Aqueous solution data partition coefficients

Tomlinson, E., Hafkenscheid, T. L. (1986) Aqueous solution and partition coefficient estimation from HPLC data. In Partition Coefficient, Determination and Estimation. Dunn, III, W. J., Block, J. H., Pearlman, R. S., Eds., pp. 101-141, Pergamon Press, New York. 

The chemical mixtures composed of 32 probe compounds were prepared in aqueous solutions. The partition coefficients of chemicals were determined by PDMS membrane-coated fibers. The partition coefficient can be obtained by equilibrium or regression methods. In the equilibrium method, the absorption amount n° is determined at a predetermined equilibrium time. The partition coefficient of a chemical can be calculated from the equilibrium absorption amount n° and the initial concentration Q (Xia et al., 2003). The equilibrium absorption amount n° can also be obtained by regression of the absorption data sampled before the equilibrium with a kinetic model. The absorption experiment time can be reduced considerably with the regression method (Xia et al., 2004). 

The kinetic data are essentially always treated using the pseudophase model, regarding the micellar solution as consisting of two separate phases. The simplest case of micellar catalysis applies to unimolecTilar reactions where the catalytic effect depends on the efficiency of bindirg of the reactant to the micelle (quantified by the partition coefficient, P) and the rate constant of the reaction in the micellar pseudophase (k ) and in the aqueous phase (k ). Menger and Portnoy have developed a model, treating micelles as enzyme-like particles, that allows the evaluation of all three parameters from the dependence of the observed rate constant on the concentration of surfactant". ... 

Many additional consistency tests can be derived from phase equiUbrium constraints. From thermodynamics, the activity coefficient is known to be the fundamental basis of many properties and parameters of engineering interest. Therefore, data for such quantities as Henry s constant, octanol—water partition coefficient, aqueous solubiUty, and solubiUty of water in chemicals are related to solution activity coefficients and other properties through fundamental equiUbrium relationships (10,23,24). Accurate, consistent data should be expected to satisfy these and other thermodynamic requirements. Furthermore, equiUbrium models may permit a missing property value to be calculated from those values that are known (2). 

The ionization potentials of some of the bipyridines have been investigated. Solubility data for 2,2 -bipyridine in aqueous solution, in aqueous solvent mixtures, and in various aqueous salt solutions have been obtained, whereas the heat of solution, heat capacities, and related data for 2,2 - and 4,4 -bipyridines in water have been measured. The enthalpies of solution of 2,2 -bipyridine in water and aqueous solvent mixtures have also been obtained. Dielectric relaxation studies of 2,2 -bipyri-dine in carbon tetrachloride have been reported in connection with hindered internal rotation. Partition coefficients for 2,2 -bipyridine between water and various organic solvents have been measured. ... 

The concentration of the weak acid or of the weak base can be determined by distribution between water and another solvent, such as benzene or chloroform the partition coefficient of the acid or base between the water and the other solvent must, of course, be known. The degree of hydrolysis may then be calculated from the concentration of the salt and the determined concentration of the weak acid or base. An example of such a salt is aniline hydrochloride. This is partially hydrolysed into aniline and hydrogen chloride. On shaking the aqueous solution with benzene the aniline will distribute itself between the water and benzene in the ratio of the distribution coefficient. The initial concentration of aniline hydrochloride is known, the concentration of the free aniline in the aqueous solution can be computed from that found in the benzene solution, and from this the total concentration of aniline, produced by hydrolysis, is deduced. Sufficient data are then available for the calculation of the degree of hydrolysis. 

The micelle/water partition coefficient for many solutes have been shown to correlate to the octanol/ water partition coefficientJ ° Data in Table 8, from Azaz and Donbrow, show that the micellar partition coefficients of the methylphenols increase with the number of methyl groups. Collett and Tobin showed that the micellar partition coefficients of several benzoic acid derivatives are proportional to their octanol-water partition coefficient for poloxamers (Table 9). Tomida et al. also illustrated that most of the 34 monosubstituted benzoic acids with Brij 35 have micellar partition coefficients that are inversely proportional to their aqueous solubilities and proportional to their octanol-water partition coefficients. The data of Tomida, Yotsuyanagi, and Ikeda[ °l for some steroid hormones (Table 10), further illustrate the parallelism between octanol-water and micelle-water partition coefficients. 

