Hexadecane-water partition constant

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. 

Figure 3.6 Plot of the natural logarithms of the partition constants at 25°C of a series of apolar, monopolar, and bipolar organic compounds between air and (a) n-hexadecane (n-C H34) and (b) water versus the dispersive vdW- parameter of the compounds defined by Eq. 3-10. Note that from Eq. 3-10 only the compound part is used because the solvent part (1) is the same for all compounds, and that TSA, is in cm2 mol-1.

Table 3.4 Air-Hexadecane, Air-Water, and Hexadecane-Water Equilibrium Partitioning of Hexane, Benzene, Diethylether, and Ethanol Free Energies, Enthalpies, and Entropies of Transfer, as well as Partition Constants Expressed on a Molar Base (i.e., mol U phase 1/mol L/ phase 2)...

It should be noted that when replacing the London dispersive interactions term by other properties such as, for example, the air-hexadecane partition constant, by expressing the surface area in a more sophisticated way, and/or by including additional terms, the predictive capability could still be somewhat improved. From our earlier discussions, we should recall that we do not yet exactly understand all the molecular factors that govern the solvation of organic compounds in water, particularly with respect to the entropic contributions. It is important to realize that for many of the various molecular descriptors that are presently used in the literature to model yiw or related properties (see Section 5.5), it is not known exactly how they contribute to the excess free energy of the compound in aqueous solution. Therefore, when also considering that some of the descriptors used are correlated to each other (a fact that... 

Figure 7.1 Plot of the decadic logarithms of the hexadecane-wa-ter partition constants versus the octanol-water constants for a variety of apolar, monopolar, and bipolar compounds. Data from Abraham (1994b). The a and b values for some LFERs (Eq. 7-7) are apolar and weakly monopolar compounds (a = 1.21, b = 0.43 Eq. 7-8), aliphatic carboxylic acids (a = 1.21, b = -2.88), and aliphatic alcohols (a = 1.12, b = -1.74).

Calculate the -hexadecane-water (In K ) and the -octanol-water (In Kiow) partition constants at 25°C of -octane (Oct), 1 -methylnaphthalene (1-MeNa), and 4-f-butylphenol (4-BuPh) using the polyparameter LFER, Eq. 7-9, with the coefficients given in Table 7.2. Compare and discuss the contributions of the various terms in Eq. 7-9 for the three compounds in the two solvent-water systems. Note that the three compounds have already been used in Illustrative Example 5.2 to evaluate the polyparameter LFER describing the aqueous activity coefficient. 

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