

The absorption, distribution, storage, and excretion of tetrachloroethylene are largely determined by its lipophilic nature. The blood/air partition coefficient estimated for humans is 10-20, the fat/air partition coefficient is 1,450-1,638, and the fat/blood partition coefficient is 125-159 (Byczkowski and Fisher 1994 Gearhart et al. 1993 Ward et al. 1988). Therefore, tetrachloroethylene is readily taken up by blood and is then distributed to fatty tissues where it is retained with a half-life of about 55 hours. The affinity of tetrachloroethylene for fat also results in its translocation into milk (Byczkowski and Fisher 1994). 

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