Air-octanol partition constant

Air-particle partition coefficient and air-octanol partition constant gKl3p = a ogKl30 +b 11... 

Figure 6.7 Plot of the decadic logarithms of the air-olive oil partition coefficients versus the air-octanol partition constants for various sets of structurally related apolar, monopolar, and bipolar compounds. Note that olive oil is a mixture of compounds that may vary in composition. Therefore, we refer to A" a oUve oi] as the air-olive oil partition coefficient (and not constant, see Box 3.2). Adapted from Goss and Schwarzenbach (2001). The a and b values for the LFERs (Eq. 6-12) are alkanes (a - 1.15, b = 0.16), alkyl aromatic compounds (a = 1.08, b = 0.22), ethers (a = 0.97, 6 = 0.01), esters (a = 0.88, b = -0,14), ketones (a = 1.21, b = 1.06), alcohols (a = 0.98, b = 1.07).

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). 

For estimating the air-olive oil partition coefficient, calculate first the air-octanol partition constant from the air-water (Kisw) and octanol-water (Ki0Vi) partition constants given in Appendix C (Eq. 6-11) ... 

Within a given class of apolar or weakly polar compounds (e.g., alkanes, chlorobenzenes, alkylbenzenes, PCBs), the variation in the air-octanol partition constants (Kiao) is much larger than the variation in the air-water partition constants (Kiaw). For example, the Kim values of the chlorinated benzenes vary between 10 3 5 (chlorobenzene) and 10-7 (hexachlorobenzene, see Hamer and Mackay, 1995), whereas their A, aw values are all within the same order of magnitude (Appendix C). Try to explain these findings. 

Table 10.4 LFERs Relating Air-Plant Partition Coefficients, K,ap, and Air-Octanol Partition Constants, Kiao, of PCBs for Various Herbs and Grasses at 25°C.(A, aP values are in kg dry weight-L"1.)a...

Air-Octanol Partition Constants (Kiao), Average Enthalpies of Air-Pasture Transfer (Aap//,), and Average Concentrations in Pasture (Cjp) and Air (Cja) of Three PCB Congeners... 

In the Appendix C you find the air-water partition constant (Kiaw) of naphthalene and its octanol-water partition coefficient (ATI0W) that you use as surrogate for the fat-water partition coefficient, Kifw). Note that these entities are given as ratios of molar concentrations. Use the fat (octanol) as phase 1 and calculate the fat-air (octanol-air) partition constant, Ki ... 

Figure 6.2 Experimentally determined air- dry octanol partition constants versus calculated (Eq. 6-11) air- wet octanol partition constants. Data from Hamer and Mackay (1995), Gruber et al. (1997), Hamer and Bidleman (1998a,b), Abraham et al. (2001).

The major differences between behavior profiles of organic chemicals in the environment are attributable to their physical-chemical properties. The key properties are recognized as solubility in water, vapor pressure, the three partition coefficients between air, water and octanol, dissociation constant in water (when relevant) and susceptibility to degradation or transformation reactions. Other essential molecular descriptors are molar mass and molar volume, with properties such as critical temperature and pressure and molecular area being occasionally useful for specific purposes. A useful source of information and estimation methods on these properties is the handbook by Boethling and Mackay (2000). 

Reported Henry s law constants and octanol-air water partition coefficients of n-heptane at various temperatures and temperature dependence equations... 

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. 

Also of interest is the maximum capacity of each phase for chemicals, i.e., the saturation concentration above which phase separation occurs. For water, this is obviously the solubility in water. For many polar substances, the chemical and water are miscible (e.g., ethanol) and no solubility limit exists. Similarly, a solubility limit in octanol may or may not exist. For air, the solubility corresponds to the saturation vapor pressure Ps. This can be converted to a solubility in units of mol / m3 by dividing by RT, the gas constant — absolute temperature product. Chapter 7 discusses solubility in water. Solubility in octanol is not by itself of comparable interest and is not treated. Vapor pressure and solubility in water are not only of fundamental interest, but their ratio H is essentially the Henry s Law constant or air-water partition coefficient, as Chapter 4 discusses. 

Muller et al. (1994) developed from theoretical concepts a multi-term expression for plant-air partitioning (Kpa), the essence of which is that Kpa should be proportional to Kow/H, where H = Henry s law constant (air-water partition coefficient) this means that Kpa should therefore be proportional to Koa, the octanol-air partition coefficient. They found a reasonable correlation between measured and predicted log Kpa values for example, for 20 chlorohydrocarbons they found R2 = 0.79. Bacci et al. (1990) also used log Kow and log H to correlate Azalea indica leaf-air partitioning of 10 chemicals (mostly pesticides) with R2 = 0.92. 

Octanoi-air partition coefficient calculated from KowI aw> where Kow is the octanol-water partition coefficient and Taw is the air-water partition coefficient or unitless Henry s law constant (Kaw = HLC/RT, where HLC = Henry s law constant, R = gas constant 8.319 Pa m /mol and r = 293 K)... 

Values of Henry s law constant k =plc, where p is the partial pressure of the solute in the gas above the solution and c is the concentration of the solute) is a quantity frequently apphed in the thermodynamic description of dilute aqueous solutions, which is used in environmental chemistry and atmospheric physics as a major criterion for describing air-water partitioning of solutes at near ambient conditions. It plays amajor role in evaluating the transport of pollutants between atmosphere and aquatic systems, rainwater and aerosols. The octanol-water partition coefficient is a dimensionless number defined as the ratio of the compound s concentration in a known volume of octan-l-ol (Cq) to its concentration in a known volume of water (c ) after the octan-l-ol and water have reached equihbrium. It has been found to be related to water solubility, soil/sediment absorption coefficients and bioconcentration factors of pollutants for aquatic life. The adsorption coefficient normalised to the organic carbon content of the soil (sediment) is a useful indicator of the binding capacity of... 

Swann R, Laskowski D, McCAll P, et al. 1983. A rapid method for the estimation of the environmental parameters octanol/water partition coefficient, soil sorption constant, water to air ratio, and water solubility. Residue Rev 85 18-28. 

Sutton. C., Calder, J.A. (1975) Solubility of alkylbenzenes in distilled water and seawater at 25°C. J. Chem Eng. Data 20, 320-322. Swann, R.L., Laskowski, D.A., McCall, P.J., Vender Kuy, K., Dishburger, J.J. (1983) A rapid method for the estimation of the environmental parameters octanol/water partition coefficient, soil sorption constant, water to air ratio, and water solubility. Res. Rev. 85, 17-28. 

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