Solvents specific/nonspecific interaction

The idea of solvent polarity refers not to bonds, nor to molecules, but to the solvent as an assembly of molecules. Qualitatively, polar solvents promote the separation of solute moieties with unlike charges and they make it possible for solute moieties with like charges to approach each other more closely. Polarity affects the solvent s overall solvation capability (solvation power) for solutes. The polarity depends on the action of all possible, nonspecific and specific, intermolecular interactions between solute ions or molecules and solvent molecules. It covers electrostatic, directional, inductive, dispersion, and charge-transfer forces, as well as hydrogen-bonding forces, but excludes interactions leading to definite chemical alterations of the ions or molecules of the solute. 

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. 

More reactive atoms especially those with lone pair electrons, such as, 4N, l5N,, 70 and 19F, are very likely to have their nuclear shieldings influenced by interactions with solvent molecules. Such interactions rnay be specific, e.g. hydrogen bonding, or nonspecific, e.g. polarisability/polarity, or perhaps a combination of specific and nonspecific solute-solvent interactions. An empirical procedure has been developed for quantitatively unraveling the contributions made to the shielding of solute nuclei by specific and nonspecific interactions. 

Typically, solute-solvent interactions are divided into two broad categories Specific and nonspecific interactions. Specific interactions include phenomena such as hydrogen bonding and ji-ji interactions, which depend on the presence of particular functional groups or steric structures. They are short ranged, and are specific in the sense that they involve individual solvent species within the first solvation shell of the liquid. In contrast, nonspecific interactions represent interactions that are not associated with the presence of individual functional groups. In molecular liquids, these include dispersion and electrostatic interactions, such as dipole-dipole forces. We will discuss the nature of each type of interaction in ionic liquids in the sections that follow. 

A solvated molecule is rather difficult to describe since there can be specific interactions with one or more solvent molecules apart from nonspecific interactions with one or more solvent shells. Although the investigation of specific solvent complexes provides a description of major solvent effects it is far from giving an exact picture of a solvated molecule. This can only be obtained by excessive solvent modelling using Monte Carlo methods which becomes too costly when carried out for a larger number of silylium cations in different solvents. 

Drago s EJC analysis and Gutmann s donor/acceptor approach [53, 67] have been compared [200, 217, 218], Eq. (2-12) has been extended for specific and nonspecific interactions between solutes and polar solvents [219]. Various Lewis acidity and basicity scales for polar solvents have been examined and compared by Fawcett, who concluded that the donor/acceptor scales of Gutmann seem to be the most appropriate [341],... 

The intermolecular solute/solvent interactions may arise from nonspecific interaction forces such as dispersion, dipole-dipole, dipole-induced dipole, etc., as well as from specific interactions found in protic and aromatic solvents. Solvent effects on NMR spectra were first observed by Bothner-By and Glick [226] and independently by Reeves and Schneider [227] in 1957. Since then, the influence of solvent on chemical shifts (and coupling constants) has been extensively studied by scores of workers and has been thoroughly reviewed by several specialists [1-4, 288-237]. 

Romero, S., Reillo, A., Escalera, J.-B., Bustamante, R, 1996. The behavior of paracetamol in mixtures of amphiprotic and amphiprotic-aprotic solvents. Relahonship of solubihty curves to specific and nonspecific interactions. Chem. Pharm. Bull. 44, 1061-1064. 

The Isotropic Surface Area ISA) is the surface of the molecule accessible to nonspecific interactions with the solvent, that is, the surface of the molecule involved in specific hydrogenbonding with water is not considered [Gollantes and Dunn III, 1995 Koehler, Grigoras et al, 1988]. A hydration complex model needs to estimate the isotropic surface area. The Polar Surface Area (PSA) is defined as the part of the surface area of the molecule associated with... 

The interaction that occurs between the solvent and the transition state is sometimes described in terms of specific and nonspecific, depending on the nature of the interaction. Specific interaction refers to hydrogen bonding or charge transfer complexation. Nonspecific interaction is the result of general attraction due to van der Waals forces. Some of the correlations that have been devised are restricted to only nonspecific solvation of the transition state by the solvent. 

In order to predict the likely solubihties (and other properties) of compounds that have not been tested and to compare ILs to other solvents it is necessary to have some measure of how they interact with solute species. Polarity is the sum of all possible (nonspecific and specific) intermolecular interactions between the solute and the solvent, excluding such interactions leading to definite chemical changes (reactions) of the solute. In ILs any ion-ion, dipole-ion, dipole-dipole, and dipole-induced-dipole interactions, dispersion interactions, hydrogen bonding, and/or 7i-interactions may be important. The contribution of each in any given solution is dependent upon both the solvent and the solute, each with its own particular characteristics. So it is not possible to measure the properties of ILs in isolation and some other answer to the problem is required. 

These nonspecific interactions (independent of the chemical nature of the solvents) are much easier to consider during modeling than specific interactions like, e.g., hydrogen bonds. Therefore for example reactions of polar species in supercritical water are a special challenge. A recent discussion of different solvent effects in supercritical fluids is given by Kajimoto [25]. 

FIGURE 10.5 Comparison between experimental (o) and calculated (solid lines) solubilities of paracetamol (S is the mole fraction of paracetamol) in the mixed solvent water/ethanol is the mole fraction of ethanol) at room temperature. The solubility was calculated using Equation 10.29. 1-activity coefficients expressed via the Flory-Huggins equation, 2-activity coefficients expressed via the Wilson equation. (From S. Romero, A. Reillo, B. Escalera, and P. Bustamante, 1996, The Behavior of Paracetamol in Mixtures of Amphiprotic and Amphiprotic-Aprotic Solvents. Relationship of Solubility Curves to Specific and Nonspecific Interactions, Chemical and Pharmaceutical Bulletin, 44, 1061. Reprinted from E. Ruckenstein, and I. L. Shulgin, 2003c, Solubility of Drugs in Aqueous Solutions. Part 2 Binary Nonideal Mixed Solvent, International Journal of Pharmaceutics, 260, 283, With permission from Elsevier.)... 

Four cases are of special Interest, the precipitation of a dissolved polymer melt by addition of a non-solvent, the separation of chemically different polymers by mixtures of several solvents (below their CST), (11) the dissolution of a polymer melt by mixture of two non-solvents, and the swelling of a crosslinked polymer melt by monomeric solvents. The incidence of the first three cases is described in Figures 5 and 6, respectively, for nonspecific Interactions between polymer and solvent(s). Specific interactions are qualitatively just as predictable as in the case of specific interactions between monomeric liquids, say molecular compound formation or acid/base interaction. But quantitative predictions are even more difficult here than in the monomer case. 

Solvent effects can be produced by specific interactions, such as protonation or hydrogen bonding and nonspecific interactions which may arise from solvent polarity effects. Both of these types of interaction are found in studies of solvent effects on nitrogen nuclear shieldings. The extent of substituent effects depends upon the position of substitution and the electronic nature of the substituent. 

---