It should be expected that calculated values of 6jjp correlate better with equilibrium properties of the membranes in aqueous solution than with transport properties. Che of the few such equilibrium measurements that have been published is by Anderson et al (.2J). Their measured partition coefficients (K), diffusion coefficients (D), and reverse osmosis rejection (R) of the organic solutes are shown in Table III for cellulose acetate membranes. Their data for cellulose acetate butyrate was similar and is not shown here. Aqueous solutions of the organic solutes, usually at concentrations of about 10 g 1 , were used in the measurement of partition coefficients by UV absorption. In Table III,... 

Computerized method provides convenient and accurate determination of the ionization constant in aqueous solution and of the apparent ionization constant in the presence of octanol. From these parameters, partition coefficients and apparent partition coefficients are easily calculated and agree with data reported using the shaker technique or HPLC. The curve-fitting method has been applied to the differential titration technique in which the solvent curve is subtracted from the solution curve before calculations are begun. This method has been applied to the potentiometric titration of aqueous solutions of the salts of bases with very low solubility in water. 

In this chapter the main aim is to present partition coefficients for solutes in aqueous surfactant systems, having carried out a critical evaluation of the data. We concentrate on polar solutes. Energetics of solubilization and solubilization sites in the micelles are also discussed. 

Initially, the vapor pressure measurements appear to be the most direct, but even here some assumptions are needed. The amount of alcohol in the micellar phase needs to be determined. To do this the difference in vapor pressure between a pure aqueous and a micellar solution is measured. If the ions of the surfactant salts out alcohol, the vapor pressure of pure water is not the correct comparison, and this could lead to lower partition coefficients. Thermodynamic data are well suited for model calculations, and both the models of DeLisi et al. ° ° and Hetu et al. fit the data well. Although in reasonable internal agreement, the partition coefficients calculated from partial molar volumes differ from those calculated from enthalpies the first is 927 or 944, the latter... 

Determination of solubility by headspace analysis offers several advantages over spectrophotometric techniques. First, because of the selectivity of chromatographic analysis, compound purity is not a critical factor second, absolute calibration of the gas chromatographic detector is not necessary if the response is linearly related with concentration over the range necessary for the measurements and finally, this method does not require the preparation of saturated solutions, since a partition coefficient, not a solubility, is actually measured. However, headspace methodology would probably not be applicable for determining PAH solubilities for three reasons. First, there is little data in the literature on the vapor pressures of PAHs. Second, the aqueous solubilities of most PAHs are too low to be measured by this procedure. Finally, adsorptive losses of PAHs to glass surfaces from the vapor phase would cause errors. 

Corresponding treatment of the kinetics for hydrophobic compounds residing in the continuous hydrocarbon domain predicts that both /Teat and are independent of the volume fraction of water in the microemulsion [55]. The K t value in microemulsion is predicted to be larger than the value in aqueous solution by a factor approximately equal to the oil-water partition coefficient of the substrate. The predicted kinetics seems to be supported by previously published experimental data. 

A more complex kind of partition is that of the distribution of small organic molecules between proteins and an aqueous phase. A number of such studies 41) have been correlated by an equation similar to Eq. (16) with the octanol-water partition coefficient and K2 the binding constant of the solute with protein or the ratio of the percent bound solute to the percent free solute. In other studies (47), the binding constant is expressed as 1/C, where C is the molar concentration of the solute necessary to produce a 1 1 (or higher) complex of protein and the solute molecule. The way that K2 is defined greatly affects the intercept and makes it difficult to compare with the data of ordinary solvent-... 

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