Partition coefficient air/water

The equilibrium air-water partition coefficient (AWPC), can be defined in different forms. Frequently used is the concentration to concentration ratio, Kiw  

The experimental techniques available to determine AWPCs and their limitations have been discussed by Staudinger and Roberts [2]. These authors also evaluated the effects of pH, compound hydration, compound concentration, cosolvent, cosolute, and salt effects, suspended solids, dissolved organic matter, and surfactants. The experimental data have been compiled by a number of different authors [2-11]. 

For liquids with low water miscibility, AWPCs can be calculated as the ratio of the solute vapor pressure to the water solubility  

Replacing Cw by the infinite dilution activity coefficient in water, 7, the following relation is obtained  

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Oxidizing Chemical Air-Water Partition Coefficient (Atm 20°C) Relative Volatility (Normalized)... 

The simplest way to predict the lipid/ water partition coefficient, Kiw, of a drug is based on measurements of the surface pressure, ttd, of the drug as a function of its concentration in the aqueous subphase (Gibbs adsorption isotherm). The Gibbs adsorption isotherm provides the air/water partition coefficient, Kaw, and the cross-sectional area, Ad of the drug and allows calculation of the lipid/water partition coefficient, K]w, according to Eq. (6) [59] ... 

As seen in Fig. 20.1, a linear correlation was observed between the logarithm of the air/water partition coefficient, log Kaw, and the log(l/Km) [57, 58], This strong correlation implies that the membrane concentration of substrates is indeed essential for activation of P-gp and leads to the following empirical correlation ... 

Fig. 20.1. Correlation between the air/water partition coefficient, Kaw, determined from measurements of the surface pressure as a function of drug concentration (Gibbs adsorption isotherm) in buffer solution (50 mM Tris/HCI, containing 114 mM NaCI) at pH 8.0 and the inverse of the Michaelis Menten constant, Km obtained from phosphate release...

Fig. 20.11. Substrate quality obtained by comparing basolateral-to-apical with apical-to-basolateral transport of substrates in polarized cell monolayers of MDR1-transfected cell lines [86] plotted versus (A) the log of the air/water partition coefficient, or (B) H-bond energy (arbitrary units, EUh cf. text). Units of the air/ water partition coefficient were [M ]. Compound (concentrations in Ref. [86] in brackets) were clozapine (50 nM) (1) cyclosporin A (2 tM) (2) daunorubicin (3) dexamethasone (2 tM) (4) digoxin (2 pM) (5) domperidone (2 pM) (6) etoposide (7) flunitrazepam (500 nM) (8) haloperidol (50 nM) (9) ivermectin (50 nM) (10) loperamide (2 pM) (11) morphine (2 pM) (12) ondansetron...

Dissociation of the neutral acid in water necessitates modifications for air-sea exchange in the model, which is based on Henry s law. Other possible pathways, e.g. sea spray, are neglected. Henry s law is restricted to concentrations of physically solved, non dissociated substances. Since only the non-dissociated acid is volatile, it is important to correct the air-water partition coefficient as to reflect the relative proportions of volatile and non-volatile components. The corrected parameter is the effective Henry s law coefficient, which is related to the Henry s law coefficient as a function of pH (modified Henderson-Hasselbalch equation) ... 

Solubility in water and vapor pressure are both saturation properties, i.e., they are measurements of the maximum capacity that a solvent phase has for dissolved chemical. Vapor pressure P (Pa) can be viewed as a solubility in air, the corresponding concentration C (mol/m3) being P/RT where R is the ideal gas constant (8.314 J/mol.K) and T is absolute temperature (K). Although most chemicals are present in the environment at concentrations well below saturation, these concentrations are useful for estimating air-water partition coefficients as ratios of saturation values. It is usually assumed... 

When solubility and vapor pressure are both low in magnitude and thus difficult to measure, it is preferable to measure the air-water partition coefficient or Henry s law constant directly. It is noteworthy that atmospheric chemists frequently use Kwa, the ratio of water-to-air concentrations. This may also be referred to as the Henry s law constant. 

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. 

Most conventional organic contaminants are fairly hydrophobic and thus exhibit a low but measurable solubility in water. Solubility is often used to estimate the air-water partition coefficient or Henry s law constant, but this is not possible for miscible chemicals indeed the method is suspect for chemicals of appreciable solubility in water, i.e., exceeding 1 g/100 g. Direct measurement of the Henry s law constant is thus required. 

The Henry s law constant is essentially an air-water partition coefficient which can be determined by measurement of solute concentrations in both phases. This raises the difficulty of accurate analytical determination in two very different media which usually requires different techniques. Accordingly, effort has been devoted to devising techniques in which concentrations are measured in only one phase and the other concentration is deduced from a mass balance. These methods are generally more accurate. The principal difficulty arises with hydrophobic, low-volatility chemicals which can establish only very small concentrations in both phases. 

