Properties of the solute and solvent

This takes us back to the different types of bonding in the two components (Units 2.2 and 2.3). A compound containing ionic bonds will usually dissolve in a solvent which is also polar (see Section 2.3.4) and not in a solvent with purely covalent bonding. Similarly, a compound which is covalent will usually dissolve in a covalently-bonded organic solvent. Thus sodium chloride, an ionic compound, will dissolve in water, which is polar, but not in petrol, which contains no polar bonda Sodium chloride wiU have slight solubility in some organic solvents that have a small amount of ionic character, such as ethanol, due to solvation. We can summarise solubihty in the phrase  

For the great majority of solutions of solids in hquids and liquids in liquids, increasing the temperature usually increases the solubility. Indeed, lead chloride is insoluble in cold water, but soluble in hot water. 

Formula 1 race winners shake the bottle before loosening the cork. . .  

---

Solubility Properties. Fats and oils are characterized by virtually complete lack of miscibility with water. However, they are miscible in all proportions with many nonpolar organic solvents. Tme solubiHty depends on the thermal properties of the solute and solvent and the relative attractive forces between like and unlike molecules. Ideal solubiHties can be calculated from thermal properties. Most real solutions of fats and oils in organic solvents show positive deviation from ideaHty, particularly at higher concentrations. Determination of solubiHties of components of fat and oil mixtures is critical when designing separations of mixtures by fractional crystallization. 

Otherwise, prediction of partition coefficient from the known physical properties of the solute and solvents have been developed by Taft, Abraham, Kamlet, and coworkers [207-210]. They have introduced solvatochromic parameters to assess the intermolecu-lar forces governing the partition mechanisms of neutral organic solutes nonspecific... 

The cohesive energy density of the binary mixture, representediyfihnot be easily predicted from the physicochemical properties of the solute and solvent. Instead, the cohesive energy density of the mixture is estimated using the geometric mean of the cohesive energy densities of the pure components ... 

In this chapter we describe the methods used to calculate solubility isotherms as well as the entire phase diagram for binary and ternary solute-SCF mixtures. The objective of the first part of the chapter is to discuss the relevant physical properties of the solute and solvent pair that are needed to describe the intermolecular forces in operation between molecules in a mixture that ultimately fix solubility levels. A brief description is provided on the application of solubility parameters to supercritical fluids. 

Liquid solutions are clear and transparent with no visible particles of solute. They may be colored or colorless, depending on the properties of the solute and solvent. Note that the terms clear and colorless do not mean the same thing a clear solution has only one state of matter that can be detected colorless simply means the absence of color. 

Liquid solutions are clear and transparent with no visible particles of solute. They may be colored or colorless, depending on the properties of the solute and solvent. 

The diffusion coefficient depends upon the ease with which the solute molecules can move. In biology we are mainly concerned with aqueous systems, and the diffusion coefficient of a solute in aqueous solution is a measure of how readily a solute molecule can push aside its neighboring water molecules and move into another position. An important aspect of the theory of diffusion is how the magnitudes of the frictional coefficients f and hence of the diffusion coefficients D (see (11.63)), depend upon the properties of the solute and solvent molecules. 

The solubility of non-electrolytes has been treated in several books, including those of Hildebrand and Scott (1950), Gerrard (1976), Shinoda (1978), and especially for water as a solvent by Yalkowsky and Banerjee (1992). Several approaches have been employed for relating the solubility to properties of the solutes and solvents. 

These diagrams can take various forms depending on the properties of the solute and solvent molecules. A very thorough study of the possibilities has been made by van Konynenburg and Scott [15] who identified six principal types of fluid/fluid equilibrium behaviour in binary systems. For a complete discussion, reference should be made to the above study and also to the works of Rowlinson [16] and other authors [17-19]. In the discussion below a simplified classification is used to describe typical forms of fluid/fluid phase behaviour in the near-critical region. (Some readers may find it helpful to read... 

There are many equations that relate diffusivity to various physical and molecular properties of both solute and solvent [2-5], but the one that appeared to fit the data best was that of Arnold [2] that gave an expression for (Dm) of the following form. 

