Hydrophobic substance partition coefficient

In the multimedia models used in this series of volumes, an air-water partition coefficient KAW or Henry s law constant (H) is required and is calculated from the ratio of the pure substance vapor pressure and aqueous solubility. This method is widely used for hydrophobic chemicals but is inappropriate for water-miscible chemicals for which no solubility can be measured. Examples are the lower alcohols, acids, amines and ketones. There are reported calculated or pseudo-solubilities that have been derived from QSPR correlations with molecular descriptors for alcohols, aldehydes and amines (by Leahy 1986 Kamlet et al. 1987, 1988 and Nirmalakhandan and Speece 1988a,b). The obvious option is to input the H or KAW directly. If the chemical s activity coefficient y in water is known, then H can be estimated as vwyP[>where vw is the molar volume of water and Pf is the liquid vapor pressure. Since H can be regarded as P[IC[, where Cjs is the solubility, it is apparent that (l/vwy) is a pseudo-solubility. Correlations and measurements of y are available in the physical-chemical literature. For example, if y is 5.0, the pseudo-solubility is 11100 mol/m3 since the molar volume of water vw is 18 x 10-6 m3/mol or 18 cm3/mol. Chemicals with y less than about 20 are usually miscible in water. If the liquid vapor pressure in this case is 1000 Pa, H will be 1000/11100 or 0.090 Pa m3/mol and KAW will be H/RT or 3.6 x 10 5 at 25°C. Alternatively, if H or KAW is known, C[ can be calculated. It is possible to apply existing models to hydrophilic chemicals if this pseudo-solubility is calculated from the activity coefficient or from a known H (i.e., Cjs, P[/H or P[ or KAW RT). This approach is used here. In the fugacity model illustrations all pseudo-solubilities are so designated and should not be regarded as real, experimentally accessible quantities. 

Wasik, S. P., Miller, M. M., Tewari, Y. B., May, W. E., Sonnefeld, W. J., DeVoe, H., Zoller, W. H. (1983) Determination of the vapor pressure, aqueous solubihty, and octanol/water partition coefficient of hydrophobic substances by coupled generator column/ liquid chromatographic methods. Res. Rev. 85, 29 42. 

DeVoe, H., Miller, M.M., Wasik, S.P. (1981) Generator columns and high pressure liquid chromatography for determining aqueous solubilities and octanol-water partition coefficients of hydrophobic substances. J. Res. Natl. Bur. Std. 86, 361. 

LogPoctanoi/water partition Coefficient (logPo/w), provides information on the relative solubility of the substance in water and the hydrophobic solvent octanol (used as a surrogate for lipid) and is a measure of lipophilicity... 

In order for a substance to be absorbed, it must cross biological membranes. Most substances cross by passive diffusion. This process requires a substance to be soluble both in lipid and water. The most useful parameters providing information on the potential for a substance to diffuse across biological membranes are the octanol/water partition coefficient (Log P) value and the water solubility. The Log value provides information on the relative solubility of the substance in water and the hydrophobic solvent octanol (used as a surrogate for lipid) and is a measure of lipophilicity. Log P values above zero indicate that the substance is more soluble in octanol than water, i.e., is lipophilic, and values below zero (negative values) indicate that the substance is more soluble in water than octanol, i.e., is hydrophilic. In general, moderate Log P values (between 0 and 4) are favorable for absorption. However, a substance with a Log P value around 0 and low water solubility (around 1 mg/1) will also be poorly soluble in lipids and hence not readily absorbed. It is therefore important to consider both the water solubility of a substance and its Log P value when assessing the potential of that substance to be absorbed. 

The partition coefficient relates to the hydrophobic character of a substance. Its logarithm is proportional to the free energy of the transfer of a substance from the aqueous phase to the micellar phase. Correlation of these partition coefficients with the partition coefficients of other systems, such as octanol/water, results in a straight line on a logarithmic scale. Some correlations are given in Chapters 5 and 6. 

