Solubility, thermodynamic aspects

This section describes further developments to May, 2007. A new topic relevant to thermodynamic aspects (Section 8.03.4.1) is solubility of pyrazine and its derivatives in supercritical carbon dioxide <2006CED2056>. A notable example of nucleophilic displacement of substituents (Section 8.03.5.4.2) is represented by highly selective monosubstitution of chloro substituent in 2,3-dichloropyrazine, which is converted by treatment with a-lithio ketones into a-(3-chloropyrazin-2-yl) ketones <2006T9919>. Similarly a-(chloropyrazinyl) acetic ester or acetonitrile derivatives are synthesized by using a-lithio acetic esters or acetonitriles, respectively. 

The next main section deals with thermodynamic aspects. It starts by consideration of the intramolecular forces between heterocyclic molecules which influence melting and boiling points, solubility and chromatographic characteristics. This is followed by a section on stability and stabilization, including thermochemistry and conformation of the saturated ring systems, and then a discussion of aromaticity. 

For a vary thorough discusson of enthalpy, entropy, and the solubility of ionic compounds, see Johnson. D. A. Some Thermodynamic Aspects of Inorganic Chemistry Cambridge University London. 1968. Chafrter 5. 

Thermodynamic Aspects (Melting and Boiling Points, Solubility, Aromaticity and... 

Comprehensive coverage of thermodynamic aspects for the 1,4-diazocines was presented in CHEC-II(1996) and included discussion on physical properties, such as melting points, solubility, and chromatographic behavior,... 

The point of this discussion has been to consider some of the thermodynamic aspects of the process that occurs when a solute is dispersed in a solvent to form a solution. Our observations tell us that like dissolves like. However, the dissolution process is so complex that predicting whether a particular solute will dissolve in a given solvent is risky. Solubility is difficult to explain and even more difficult to predict, especially when water is the solvent. The only way to be certain about the compatibility of a given solute and solvent is to do the experiment. 

D. A. Johnson (1982) Some Thermodynamic Aspects of Inorganic Chemistry, 2nd edn, Cambridge University Press, Cambridge - Contains a useful discussion of the solubility of ionic salts in aqueous solution. 

The goal of this review was to present aspects of the preferential adsorption phenomena of solvents and polymers with a focus on their thermodynamic aspects. The idea behind this was to attract the attention of polymeric physico-chemists to this area, which is sufficiently related to a lot of different effects. In fact, preferential adsorption occupies a special place in the solubility of polymers in mixed solvents. 

Since the photoactive layer is deposited from solution, the morphology determining features are of thermodynamic and kinetic nature, the latter mainly playing a role during the film formation process. Thermodynamic aspects are reflected in the chemical structure of the donor and acceptor compounds determining to a large extent their solubility in different solvents and the interaction (miscibility)... 

Thermodynamic Aspects of Solubility At equilibrium in a saturated solution, the chemical potential, or partial molal free energy, of the solute must be the same in the solution as in the solid phase. If we consider two different saturated solutions, there-fore, both in equilibrium with the same solid phase, the chemical potential of the solute must be the same in both. The chemical potential ( ) and activity (c) are related by the equation p — po — RT In o, where Po is the chemical potential of the substance in the standard state. Hence, if the same standard state is chosen for all the solutions considered, the activity of the solute must be the same in all. 

These are but a few examples of the roles that solubility plays in technology, in physical chemistry, and in colloid science. Solubility is one of the truly basic thermodynamic parameters of any system, and so it is of pervasive importance. Some of the thermodynamic aspects of solubility will now be considered. 

One last concept must be considered with respect to the thermodynamic aspects of solubility—the condition of equilibrium in mixtures that contain two or more phases. What specific conditions must be met for a particular mixture to be regarded as being in thermodynamic equilibrium A particularly important requirement is that the chemical potential of each component must be the same in all the phases that are present. Numerous boundary conditions apply to this requirement, which have been discussed elsewhere [34]. By introducing the concepts of field and density thermodynamic variables, Griffiths and Wheeler were able to restate the condition of equilibrium for heterogeneous mixtures in a particularly simple, rigorous, and elegant form [35,36]. 

With these concepts in hand, we may now briefly consider some thermodynamic aspects of solubility. Suppose (as above) one starts with pure solute (say, sodium chloride crystals) and pure solvent (water) and adds the salt crystals to the water at constant temperature. Just at the point when they are brought together, a nonequilibrium state exists, because salt has a finite solubility in water but has not yet dissolved. The concentration profile at this time is the step-function described in Fig. 1. The process of dissolving salt in water has a negative free energy, and thus occurs irreversibly until the liquid is saturated. As the concentration of salt in the liquid phase increases, so does its chemical potential, as seen from Eq. (4), and so the driving force for dissolution (the difference between the chemical potential at any given time and the equilibrium chemical potential) steadily decreases. Finally, a concentration is reached at which the chemical potential of sodium... 

Thermodynamically, intermixing as above arises from achieving a negative free energy from the process in mm, this arises by balancing a contribution from enthalpy (heat content) with another from entropy (involving material stmctural aspects). The solubility parameter reflects the enthalpic term. 

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