Air-organic solvent partition constant

LFERs Relating Partition Constants in Different Air-Solvent Systems Model for Description of Air-Solvent Partitioning Temperature Dependence of Air-Organic Solvent Partition Constants Applications... 

Temperature Dependence of Air-Organic Solvent Partition Constants... 

Furthermore, air-organic solvent partition constants, in particular the air-octanol partition constant, are widely used to evaluate and/or predict the partitioning of organic compounds between air and natural organic phases. Such organic phases are present, for example, in aerosols or soils (Chapters 9 and 11) or as part of biological systems (Chapter 10). 

In this chapter we will focus on another special case, that is, the case in which we assume that Yu is different from 1 but is constant over the concentration range considered. This situation is primarily met when we are dealing with dilute solutions. As we have seen for the solvent water (Table 5.2), for many organic compounds of interest to us, Ylw does not vary much with concentration, even up to saturated solutions. Hence, for our treatment of air-water partitioning, as well as for our examples of air/organic solvent partitioning at dilute conditions, we will assume that Yu is constant. This allows us to modify Eq. 6-1 to a form known as Henry s Law ... 

Furthermore, for most compounds of interest to us, the octanol molecules present as cosolutes in the aqueous phase will have only a minor effect on the other organic compounds activity coefficients. Also, the activity coefficients of a series of apolar, monopolar, and bipolar compounds in wet versus dry octanol shows that, in most cases, Yu values changes by less than a factor of 2 to 3 when water is present in wet octanol (Dallas and Carr, 1992 Sherman et al., 1996 Komp and McLachlan, 1997a). Hence, as a first approximation, for nonpolar solvents, for w-octanol, and possibly for other solvents exhibiting polar groups, we may use Eq. 6-11 as a first approximation to estimate air- dry organic solvent partition constants for organic compounds as illustrated in Fig. 6.2. Conversely, experimental KM data may be used to estimate K,aw or Kitvi, if one or the other of these two constants is known. 

Air-Organic Solvent and Other Partition Constants Comparison of Different Organic Solvents... 

Note that many of the activity coefficients of organic compounds in dilute aqueous solution, y , that we used in Chapter 5 were derived from experimental air-water partition constants (Ki lv/) using Eq. 6-7. Finally, we should point out that in the literature, similar to air-solid surface partitioning (Section 11.1), partition constants are quite often reported as the reciprocal quantity of the air-solvent partition constants as defined above, that is, as solvent-air partition constants. However, it does not really matter in what form such constants are given, as long as we pay... 

Table 6.1 Measured Air-Solvent Partition Constants (KM) and Calculated Activity Coefficients (y,., Eq. 6-7) at 25°C of Five Model Compounds Exhibiting Different H-Donor, H-Acceptor, and Polarity/Polarizability Properties for Some Organic Solvents...

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. 

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.

Note that if Kf has been determined from solubility measurements, ywsaU is strictly valid only for saturated conditions. For dilute solutions y,wsalt can be determined from measurements of air-water or organic solvent-water partition constants at different salt concentrations. From the few compounds for which ywsa,t has been determined by both solubility and air-water or solvent-water partitioning experiments, because of the large scatter in the data, it is not clear whether Kf varies with organic solute concentration. It can, however, be concluded that, if there is an effect, it is not very large. 

Figure 5.6 Illustration of the effect of a completely water-miscible solvent (CMOS, i.e., methanol) on the activity coefficient of organic compounds in water-organic solvent mixtures decadic logarithm of the activity coefficient as a function of the volume fraction of methanol. Note that the data for naphthalene (Dickhut et al., 1989 Fan and Jafvert, 1997) and for the two PCBs (Li and Andren, 1994) have been derived from solubility measurements whereas for the anilins (Jayasinghe etal., 1992), air-water partition constants determined under dilute conditions have been used to calculate y,f.

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 ... 

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