The calculation is illustrated in Table 1.5.5 for pentachlorophenol. The experimental aqueous solubility is 14.0 g/m3 at a pH of 5.1. The environmental pH is 7. Higher environmental pH increases the extent of dissociation, thus increasing the Z value in water, increasing the apparent solubility, decreasing the apparent KqW and Henry s law constant and the air-water partition coefficient, and decreasing the soil-water partition coefficient. 

Calculated Zw values and some partition coefficients at different environmental pHs for pentachlorophenol (PCP), 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP) and p-cresol at 25°C. Kaw is the air-water partition coefficient and Ksw is the soil-water partition coefficient... 

The Flenry s law constant data calculated as the ratio of vapor pressure to solubility in Figure 1.7.13 are quite scattered. There is little systematic variation with molar volume. Most values of log H lie between -0.1 to -0, i.e., H lies between 0.8 and 0.08, and the resulting air-water partition coefficient KAW or H/RT thus lies between 3 x 10-4 and 3 x 10-5. 

The Level I calculations for environmental pHs of 5.1 and 7 suggest that if 100,000 kg (100 tonnes) of pentachlorophenol (PCP) are introduced into the 100,000 km2 environment, most PCP will tend to be associated with soil. This is especially the case at low pH when the protonated form dominates. Very little partitions into air and only about 1% partitions into water. Soil contains most of the PCP. Sediments contain about 2%. There is evidence of bioconcentration with a rather high fish concentration. Note that only four media (air, water, soil and bottom sediment) are depicted in the pie chart therefore, the sum of the percent distribution figures is slightly less than 100%. The air-water partition coefficient is very low. As pH increases, dissociation increases and there is a tendency for partitioning to water to become more important. Essentially, the capacity of water for the chemical increases. Partitioning to air is always negligible. 

Such simulations suggest that because of their relatively high water solubility which in combination with low vapor pressure causes low air-water partition coefficients, the phenols tend to remain in water or in soil and show little tendency to evaporate. Their environmental fate tends to be dominated by reaction in soil and water, and for the more sorptive species, in sediments. Their half-lives are relatively short, because of their susceptibility to degradation. 

Ashworth, R. A., Howe, G. B., Muhins, M. E., Roger, T. N. (1988) Air-water partitioning coefficients of organics in dilute aqueous solutions. J. Hazard. Materials 18, 25-36. 

Jonsson, J. A., Vejrosta, J., Novak, J. (1982) Air/water partition coefficients for normal alkanes (n-pentane to n-nonane). Fluid Phase Equil. 9, 279-286. 

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

Ryu, S.-A., Park, S.-J. (1999) A rapid determination method of the air/water partition coefficient and its application. Fluid Phase Equil. 161, 295-304. 

Sarraute, S., Delepine, H., Costa Gomes, M.F., Majer, V.(2004) Aqueous solubility, Henry s law constants and air/water partition coefficients of ra-octane and two halogenated octanes. Chemosphere 57, 1543-1551. 

Surface Water t,/2 = 16 h (calculated for river water 1 m deep, water velocity 0.5 m/s, wind velocity 1 m/s from air-water partition coefficients (Southworth 1979 Hallett Brecher 1984)... 

Henry s law constant Sometimes referred to as the air-water partition coefficient, the Hemy s law... 

Vapor pressure The vapor pressure of a substance is defined as the pressure exerted by the vapor (gas) of a substance when it is under equihbrium conditions. It provides a semi-quantitative rate at which it will volatilize from soil and/or water. The vapor pressure of a substance is a required input parameter for calculating the air-water partition coefficient (see Henry s law constant), which in turn is used to estimate the volatilization rate of compounds from groundwater to the unsaturated zone and from surface waterbodies to the atmosphere. 

Cottrell, T. and Mazza, G. Air/water partition coefficient, solnbility and vaponr pressure of selected food volatiles, in Annual Conference of the Canadian Institute Food, Science and Technology, September 20-23, 1997, Paper 15. 

Kawamoto, K. and Urano, K. Parameters for predicting fate of organochlorine pesticides in the environment (I) octanol-water and air-water partition coefficients, Chemosphere, 18(9/10) 1987-1996, 1989. 

Kochetkov, A., Smith, J.S., Ravikrishna, R., Valsaraj, K.T., and Thibodeaux, LJ. Air-water partition coefficients for volatile methyl siloxanes. Environ. Toxicol Chem., 20(10) 2184-2188, 2001. 

Park. Y.-S. and Park, S.-J. Determination and prediction of octanol/water partition coefficients and air-water partition coefficients for environmental toxic chemicals. Kongop Hwahak, ll(7) 773-779, 2000. 

Tow the octanol-water partition coefficient the octanol-air partition coefficient the air-water partition coefficient... 

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