Solvent and Solution Properties

In Chapters 1 and 2 we covered molecular polarizabilities, dipoles, and conformations. We are now ready to explore how these properties dictate the properties of solvents, the interactions of solutes with the solvent, and the interactions between solutes. Since the vast majority of reactions performed by organic chemists occurs in solution, the choice of solvent can play an extremely important role in controlling the reactions. We need to choose solvents that not only solubilize the reactants, but also accelerate the desired reaction and /or impede undesirable reactions. Moreover, we can change the solvent to probe reaction mechanisms and look for the existence of various intermediates (see Grunwald-Winstein scales in Chapter 8). Finally, the interactions between the molecules of a solvent, and the interactions between solvent and solute, are some of the same interactions that occur between enzyme and substrate, antibody and antigen, and synthetic receptors and various target molecules— all topics of the next chapter. 

Before exploring the forces that cause solvents to stick together, it is instructive to give a general picture of the structure of liquids. Liquids are best described by a state of rapidly changing molecular order, which retains a high degree of cohesive interactions between the molecules. 

Schematic representation of the radial distribution function g r) for a typical solid. 

Schematic representation of g(.r) for a typical liquid. After a few solvent spheres, there is no longer any spatial correlation to another solvent molecule. The origin on the y axis represents a 50% chance of finding another solvent molecule. 

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Besides these special physical properties, hydrogen-bonded liquid water also has unique solvent and solution properties. One feature is high proton (H ) mobility due to the ability of individual hydrogen nuclei to jump from one water molecule to the next. Recalling that at temperatures of about 300 K, the molar concentration in pure water of H3O ions is ca. 10 M, the "extra" proton can come from either of two water molecules. This freedom of to transfer from one to an adjacent "parent" molecule allows relatively high electrical conductivity. A proton added at one point in an aqueous solution causes a domino effect, because the initiating proton has only a short distance to travel to cause one to pop out somewhere else. 

The goal of theory and computer simulation is to predict S i) and relate it to solvent and solute properties. In order to accomplish this, it is necessary to determine how the presence of the solvent affects the So —> Si electronic transition energy. The usual assmnption is that the chromophore undergoes a Franck-Condon transition, i.e., that the transition occurs essentially instantaneously on the time scale of nuclear motions. The time-evolution of the fluorescence Stokes shift is then due the solvent effects on the vertical energy gap between the So and Si solute states. In most models for SD, the time-evolution of the solute electronic stracture in response to the changes in solvent environment is not taken into accoimt and one focuses on the portion AE of the energy gap due to nuclear coordinates. 

The rates of Intramolecular processes in anisotropic media (such as cholesteric liquid-crystalline phases) are a function of the same solvent and solute properties mentioned above and, additionally, the exigencies imposed by solvent order on the frequency and orientations of head-to-tall coll is ions(36). The importance of the latter considerations is demonstrated by the... 

The temperature dependence of the partition coefficient has so far been largely neglected since the temperature coefficient is usually small. Most partition coef-Hcients were reported without specific reference to temperature, or simply with an indication that the measurement was made at room temperature. It seems appropriate on theoretical grounds to consider the temperature dependence of the partition coefficient and its relation to solvent and solute properties. 

The starting point for such analytical efforts is linear response theory. Different approaches include the dynamical mean spherical approximation (MSA), " generalized transport equations, and ad hoc models for the frequency and wavevector dependence of the dielectric response function e(k,w). These linear response theories are very valuable in providing fundamental understanding. However, they carmot explore the limits of validity of the imderlying hnear response models. Numerical simulations can probe nonlinear effects. They are very useful in the direct visualization and examination of the interplay between solvent and solute properties and the different relaxation times associated... 

Solubility and Solution Properties. Poly(vinyhdene chloride), like many high melting polymers, does not dissolve in most common solvents at ambient temperatures. Copolymers, particularly those of low crystallinity, are much more soluble. However, one of the outstanding characteristics of vinyUdene chloride polymers is resistance to a wide range of solvents and chemical reagents. The insolubiUty of PVDC results less from its... 

Another advantage of HdC is its generosity in terms of mobile-phase selection. The polymer size and solution properties of a polymer can be studied using HdC, especially OTHdC, in almost any solvent. In SEC, by comparison, the packing material and mobile phase have to be selected to prevent the nonsize exclusion effect. Because the instrumentation of HdC is similar to SEC, and the packing material and columns have become available commercially, this technique will gain in popularity. 

In the case of nonionic but polar compounds such as sugars, the excellent solvent properties of water stem from its ability to readily form hydrogen bonds with the polar functional groups on these compounds, such as hydroxyls, amines, and carbonyls. These polar interactions between solvent and solute are stronger than the intermolecular attractions between solute molecules caused by van der Waals forces and weaker hydrogen bonding. Thus, the solute molecules readily dissolve in water. 

