Partition constant of compound

Using the numbers given in Table 3.5 we can now inspect Table 3.4 in order to get some feeling of the temperature dependency of partition constants. Except for the hexadecane/water partitioning of hexane and benzene, there is a significant effect of temperature on the partition constants, particularly if one of the phases is the gas phase. For example, the air/water partition constant of diethylether is about 4 times larger at 25°C as compared to 5°C (An// = 46.8 kJ-mol 1). As we will see later in various other chapters, in cases in which equilibrium is not established, temperature may have an important effect on the direction of fluxes of compounds between environmental compartments. 

A second, very different general approach to predict the partition constant of a compound in a given two-phase system assumes that the free energy of transfer term for... 

The special interaction terms are necessary to describe intramolecular interactions between different parts of the molecule that cannot be accounted for when considering the transfer of the isolated parts. Obviously, this type of approach has a big advantage in that it allows one to estimate a partition constant based solely on the compound s structure. Good results can be anticipated particularly in those cases where the partition constant of a structurally closely related compound is known, and thus only the contributions of the parts that are different between the two compounds have to be added and/or subtracted, respectively. The most advanced and most widely used method that is based on this concept is the structural group contribution method for estimating octanol-water partition constants. We will discuss this method in Chapter 7. 

Fig. 3.4 shows that when plotting the air-pure liquid compound partition constants of a large number of chemicals versus their dispersive vdW parameters, the apolar and monopolar compounds fall more or less on one line, while the bipolar compounds do not show this behavior. Explain these findings. For which kind of bulk liquids (give examples) would you expect that in a similar plot, all compounds (including the bipolar ones) should fit one line ... 

Another important lesson that we can leam from the data presented in Table 6.1 is that the activity coefficient of an organic compound in an organic solvent depends strongly on the prospective involvements of both the partitioning compound and the solvent for dispersive, dipolar, H-donor, and H-acceptor intermolecular interactions. This implies that we may need to represent the properties of both the solute and the solvent when we seek to correlate air-liquid partition constants of structurally diverse substances. Thus, if the types of intermolecular interactions of a variety of solutes interacting with two chemically distinct solvents 1 and 2 are very different, a one-parameter LFER for all compounds, i, of the form ... 

Figure 6.5 Plot of the decadic logarithms of the air-cyclohexane versus the air-hexadecane partition constants of a series of apolar, monopolar, and bipolar compounds. Data from Abraham et al. (1994b).

Finally, the relationships between the air-organic solvent, the air-water, and the organic solvent-water partition constants of a given compound (Eq. 6-11) will make it very easy to understand organic solvent-water partitioning, which we will treat in Chapter 7. 

We start, however, with some general thermodynamic considerations (Section 7.2). Then, using our insights gained in Chapter 6, we compare solvent-water partition constants of a series of model compounds for different organic solvents of different polarity (Section 7.3). Finally, because n-octanol is such a widely used organic solvent in environmental chemistry, we will discuss the octanol-water partition constant in somewhat more detail (Section 7.4). 

In Section 3.4, we derived the equilibrium partition constant of a compound between two bulk liquid phases (Eq. 3-40). Denoting the organic phase with a subscript we... 

Table 7.1 Organic Solvent-Water Partition Constants of a Series of Compounds for Various Organic Solvents at 25°C ... 

Kishino and Kobayashi (1994) determined the n-octane-water (Kioclw) and n-octanol-water (Kiow) partition constants of a series of chlorinated phenols (see Table below). Plot the log Kioctw values versus the log Kiov/ values of the 13 compounds. Inspect the data and derive meaningfull LFERs of the type Eq. 7-7 for subsets of compounds. Discuss your findings in terms of the molecular interactions that govern the partitioning of the chlorinated phenols in the two different solvent-water systems. 

Multiplication of Eq. 8-31 with [HA]W / [HA]W (=1) and rearrangement shows that Diavi is simply given by the product of the fraction in nondissociated form (aia) and the air-water partition constant of the neutral compound (Kiaw) ... 

F can also be separated by solvent extraction. Faure et al. (223)(224) used solvent extraction with disphenyldichiorosilane in isopropyl ether for the determination of oxygen in molybdenum and lead. The separation of F by solvent extraction using triphenyl antimony (V) derivatives is described by Chermette et al. (225)(226). It is shown that, if fluoride is extracted with an excess of reagent, the extracted salt is triphenyl-antimonyhydroxyfluoride if the pH is not too low. The partition constants of this compound in benzene-water and carbon tetrachloride-water systems are high. The kinetics of the fluoride exchange reaction ... 

The effects of pH on electrokinetic velocities in micellar electrokinetic chromatography was studied by using sodium dodecyl sulfate solutions [179]. Micellar electrokinetic capillary chromatography with a sodium dodecyl sulfate pseudostationary phase has been used to determine the partition constants for nitrophenols, thiazolylazo dyes, and metal chelate compounds [180]. A similar technique was used to separate hydroquinone and some of its ether derivatives. This analysis is suitable for the determination of hydroquinone in skin-toning creams [181]. The ingredients of antipyretic analgesic preparations have also been determined by this technique [182], The addition of sodium dodecyl sulfate improves the peak shapes and resolution in chiral separations by micellar electrokinetic chromatography [183]. 

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