The partition coefficient Kq of an organic compound in the 1-octanol/water system is used to assess the bioaccumulation potential and the distribution pattern of drugs and pollutants. The partition coefficient of imidazole and ILs strongly depends on the hydrogen bond formed by these molecules and is less than one due to the high solubility in water. The low value of the 1-octanol/water partition coefficient is required for new substances, solvents, insecticides to avoid bioaccumulation. Kqw is an extremely important quantity because it is the basis of correlations to calculate bioaccumulation, toxicity, and sorption to soils and sediments. Computing the activity of a chemical in human, fish, or animal lipid, which is where pollutants that are hydrophobic will appear, is a difficult task. Thus, it is simpler to measure the 1-octanol/water partition coefficient. This parameter is used as the primary parameter characterizing hydrophobisity. 

For example, when we are interested in the accumulation of SOCs in earthworms, we may adopt a similar approach as we used for sediment-dwelling organisms. Since earthworms have a significant lipid content (ca. 5%, Table 10.1) and we are interested in relatively hydrophobic substances, we use lipid- and organic carbon-normalized biota-soil accumulation partition coefficients CK, jpoC) and bioaccumulation factors (BSAFnipoc). These correspond exactly to the biota-sediment accumulation partition coefficients and factors defined by Eqs. 10-21 and 10-24 (for an application see Problem P 10.2),... 

Because 1-octanol is a good surrogate phase for lipids in biological organisms, the octanol-water partition coefficient, a ratio of concentrations in 1-octanol and water, represents how a chemical would thermodynamically distribute between the lipids of biological organisms and water. It further represents the lipophilicity and the hydrophobicity of the chemical substance. It usually is referred to as Kow or P, or in its 10-based logarithmic form as log I, w or log P, and is unitless. For more detail see Chapter 5. 

Partition coefficient represents the equilibrium ratio of the molar concentrations of a chemical substance (the solute) in a system containing two immiscible liquids. The octanol / water partition coefficient is expressed as either Kow or P and is a descriptor of a substance s relative affinity for lipids and water. For purposes of simplification, Kow is usually reported as its common logarithm (log Kow or log P). A large log Kow value for a chemical (relative to other substances) indicates that the chemical has a greater affinity for the n-octanol phase and, hence, is more hydrophobic (lipophilic). A negative log Kow value indicates that a chemical has a greater affinity for the water phase and, hence, is more hydrophilic. 

A commonly used partition coefficient is the 1-octanol-water partition coefficient, K(k which is the ratio of a chemical s concentration in 1-octanol to its concentration in water at equilibrium in a closed system composed of octanol and water (Bacci 1994). The 1-octanol is chosen to mimic biological lipids. For organic chemicals, log Kow ranges from -3 to 7. When log Kow exceeds 3, substances are considered hydrophobic (Elzerman and Coates 1987). The Kow partition coefficient has been extensively used as an estimate of the BCF. Under the assumptions of Landrum et al. (1996), together with an estimated lipid content of about 5% in biota and an assumed equal affinity of the compound for both body fat and octanol, the BCF can be calculated by the use of BCF = 0.048 Kow (Paasivirta 1991). This equation can vary depending on the species used. The relationship between log K()W and BCF can be viewed by scatterplot analysis (Figure 2.4). These plots show a clear relationship for... 

The partitioning of a substance between two immiscible solvents is an important property of a molecule. If the two solvents are polar and non-polar, the ratio of the concentrations (when measured at equilibrium and below saturation in either solvent) is considered to describe the hydrophobicity of a compound. The partition coefficient (Kow or P) may therefore be defined as ... 

Hydrophobicity or lipophilicity is understood as a measure of the relative tendency of an analyte to prefer a non-aqueous to an aqueous environment. The partition coefficients of the substances may differ if determined in different organic-water eluent systems but their logarithms are often linearly related [117]. Octanol-water partitioning is a common reference system which provides the most commonly recognized hydrophobicity measurement, the logarithm of the partition coefficient, log P [8,45,46]. 

The overall hydrophobicity of a molecule can be obtained as a superposition of fragment contributions. This hydrophobicity may be measured, for example, by the logarithm of the partition coefficient in an octanol/water system, where log(P)= -I TAGtransfer the transfer free energy AGt ansfer refers to one mole of substance going from one solvent to the other. 

The range of membrane permeabilities is very high. Typically, hydrophobic molecules have high partition coefficients, while hydrophilic molecules have low partition coefficients. The direction and rate of transport are determined by the concentration gradient of a substance across the membrane. 

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