Donor distribution coefficient

MD simulations in expHcit solvents are stiU beyond the scope of the current computational power for screening of a large number of molecules. However, mining powerful quantum chemical parameters to predict log P via this approach remains a challenging task. QikProp [42] is based on a study [3] which used Monte Carlo simulations to calculate 11 parameters, including solute-solvent energies, solute dipole moment, number of solute-solvent interactions at different cutoff values, number of H-bond donors and acceptors (HBDN and HBAQ and some of their variations. These parameters made it possible to estimate a number of free energies of solvation of chemicals in hexadecane, octanol, water as well as octanol-water distribution coefficients. The equation calculated for the octanol-water coefficient is ... 

Eq. 1 AGbind, free energy of binding log D14, logarithm of the distribution coefficient at pH 7.4 pATa, negative logarithm of the dissociation constant HBD, number of hydrogen bond donors. 

Apart from the hydrophobic interactions provided by the alkyl part of the molecule, octanol has also hydrogen-bond acceptor and donor functions like lipid membranes have. This property of n-octanol made the octanol-water distribution coefficient that widely used. However, n-octanol or reversed phase materials cannot mimic the interfacial character of the bilayer structure. The ionic interactions between membrane phospholipids and solute are also not represented in the properties of octanol or reversed phase materials. To overcome this issue, alternative stationary phases... 

The two values differ by the impurity distribution coefficient, dangling bonds are the only deep defects which take up donor or acceptor electrons and holes. No other charged gap states of significant density have been found. 

Therefore, monopolar solute values vary significantly between different organic liquid/water systems in a predicable way. As shown above, the complementary H-donor functionality of chloroform increases the distribution coefficient of acetone (an H-acceptor) by a factor of 10. Therefore, chloroform would be the better solvent for the extraction of acetone from water. 

The above discussion should help the reader narrow down the choices of an extraction solvent to one of four solvent classes apolar, H-donor, H-acceptor, or bipolar. However, as mentioned in the introduction to this chapter, sometimes liquid/liquid extraction is done with a solvent mixture such as crude oil. In these cases a first approximation of the distribution coefficient may be made with the following formula ... 

Figure 5. Distribution coefficients of U (VI) between 5M aqueous phosphoric acid and mixture of HDEHDTP and neutral oxygen donors in solution in do-decane as a function of the reagent concentration ratio (1) 0.5M (HDEHDTP + POX 11), (2) 0.5M (HDEHDTP + TOPO), (3) 0.5M (HDEHDTP + TBP).

Uranium transfers at a slower rate than plutonium because uranium has a lower solubility than plutonium in the donor alloy and uranium has a lower distribution coefficient that plutonium in the donor alloy-salt system. This difference in the rate of transfer is very desirable because it provides a means for enriching the plutonium content of the product to required concentrations for recycle to the reactor core. This enrichment is obtained by terminating the circulation of the transport salt between donor and acceptor alloys before complete uranium transfer has occurred. As uranium transfers, the solid UCU5 compound dissolves into the donor alloy. After plutonium and the desired amount of uranium are separated from FP-4 elements, the remaining uranium may be separated by diverting the transport salt to a second zinc-magnesium acceptor alloy. 

Apart from reflecting directly the reactivity between receptor and pharmacological agent, the MO-related descriptors may be related to the intermolecular donor-acceptor interactions responsible for bioavailability of compounds. Once again, the search for analogous correlations for the properties like solubility or distribution coefficients could be useful for determining the mechanism of biological action. 

The principle of this type of direct trapping is described by Equations 13.3 through 13.5, showing the influence of pH on the extractability of acidic (or basic) compounds. If the donor pH is selected less than or equal to the pK and the acceptor pH > pK + 3.3, the distribution coefficient between acceptor and donor at equilibrium (Equation 13.5) will be about 2000, promising potentially very high concentration enrichment factors. This is termed complete trapping. 

Figure 15.S. Theoretical concentration vs. time profiles in the acceptor eompaitment calculated for different membrane water distribution coefficients (A) logD = 1 (B) logD = 2 (C) logD = 3 (D) logD = 4. Calculations were performed using a numerical integration approach based on diffusion coefficients in water and membrane. A water/membrane volume ratio of 1000 was used. The absolute concentration in the acceptor compartment is the product of the initial concentration (in donor compartment) times the concentration factor (y axis).

On the other hand, Dyrssen and Liem (1960) report (table 7) greater variation in both distribution ratios [for americium and europium extraction by dibutyl phosphoric acid (HDBP)] and in separation factors as a function of diluent. The separation factors and distribution coefficients are correlated (more or less consistently) inversely with the distribution ratio of the extractant between the phases. In this system, the largest separation factors are observed in n-hexane, chloroform, and carbon tetrachloride. Diluents capable of direct coordination (i.e., those possessing potential oxygen-donor atoms) are correlated with reduced distribution ratios and separation factors. The observations of greater separation factors in non-complexing diluents suggest that more effective separation is observed when the inner-coordination sphere of the hydrophobic complex is not disturbed. 

The diffusion of Se was studied at 400 to 490C by removing layers. Two regions were distinguishable in the donor distribution profiles. The first had a low diffusion coefficient and a high surface concentration, near to the limit of solubility of... 

Here, AE is the difference of the redox potentials of donor D and acceptor A, = 96500 C/mol q is the distribution coefficient of the radical ion which remains in the micellar phase, and q is the product of the distribution coefficients of the parent reactants A and D. If recombination can occur in the bulk phase or within the micellar phase but not at the interface, the value of recombination rate constant depends upon the minimal energy needed to bring one of the radical ions from the bulk to the micellar phase, i.e. fe = 10 q or from the micellar to the bulk phase with 10 / " dm /mole s (the highest of these values). So, the stabilization of the radical ions needs q to be small enough (less than 10" ) and q" to be high enough (more than 10 ). 

Usually, a neutral donor ligand (L) extracts the metal ion by a solvation mechanism where, a counter anion is needed for charge neutralization of the cation. The difference in the distribution coefficient of the metal ion (Dm) between the feed and the strip is generally obtained by a concentration gradient of the counter ion, X, which is accompanying the metal ion into the membrane... 

The study of the complexes of the rare earth elements has been marked by several periods of renewed interest. As already mentioned, the first intensive work was carried out to find those ligands which would be effective in conjunction with ion exchange or in solvent extraction procedures for the separation of the elements one from another. In the initial phase these studies focused primarily on the determination of equilibrium constants and distribution coefficients (Moeller et al., 1965). The compounds that were studied as ligands in both cases were almost exclusively limited to molecules that used oxygen as the donor atom, either alone, or as in the case of the aminopolycarboxylic acids, in combination with nitrogen donors within the same molecule. This phase started in the early 1940 s and lasted into the I960 s. Toward the end of this period the equilibrium measurements were supplemented with calorimetric studies of the enthalpy of formation of the complexes in solution in order to define more clearly the nature of the complexation process (Moeller, 1973). However, during much of this time there was very little effort devoted to the study of the solid complexes which could be isolated from solutions. 

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