Air-Organic Solvent Partitioning

Temperature influences air-bulk liquid partitioning of a compound i chiefly in two ways (1) by its effect on the activity coefficient of the compound in the liquid phase and (2) by its effect on the compound s liquid vapor pressure. In the cases where Henry s law applies (Eq. 6-4), for a narrow temperature range, we may write the familiar relationship (Section 3.4)  

if no experimental value for AuHi is available (i.e., from measurements of Ka( ) at different temperatures), it can be obtained from experimental (or estimated) AvapH, and Hfe values. Finally, we should note that Eq. 6-8 applies in a strict sense only if we express the amount of the compound in the gas and liquid phase as partial pressure and mole fraction, respectively. However, if we assume that the molar volume of the liquid, Vi, is not significantly affected by temperature changes, we may also apply Eq. 6-8 to describe the temperature dependence of K,n ( ) (Eq. 6-5) with a constant term that is given by constant + In Ve Furthermore, if we express the amount of the compound in the gas phase in molar concentrations (Eq. 6-6), then we have to add the term RTm to Aa // where rav (in K) is the average temperature of the temperature range considered (see Section 3.4)  

Since thermodynamic properties are independent of the reaction pathway and only depend on the starting and ending conditions, we know that the partitioning of 

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. 

Let us now evaluate how the Kiat values of different compounds are affected by the chemical nature of the organic solvent. To this end, we consider a set of five model compounds (Fig. 6.3) exhibiting very different structures that enable them to 

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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... 

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 ... 

Table 6.2 Air-Organic Solvent Partitioning Multiparameter LFERs (Eq. 6-13) for Some Organic Solvents at 25°Ca... 

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). 

Typically, for many smaller organic molecules, His rather small (i.e., I H < 10 kJ -mol-1 Table 5.3). As a result, for such small compounds, similar to the situation encountered in air-organic solvent partitioning, Aaw//, will not be very different from the enthalpy of vaporization of the compound (Table 6.3), and therefore, the effect of temperature on air-water partitioning, will, in general, be significant (see Illustrative Example 6.2). 

Another possibility to predict /Qaw is to use our multiparameter LFER approach. As we introduced in Chapter 5, we may consider the intermolecular interactions between solute molecules and a solvent like water to estimate values of yiv (Eq. 5-22). Based on such a predictor of %w, we may expect a similar equation can be found to estimate Ki3LVl values, similar to that we have already applied to air-organic solvent partitioning in Section 6.3 (Table 6.2). Considering a database of over 300 compounds, a best-fit equation for Ki m values which reflects the influence of various intermolecular interactions on air-water partitioning is ... 

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

To evaluate the effects of salts or organic cosolvents on air-water (or more correctly, air-aqueous phase or air-organic solvent/water mixture) partitioning, we may simply apply the approaches discussed in Section 5.4 (Eqs. 5-27 and 5-29). Thus, knowing how salt affects a compound s aqueous solubility, while having no effect on its saturation vapor pressure, we deduce that the impact of salt on Ki3V/ may be expressed by ... 

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