Aqueous solubility and activity coefficient

Effect of Temperature and Solution Composition on Aqueous Solubility and Activity Coefficient... 

So far, we have focused on how differences in molecular structure affect the solubilities and activity coefficients of organic compounds in pure water at 25°C. The next step is to evaluate the influence of some important environmental factors on these properties. In the following we consider three such factors temperature, ionic strength (i.e., dissolved salts), and organic cosolutes. The influence of pH of the aqueous solution, which is most important for acids and bases, will be discussed in Chapter 8. 

At the Prague Institute of Chemical Technology, F. Jirsa studied anodic oxidation of gold [15] later he published an important paper about silver electrode for a silver-iron battery [16], Jaroslav Chloupek (1899-1975), partly with V. Danes (1907-1980) and B. Danesova, studied the electrode potential in solutions of mixed manganese salts [17], the solubility and activity coefficient of Ag2S04 in some solutions [18], the ions and deviations from the approximation of Debye-Hiickel theory [19], the liquid potentials [20], and the anomalous valency effect of strong electrolytes in aqueous solution [21]. 

Important to any measurement of citrus juice volatile flavor components is the presence of (i-limonene, since this compound is naturally present as the most concentrated component in all of the natural citrus oils. Also, the solubility of d-limonene in aqueous media must be considered, since after liquid phase saturation, the headspace concentration remains constant. It has long been established for d-limonene and similar nonpolar flavor compounds over water that meaningful headspace measurement techniques [e.g., solid-phase microextraction (SPME)] require equilibrium of the vapor and liquid phase concentrations. Equilibrium may take a number of hours for static (unstirred) experiments and less than 1 hr for stirred systems. These conditions have been discussed elsewhere, and solubility and activity coefficients of d-limonene in water and sucrose solutions have been determined [1,2]. More recently, the chemical and physical properties as well as citrus industry applications of d-limonene and other citrus essential oils have been compiled [3]. Although not specific to d-limonene, important relationships affecting behavior of flavor release and partitioning between the headspace and the liquid phase of a number of food systems have also been discussed [4]. 

Despite the work of Overton and Meyer, it was to be many years before structure-activity relationships were explored further. In 1939 Ferguson [10] postulated that the toxic dose of a chemical is a constant fraction of its aqueous solubility hence toxicity should increase as aqueous solubility decreases. Because aqueous solubility and oil-water partition coefficient are inversely related, it follows that toxicity should increase with partition coefficient. Although this has been found to be true up to a point, it does not continue ad infinitum. Toxicity (and indeed, any biological response) generally increases initially with partition coefficient, but then tends to fall again. This can be explained simply as a reluctance of very hydrophobic chemicals to leave a lipid phase and enter the next aqueous biophase [11]. An example of this is shown by a QSAR that models toxicity of barbiturates to the mouse [12] ... 

Haines, R.L.S., Sandler, S.L. (1995) Aqueous solubilities and infinite dilution activity coefficients of several polycyclic aromatic hydrocarbons. J. Chem. Eng. Data 40, 835-836. 

Emollients are often added to cream formulations to modify either the characteristics of the pharmaceutical vehicle or the condition of the skin itself to promote penetration of the active ingredient to act either locally or systemically. The stratum corneum, being keratinized tissue, behaves as a semipermeable artificial membrane, and drug molecules can penetrate by passive diffusion. The rate of drug movement depends on the drug concentration in the vehicle, its aqueous solubility, and the oil/ water partition coefficient between the stratum corneum and the product s vehicle. Commonly used emollients include glycerin, mineral oil, petrolatum, isopropyl pal-mitate, and isopropyl myristate. 

Illustrative Example 5.3 Evaluating the Effect of Temperature on Aqueous Solubilities and Aqueous Activity Coefficients Dissolved Inorganic Salts... 

Illustrative Example 5.4 Quantifying the Effect of Inorganic Salts on Aqueous Solubility and Aqueous Activity Coefficients... 

Hence, this approach is very similar to the one used for describing the effect of salt on aqueous solubility and aqueous activity coefficient (Eqs. 5-27 and 5-28). Some example calculations using Eq. 5-30 or 5-31, respectively, are given in Illustrative Example 5.5. Finally, we should note that the mole fractions of two solvents in a binary mixture are related to the volume fractions by ... 

Any general approach (i.e, any approach that is not restricted to a confined set of structurally related compounds) for prediction of aqueous solubilities and/or aqueous activity coefficients has to cope with the intrinsic difficulty of describing precisely... 

P 5.5 Evaluating the Effect of Temperature on the Aqueous Solubility and on the Aqueous Activity Coefficient of a Solid Compound... 

Many additional consistency tests can be derived from phase equilibrium 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 solubility, and solubility of water in chemicals are related to solution activity coefficients and other properties through fundamental equilibrium relationships (10,23,24). Accurate, consistent data should be expected to satisfy these and other thermodynamic requirements. Furthermore, equilibrium models may permit a missing property value to be calculated from those values that are known (2). 

Myrdal, P., A.M. Manka, and S.H. Yalkowsky. 1995. Aquafac 3 Aqueous Functional Group Activity Coefficients Application to the Estimation of Aqueous Solubility. Chemosphere. 30, No. 9 1619-1637. 

Myrdal, P.B., Manka, A.M., and Yalkowsky, S.H., Aquafac 3 Aqueous functional group activity coefficients, application to estimation of aqueous solubility, Chemosphere, 30, 1619-1637, 1995. 

Yalkowsky and Banerjee have pubhshed an extensive review of methods for estimating aqueous water solubility of organic compounds. Many methods have been developed based on measured properties such as partition coefficients, chromatographic parameters, and activity coefficients. " Purely in silico methods are based on LFER, and a variety of geometric, electronic, and topological molecular descriptors. ... 

Rard, J. A., and Archer, D. G., 1995, Isopiestic investigation of the osmotic and activity coefficients of aqueous NaBr and the solubility ofNaBr.2H20(cr) at 298.15 K. Thermodynamic properties of the NaBr+H20 system over wide ranges of temperature and pressure Journal of Chemical and Engineering Data, v. 40, p. 170-185. 

Numerous relationships exist among the structural characteristics, physicochemical properties, and/or biological qualities of classes of related compounds. Simple examples include bivariate correlations between physicochemical properties such as aqueous solubility and octanol-water partition coefficients (Jtow) and correlations between equilibrium constants of related sets of compounds. Perhaps the best-known attribute relationships to chemists are the correlations between reaction rate constants and equilibrium constants for related reactions commonly known as linear free-energy relationships or LFERs. The LFER concept also leads to the broader concepts of property-activity and structure-activity relationships (PARs and SARs), which seek to predict the environmental fate of related compounds or their bioactivity (bioaccumulation, biodegradation, toxicity) based on correlations with physicochemical properties or structural features of the compounds. Table 1 summarizes the types of attribute relationships that have been used in chemical fate studies and defines some important terms used in these relationships. 

The application of speciation-solubility in geochemistry goes back to Garrels and Thompson (1962), who calculated the aqueous speciation in seawater and saturation states with respect to mineral solubilities. Since then, this subject has been treated extensively, and hundreds of codes are available for this kind of calculation. In principle, the concentration and activity coefficients of all aqueous species can be calculated if there are the same number of equations as unknowns. Given an equilibrium constant for each complex, plus an analysis giving the total quantity of each basis species, this is usually possible. 

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