Molar water solubilities, correlations

This approach to estimating C does not require the water solubility be known or determined expirimentally. Several laboratories have studied the relationship between the water solubility of a compound and its log P (octanol) value. Hansch, et al. (36) showed that for 156 organic liquids, the molar water solubilities (S ) were correlated to P by equation 21a. The correlation coefficient was increased to as high as 0.99 by segregating compounds by chemical... 

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. 

The correlation between aqueous solubility and molar volume discussed by McAuliffe [5] for hydrocarbons, and the importance of the cavity term in the solvatochromic approach, indicates a significant solubility dependence on the molecular size and shape of solutes. Molecular size and shape parameters frequently used in quantitative structure-water solubility relationships (QSWSRs) are molecular volume and molecular connectivity indices. Moriguchi et al. [33] evaluated the following relationship to estimate Cw of apolar compounds and a variety of derivatives with hydrophilic groups ... 

The correlation between the molar aqueous solubility of solvents log(s/mol dm"3), and the 1 -octanol/water distribution ratio, log P... 

All that which has been said about empirical models based on the 1-octanol/water partition coefficients is also true for models based on the water solubilities (a) the measured water solubility data are of low precision (see TABLE 4), (b) there exists a wide variety of quantitative linear models correlating soil sorption coefficients and water solubility (Wijayaratne and Means 1984 Chiou et al. 1983 Banwart et al. 1982 Means et al. 1982 Briggs 1981 Kenaga and Goring 1980 Karickhoff etal. 1979), and (c) the statistical requirements for linear regression models are violated by the large errors in the experimental water solubility data. That some quantitative models (Wijayaratne and Means 1984 Banwart et al. 1982 Means et al. 1982) use non-molar units ( ig mL ) for water solubility data, further impedes their use. 

The binding constants of a number of compounds were measured using dialysis, solubility and sorption techniques. The solubility technique was used for compounds which were not radiolabeled. All data was collected at pH = 8.3. The binding constants were then compared to the octanol/water partition coefficients for the compounds and the molar solubilities of the compounds. The data is presented in Table II. The Kow values were taken from the literature.18 22-2 The solubility values were determined in this research with the exception of DDT and Lindane, which were taken from the literature. A plot of log Kc vs. log Kow is presented in Figure 5. The slope of this line is 0.71, the intercept is 0.75 and the value of the correlation coefficient is 0.9258. The regression is highly significant... 

There exist several indications of correlations between hydrophobicity or solubility in water and molecular volume or surface, parachlor, molar refraction, or molecular connectivity index32 40). 

Equation 15 can be used to correlate the gas solubility in the presence of a salt if the composition dependence of the molar volume of the binary electrolyte—water mixture is known. Such data are available in the literature for numerous aqueous salt solutions. Like that of Sechenov, eq 15 is a one-parameter equation whose parameter B has to be determined from the solubility data. The two equations provide almost the same results (see Figure 1). 

PEG-200 and PEG-400. It was shown that the coupling of our Eq. (4) with the Flory-Huggins equation for the activity coefficient of the water in the binary mixed solvent provides an accurate correlation for the gas solubility with a single adjustable parameter. However, the more simple Eq. (2) has a satisfactory accuracy and is recommended because it requires only the gas solubilities in the individual solvents and the molar volumes of the latter. 

Eq. (28) thus obtained can be used to represent the solubility of poorly soluble drugs in aqueous mixed solvents if information about the properties of the binary solvent (composition, phase equilibria and molar volume), the nonideality parameters and the constant A is available. These parameters can be considered as adjustable, and determined by fitting the experimental solubilities in the binary solvent. We applied such a procedure to the solubilities of caffeine in water/AW-dimethylformamide (Herrador and Gonzalez, 1997) and water/1,4-dioxane (Adjei et al., 1980), of sulfamethizole in water/1,4-dioxane (Reillo et al., 1995) as well as of five solutes in water/ propylene glycol (Rubino and Obeng, 1991). It was shown that Eq. (28) provides accurate correlations of the experimental data. 

The equilibrium water saturation of a polymeric system increases with the number of polar groups present in the polymeric matrix. Circumstances like the accessibility of the polar groups, the relative strength of the water-water versus water-polymer bonds and for semi-crystalline polymers the degree of crystallinity, hamper a straightforward correlation between the number of polar groups and the solubility. Van Krevelen [1] presents the amount of water per structural group at equilibrium (expressed as molar ratio), as what he calls the best possible approach to the sorptive capacity of (amorphous) polymers versus water. 

Physical-Chemical Properties of Chlorinated Dibenzo-p-dioxins Reported and experimental data for aqueous solubility, octanol-water partition coefficient, vapor pressure, and Henry s law constant are compiled and correlated with molar volumes and chlorine number. 

Here is the solubility of a dmg in water on the molar scale (M at 25 °C log s = 1.74 + logxssat for poorly soluble solutes, see above). The first term in Eq. (1.25) assumes that the dmg molecule has a molar cohesive energy similar to that of 1-octanol, so that the logarithm of the activity coefficient of the dmg in water should be equal to P°w, the logarithm of the distribution ratio between 1-octanol and water. This, in turn, can be related to the constitution and stmcture of the dmg (Hansch and Leo 1979). The second term, involving the molar entropy of fusion of the dmg molecule, is taken as zero for liquid solutes (at 25 °C) but non-zero for crystalline drugs. In summary, Eq. (1.25) predicted the aqueous solubility of 167 dmg and related organic solutes with a correlation coefficient of 0.994 and a standard deviation of 0.24 in log s. 

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