Partition coefficients factors affecting

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. 

Absorption, in general, is treated as a physicochemical transport process based on computations of logP (the octanol/water partition coefficient) and solubility governed by factors such as polar surface area on the molecule. It is conceivable that SNPs in drug transporter genes will affect the pharmacokinetic properties of compounds and, therefore, these may have to be taken into consideration in the design process. 

Despite these reservations, environmental distribution values may be considered valid for the sorption process, to a first approximation. On this basis, it can be concluded that detected environmental partition coefficients show the clear affinity of surfactants to particulate material. The affinity is higher for cationic surfactants than for other surfactants, as shown by the high partition coefficient values (Table 5.4.1). Partition coefficients are also higher for the water column than for sediments (Table 5.4.1), and it is difficult to offer an explanation for this, bearing in mind the many factors affecting the partition coefficient in both natural water and sediment. 

If an ideal solution is formed in the octanol phase, and the solute in the aqueous phase is not affected by the dissolved octanol, then the last two factors in Equation 10 equal zero. Under these assumptions, an ideal octanol/water partition coefficient (K w) can be defined by (19) ... 

Martin has considered the chemical and physical factors affecting partition coefficients.20 Restricting his discussion to ideal solutions, he considered a solute, A, distributed between two phases in equilibrium with each other. The partition coefficient, a, of the solute A is related to the free energy required to transport one mole of A from one phase to... 

It may be deduced from KP = Koc x foc that partition coefficients of hydro-phobic organic compounds in general are dependent upon the chemical of interest (compound-specific properties affect the value of Koc) and the matrix properties of the medium in which it resides. In addition to the fraction of organic carbon present in the sorption phase, additional environmental factors affect partitioning. These factors include temperature, particle size distribution, the surface area of the sorbent, pH, ionic strength, the presence of suspended material or colloidal material, and the presence of surfactants. In addition, clay minerals may act as additional sorption phases for organic compounds. Nevertheless, organic carbon-normalized partition... 

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. 

Competition Between Pollutants. Competition between several organic pollutants may affect the photocatalytic degradation rate of each species, depending on whether the process is limited by the irradiation or by the total organic matter. The factors intervening in the competition are the respective concentrations, the partition coefficients between the fluid phase and the adsorbed phase, and the relative reactivities with respect to the active species. Consequently, interference effects may or may not be observed. 

Kinetics of Immobilized Enzymes. Another major factor in the performance of immobilized enzymes is the effect of the matrix on mass transport of substrates and products. Hindered access to the active site of an immobilized enzyme can affect the kinetic parameters in several ways. The effective concentration of substrates and products is also affected by the chemistry of the matrix especially with regard to the respective partition coefficients between the bulk solution and the matrix. In order to understand the effects of immobilization upon the rate of an enzyme-catalyzed reaction one must first consider the relationship between the velocity of an enzyme-catalyzed reaction and the... 

In addition to the factors listed in Table VIII, the nature of the surfactant-modified stationary phase affects P (partition coefficient for distribution of solute between bulk solvent and modified stationary phases) and thus will influence the retention observed. It should be realized that most of the normal and reversed-phase packing materials will adsorb/absorb surfactant molecules from the mobile phase solution and become coated to different degrees when surfactant mobile phases are passed through them. Numerous adsorption isotherms have been reported for various surfactant - stationary phase combinations illustrating this point (82,85,106,115-128,206). The presence of additives can mediate the amount of surfactant surface coverage obtained (110-129,175,206). It has been postulated that the architecture which adsorbed surfactant molecules can assume on conventional stationary phases can range from micellar, hemi-micellar, or admicellar to mono-,bi-, or multilayered, and/or other liquid crystalline-type structures (93,106,124,128,129,... 

Water solubility is one of the major parameters which affect the fate and distribution in the environment. Hydrophobic compounds with high octanol-water partition coefficients tend to bio accumulate. Opperhuizen and Voors [63] have shown that hydrophobicity of PCDEs determines the bio concentration factor of PCDEs and that bioconcentration kinetics of PCDEs resemble those of PCBs. 

The lipid-aqueous partition coefficient of a drug molecule affects its absorption by passive diffusion. In general, octanol/pH 7.4 buffer partition coefficients in the 1-2 pH range are sufficient for absorption across lipoidal membranes. However, the absence of a strict relationship between the partition coefficient of a molecule and its ability to be absorbed is due to the complex nature of the absorption process. Absorption across membranes can be affected by several diverse factors that may include the ionic and/or polar characteristics of the drug and/or membrane as well as the site and capacity of carrier-mediated absorption or efflux systems. 

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