In the last two decades experimental evidence has been gathered showing that the intrinsic properties of the electrolytes determine both bulk properties of the solution and the reactivity of the solutes at the electrodes. Examples covering various aspects of this field are given in Ref. [16]. Intrinsic properties may be described with the help of local structures caused by ion-ion, ion-solvent, and solvent-solvent interactions. An efficient description of the properties of electrolyte solutions up to salt concentrations significantly larger than 1 mol kg 1 is based on the chemical model of electrolytes. 

In common with similar approaches that relate solvent accessible surface to cavity free energy90-93, the simple SMI model required careful parameterization, and assumed that atoms interacted with solvent in a manner independent of their immediate molecular environment and their hybridization76. In more recent implementations of the SMx approach, ak parameters are selected for particular atoms based on properties determined from the SCF wavefunction that is evaluated during calculation of the solute and solvent polarization energies27. On the other hand, the inclusion of more parameters in the solvation model requires access to substantial amounts of experimental data for the solvation free energies of molecules in the training set94 95. 

In conclusion, therefore, it should be perceived that this groundwork of solution chemistry ultimately leads to the ability to predict at least semi-quanti-tatively the solubility and two-phase distribution in terms of some simple properties of the solute and the solvents involved. For this purpose, Eqs. (2.12), (2.53), (2.61), and (2.63) and the entries in Tables 2.3 and 2.5 should be particularly useful. In the case of inorganic ions, the entries in Table 2.4 are a rough guide to the relative extractabihties of the ions from aqueous solutions. The more negative the values of A, ydG° of the ions, the more difficult their removal from water becomes, unless complexation (see Chapter 3) compensates for the ion hydration. 

In the case of polar SD, the main change in solute properties is in its charge distribution. If the eharge distributions of the solute and solvent molecules are represented as sets of partial charges, as is usually done in computer simulation studies of SD, A Wp, is a smn of Coulombic interactions... 

Since the dilute solution theory is considered as the basis for the indicated treatment, it will receive considerable attention. Influences of several parameters as molecular weight, molecular weight distribution, thermodynamic and kinetic chain stiffness, intramolecular hydrodynamic inter-action, optical properties of the chain and solvent power will be considered. 

Since m s a property of the solution and therefore not a convenient input quantity for a continuum solvation theory, further work is needed to develop an expression that includes instead the pure-solvent dielectric susceptibility. Song and co-workers [43,44,47] have... 

The Physicochemical Properties of Solvents and Their Relevance to Electrochemistry. The solvent properties of electrochemical importance include the following protic character (acid-base properties), anodic and cathodic voltage limits (related to redox properties and protic character), mutual solubility of the solute and solvent, and physicochemical properties of the solvent (dielectric constant and polarity, donor or solvating properties, liquid range, viscosity, and spectroscopic properties). Practical factors also enter into the choice and include the availability and cost of the solvent, ease of purification, toxicity, and general ease of handling. 

The cavitation contribution is determined following Pierotti s scaled particle theory, where AGcav is expressed as an expansion series in powers of the radius of the sphere which excludes the centers of the solute molecules from the solute, Rms (i.e., the sum of the solute and solvent radii Eq. 4-6). The expansion coefficients Kt (i = 0-3) are expressed in terms of properties of the solvent (the radius of the solvent molecule and the numeral density) and of the solution (temperature and pressure) ... 

Further development of theory of reorganization energy consists in taking to consideration the properties of medium and manner in which it interfaces with the solute (Newton, 1999). These properties must include both size and shape of the solute and solvent molecules, distribution of electron density in reagents and products and the frequency domain appropriate to medium reorganization. 

Although the composition of an ideal solution can be predicted theoretically, few solutions are ideal, and fugacities and activity coefficients are seldom available for real systems. Hence, in general, too little is known of the direct relationships between solubilities and the specific properties of the solute and the solvent to permit prediction of solubility curves. The characteristics of each system must be determined experimentally. In many cases, it is not even possible to predict the effect of temperature on the solubility values of a given solute-solvent system. 

