Solvent-water partition constants for

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. 

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). 

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

What are the major difficulties of any atom/fragment contribution method for estimation of solvent-water partition constants from structure ... 

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.

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. 

LFERs Relating Partition Constants in Different Solvent-Water Systems Model for Description of Organic Solvent-Water Partitioning Illustrative Example 7.1 Evaluating the Factors that Govern the Organic Solvent-Water Partitioning of a Compound... 

The driving force for the mass transfer of the solute in the three-phase system can be determined with the solvent/water partition coefficient, just as the partition coefficient for gas/liquid phases, the Henry s Law constant, is used to determine the driving force for the mass transfer of ozone. A solute tends to diffuse from phase to phase until equilibrium is reached between all three phases. This tendency of a solute to partition between water and solvent can be estimated by the hydrophobicity of the solute. The octanol/water partition coefficient Kow is a commonly measured parameter and can be used if the hydrophobicity of the solvent is comparable to that of octanol. How fast the diffusion or transfer will occur depends not only on the mass transfer coefficient in addition to the driving force but also on the rate of the chemical reaction as well. 

Korenman et al. [45] studied the solvent-water partition coefficients Ksv/ of ethylamine, n-propylamine, and n-butylamine at 20°C. Based on the data for these amines with a short, linear alkyl chain, the contributions to the logarithm of the solvent-water partition coefficients were found to be constant among particular solvent classes. The contribution values are shown in Table 13.4.1. Accordingly, A log K0VI for CH2 is 0.50. 

Yes, and the predictions are quite good. Two different methods emerge from a host of others and are most commonly used to predict octanol-water partition coefficients for the many organic compounds that exist. One approach is to calculate from a knowledge of structural constants, whereas the second approach requires that a chemical s partition coefficient be measured between a solvent other than octanol and water, Kf, . Kf, can then be calculated from linear regression equations that relate log (for a particular solvent) and log Kf, . Two forms of the structural constant approach are most popular (1) the Hansch n hydrophobic character of... 

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