The properties of a solution differ considerably from those of the pure solvent Those solution properties that depend primarily on the concentration of solute particles rather than their nature are called colligative properties. Such properties include vapor pressure lowering, osmotic pressure, boiling point elevation, and freezing point depression. This section considers the relations between colligative properties and solute concentration, with nonelectrolytes that exist in solution as molecules. 

The GIPF technique has been used to establish quantitative representations of more than 20 liquid, solid and solution properties,31 34 including boiling points and critical constants, heats of phase transitions, surface tensions, enzyme inhibition, liquid and solid densities, etc. Our focus here shall be only upon those that involve solute-solvent interactions. 

Association and mobilities are related in a complex way to the bulk properties of the solvent and solute. These properties include the charge density and distribution on the ions and the Lewis base properties, the strength and nature of the solvent molecule dipole, the hydrogen-bonding capability, and the intermolecular structure of the solvent. Some correlations can be made on the basis of mobility and association trends in series such as the halides and alkali metals within a single solvent others can be drawn between solvents for a given ion. It appears that conductance measurements provide a clear measure of the sum of ion-solvent interactions, but that other techniques must be used in conjunction with conductance if assessments of individual contributions from specific factors are to be made. 

Moreover, the unique adsorption properties of GEC allowed the very sensitive electrochemical detection of DNA based on its intrinsic oxidation signal that was shown to be strongly dependent of the multi-site attachment of DNA and the proximity of G residues to GEC [100]. The thick layer of DNA adsorbed on GEC was more accessible for hybridization than those in nylon membranes obtained with genosensors based on nylon/GEC with a changeable membrane [99,101,102]. Allhough GEC has a rough surface, it is impermeable, while nylon is more porous and permeable. DNA assays made on an impermeable support are less complex from a theoretical standpoint [7] the kinetics of the interactions are not compUcated by the diffusion of solvent and solutes into and out of pores or by multiple interactions that can occur once the DNA has entered a pore. This explained the lower hybridization time, the low nonspecific adsorplion and the low quantity of DNA adsorbed onto GEC compared to nylon membranes. 

Physical properties of polymers, including solubility, are related to the strength of covalent bonds, stiffness of the segments in the polymer backbone, amount of crystallinity or amorphousness, and intermolecular forces between the polymer chains. The strength of the intermolecular forces is directly related to the CED, which is the molar energy of vaporization per unit volume. Since intermolecular attractions of solvent and solute must be overcome when a solute (here the polymer) dissolves, CED values may be used to predict solubility. 

The property of adsorption from solutions of a particular solute is in general, apart from the fact that both solvent and solute are adsorbed (see p. 181), complicated by the fact that the adsorbing surface presented to the liquid is not uniform but broken up into a series of fissures or capillaries as is the case with solids such as charcoal and pumice or gels such as those of silica and alumina with the result that true equilibrium between solution and adsorbent may not result until after long periods of time, necessary for the intradiffusion of the solution into the absorbent during which period secondary chemical action may take place. For comparative purposes adsorption as distinguished from absorption or sorption (J. W. McBain, Phil. Mag. xvili. 6,1909) is considered to take place rapidly in solutions as well as in gases (see p. 123). 

The characterization of a solvent by means of its polarity is an unsolved problem since the polarity itself has, until now, not been precisely defined. Polarity can be understood to mean (a) the permanent dipole moment of a compound, (b) its dielectric constant, or (c) the sum of all those molecular properties responsible for all the interaction forces between solvent and solute molecules (e.g., Coulombic, directional, inductive, dispersion, hydrogen bonding, and EPD/EPA interaction forces) (Kovats, 1968). The important thing concerning the so-called polarity of a solvent is its overall solvation ability. This in turn depends on the sum of all-specific as well as nonspecific interactions between solvent and solute. 

Many efforts have been made to correlate solute-solvent and solute-solute interactions in solutions with such polarity scales as relative pennittivity and dipole moment but they have often been unsuccessful. The chemical properties of solvents, as described below, often play more important roles in such interactions. 

An approach to quantifying the interaction between solute and solvent and hence to solvent effects on redox potentials is that developed by Gutmann.41 Interactions between solvent and solute are treated as donor-acceptor interactions, with each solvent being characterized by two independent parameters which attempt to quantify the electron pair donor properties (donor number)... 

The electrical transport properties of alkali metals dissolved in ammonia and primary amines in many ways resemble the properties of simple electrolytes except that the anionic species is apparently the solvated electron. The electrical conductance, the transference number, the temperature coefficient of conductance, and the thermoelectric effect all reflect the presence of the solvated electron species. Whenever possible the detailed nature of the interactions of the solvated electrons with solvent and solute species is interpreted by mass action expressions. 

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