Liquid crystals possess physical properties which lie somewhere between those of solids and liquids cf. Section 3.1 and [725]. The rigidity which is present in a solid matrix is absent in liquid crystals, thus permitting molecular motion as well as conformational flexibility of the dissolved solute molecules. At the same time, due to the order in the liquid-crystalline phase, the randomness in motion and conformational flexibility of the dissolved solute molecules is to some extent restricted. If the structures of the solute and solvent molecules are compatible, then solute molecules can be incorporated into the liquid-crystalline phase without disrupting its order. Thus, the reactivity of substrate molecules incorporated into liquid crystals without destroying their order should be different from that in isotropic solvents. Apart from the first report on the influence of liquid crystals on chemical reactions by Svedberg in 1916 [726], the use of liquid crystals as... 

It was van t Hoff, winner of the very first Nobel prize in chemistry, who perceived an analogy between the properties of dilute solutions and the gas laws. We will see that many physical properties of dilute solutions, such as the amount of light scattered or the viscosity, can be written as a virial equation in the number of molecules (moles), N, or concentration of solute, c. We have written a general form of a virial equation in Equation 12-4, using the quantity P to represent some measured property of the solution and P0to represent the property of the pure solvent. 

Asphaltene content bears directly on the physical properties of the liquid product. Viscosity is of particular interest because of the importance of this parameter to operation of liquefaction plants and as a measure of the extent of liquefaction. The correlation between asphaltene content and the viscosity of the liquid has been a subject of a number of investigations (23-27). The logarithm of the viscosity ratio, In 7j/rj0 (where i and y0 are the viscosities of the solution and solvent, respectively) was found to be a linear function of concentration when asphaltene was redissolved in the pentane-soluble oil isolated from a coal-derived liquid (24). The slopes of these lines, termed the logarithmic viscosity numbers, are a measure of the contribution to the viscosity of a solution attributable to asphaltene. By comparison of logarithmic viscosity numbers of asphaltenes and their acidic and basic subfractions, it was determined that intermolecular association, which is especially strong between the acid and base subfractions, is responsible for a significant portion of the viscosity of these solutions. 

Barone et al studied the optical properties of solvated /7-benzoquinone, i.e., a benzene molecule in which two opposite C atoms have been replaced by oxygen cf. Fig. 15). They used time-dependent density-functional calculations in the determination of the optical properties of the solute and a polarizable-continuum approach for the solvent. By using a recent development that allows for optimizing the structure of the excited molecule in solution, they could determine both the ground-state structure and that of the excited state. Then, they could calculate both vertical... 

For these reasons we cannot use (7(R) as a rigid support for dynamical studies of trajectories of representative points. G(R) has to be modified, at every point of each trajectory, and these modifications depend on the nature of the system, on the microscopic properties of the solution, and on the dynamical parameters of the trajectories themselves. This rather formidable task may be simplified in severai ways we consider it convenient to treat this problem in a separate Section. It is sufficient to add here that one possible way is the introduction into G (R) of some extra coordinates, which reflect the effects of these retarding forces. These coordinates, collectively called solvent coordinates (nothing to do with the coordinates of the extra solvent molecules added to the solute ) are here indicated by S, and define a hypersurface of greater dimensionality, G(R S). To show how this approach of expanding the coordinate space may be successfully exploited, we refer here to the proposals made by Truhlar et al. (1993). Their formulation, that just lets these solvent coordinates partecipate in the reaction path, allows to extend the algorithms and concepts of the above mentioned variational transition state theory to molecules in solution. 

It follows from Eq. (2) or from its simplified form of Eq. (3) that the energy of solvation (hydration) Es should be dependent, on the one hand, upon the properties of the solute and, on the other hand, upon the properties of the solvent (water). This obvious fact is for no apparent reason often ignored in the literature on the hydro-phobic-hydrophilic properties of chemical compounds. In some cases it leads to an inadequate interpretation of the results obtained. This question will be dealt with below. 

---