Factors Influencing Partition Coefficients

If the partition coefficients (the log of the ratio of the concentration in particulates to the concentration in solution) was dependent only on the solubility or related properties of a specific chemical, and if equilibrium was always attained between the particulate and aqueous phases, then the mass of a chemical transported on the particulate phase should be readily predicted (Allan, 1986 Table 4-1)  

Controlled Solely by Partition Coefficient and Concentration of Suspended Solids). From Allan (1986). 

An example has been given by Allan (1986) for the Niagara River, which has a mean annual suspended particulate load of 8.4 mg/L. Assuming this suspended particulate concentration, some 50% or greater of toxic chemicals and elements with partition coefficients larger than 10 should be transported in the particulate phase. The mass transported by the particulate phase increases with increasing particulate concentration and with increasing partition coefficient. 

Several factors affect the partition coefficients of organic chemicals and elements in aquatic systems and examples of such influences are given here  

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Before discussing mineral-melt partition coefficients in detail, it is useful to consider other factors that may influence partition coefficients for the U-series elements. Such factors arise both because the U-series elements typically occur at very low abundances in nature, and because they are radioactive. The first feature introduces the possibility of deviations from Henry s Law, at very low concentrations. The second feature raises... 

Many U-series elements occur in nature at much lower concentrations than the more commonly used trace elements, such as the lanthanides. This fact, coupled with the highly incompatible nature of many U-series elements, means that in some cases factors other than lattice strain may influence the uptake of cations into a mineral, and so affect the partition coefficient. 

Important factors influencing the analyte volatilisation process are related to diffusion, porosity, and surface area (for solids). To obtain reproducible results it is necessary to control storage temperature and time strictly. The temperature of the sample is very important because of the specific boiling points of the various analytes. The partition coefficient, K, at equilibrium, is... 

De Wolf, W, De Bruijn, J H M, Seinen W, Hermens JLM (1992) Influence of biotransformation on the relationship between bioconcentration factors and octanol water partition-coefficients. Environ Sci Technol 26(6) 1197-1201... 

KoC is an important parameter which describes the potential for movement or mobility of pesticides in soil, sediment and groundwater. Because of the structural complexity of these agrochemical molecules, the above simple relationship which considers only the chemical s hydrophobicity may fail for polar and ionic compounds. The effects of pH, soil properties, mineral surfaces and other factors influencing sorption become important. Other quantities, KD (sorption partition coefficient to the whole soil on a dry weight basis) and KqM (organic matter-water partition coefficient) are also commonly used to describe the extent of sorption. K0M is often estimated as 0.56 KoC, implying that organic matter is 56% carbon. 

It is apparent from early observations [93] that there are at least two different effects exerted by temperature on chromatographic separations. One effect is the influence on the viscosity and on the diffusion coefficient of the solute raising the temperature reduces the viscosity of the mobile phase and also increases the diffusion coefficient of the solute in both the mobile and the stationary phase. This is largely a kinetic effect, which improves the mobile phase mass transfer, and thus the chromatographic efficiency (N). The other completely different temperature effect is the influence on the selectivity factor (a), which usually decreases, as the temperature is increased (thermodynamic effect). This occurs because the partition coefficients and therefore, the Gibbs free energy difference (AG°) of the transfer of the analyte between the stationary and the mobile phase vary with temperature. 

Both the mass transfer kinetic parameters (diffusion in the phases, D, D j, surface renewal frequency, s) and chemical reaction rate constants (kg, kj) strongly influence enhancement of the absorption rate. The particle size, dp, the dispersed liquid holdup, e and the partition coefficient, H can also strongly alter the absorption rate [42-44,46,48]. Similarly, the distance of the first particle from the gas-liquid interface, 6q is an essential factor. Because the diffusion conditions are much better in the dispersed phase (larger solubility and, in most cases, larger diffusivity, as well) the absorption rate should increase with the decrease of the (5g value. 

One of the most important factors influencing the transfer of an anesthetic from the lungs to the arterial blood is its solubility characteristics (Table 25-2). The blood gas partition coefficient is a useful index of solubility and defines the relative affinity of an anesthetic for the blood compared with that of inspired gas. The partition coefficients for desflurane and nitrous oxide, which are relatively insoluble in blood, are extremely low. When an anesthetic with low blood solubility... 

Thus the partition coefficient, the stereochemistry, and the functional groups present on a molecule may all influence the particular metabolic transformation, which takes place, and these factors are discussed in more detail later. 

The permeability, P (P = Pc x D), of a nonpolar substance through a cell membrane is dependent on two physicochemical factors (1) solubility in the membrane (Pc), which can be expressed as a partition coefficient of the drug between the aqueous phase and membrane phase, and (2) diffusivity or diffusion coefficient (D), which is a measure of mobility of the drug molecules within the lipid. The latter may vary only slightly among toxicants, but the former is more important. Lipid solubility is therefore one of the most important determinants of the pharmacokinetic characteristics of a chemical, and it is important to determine whether a toxicants is readily ionized or not influenced by pH of the environment. If the toxicant is readily ionized, then one needs to understand its chemicals behavior in various environmental matrices in order to adequately assess its transport mechanism across membranes. 

Morse J.W. and Bender M.L. (1989) Partition coefficients in calcite an examination of factors influencing the validity of experimental results and their application to natural systems. Chem. Geol., (in press). 

The influx (k+) and efflux (ka) coefficients, Michaelis-Menten constants (fCm), and resistance factors (RF) were determined for a series of anthracyclines (4-13) with different pvalues and octanol-water partition coefficients, D, at pH 7.4 [130, 135]. The results are summarized in Table 5.21. Increases in D favorably influenced k+ and unfavorably affected ka and 1/Km k+ became faster, and ka increased, as did 1/fQjj This implies that, in the case of the anthracyclines studied, lipophilicity affects uptake kinetics more than the kinetics of P-gp-mediated efflux. Their RF value therefore decreased with increasing lipophilicity. However, possible differences in the distribution pattern of these molecules as well as the influence of membrane binding on their uptake rates were not considered. 

The site preferences shown by cations in the spinel structure demonstrate that transition metal ions prefer coordination sites that bestow on them greatest electronic stability. In addition, certain cations deform their surrounding in order to attain enhanced stability by the Jahn-Teller effect. These two features suggest that similar factors may operate and cause enrichments of cations in specific sites in silicate structures, leading to cation ordering or intersite (intracrystalline ) partitioning within individual minerals which, in turn, may influence distribution coefficients of cations between coexisting phases. 

To this point, various physicochemical properties of drugs such as lipophilicity, ionization, and partition coefficient have been discussed. While these are certainly major factors, there is an additional factor that can influence drug distribution, namely chirality. Chirality is a relatively unique structural characteristic of certain molecules that can exist in two asymmetric, nonsuperimposable isomers (enantiomers) due to the presence of a chiral center (a carbon atom that is attached to four different functional groups (see Chapters 5 and 13). 

The threshold level of a substance can be decreased by the presence of less sensori-cally active substances. In a mixture of ethanol, ethylacetate, ethyleneglycol mono-ethylether and toluene, the odor threshold level of ethyl acetate was reduced to half and in the case of cookies a factor of 5 decrease was observed. A reason for this finding may be the adsorption process taking place in the solid food. Compared to the solution processes in the complete food, the influence of other components on the ethyl acetate partition coefficient during a simple adsorption on the surface is likely to be larger. The repulsion of ethyl acetate from the surface increases its partial pressure over the food. 

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