Nonspecific solvents

We discuss in this section a relatively simple solvent effect that depends only on the size or volume of the particles involved. It will be seen below that since this type of effect depends only on the sizes of the particles and not on any specific interactions between the solutes and the solvent molecules, it may be referred to as the nonspecific solvent effect. ... 

An inert solvent, such as argon or methane, would behave as a nonspecific solvent. In this section we consider only the volume of the solvent molecules. 

Nonspecific solvents extract a small amount (10%) of coal at temperatures up to about 100°C (212°F). The extract is thought to arise from the resins and waxes that do not form a major part of the coal substance. Ethanol is an example of the nonspecific solvents. 

Solvent Effects on Nuclear Magnetic Resonance Spectra 6.5.1 Nonspecific Solvent Effects on NMR Chemical Shifts... 

Katritzky, A.R., Mu, L. and Karelson, M. (1997b). QSPR Treatment of the Unified Nonspecific Solvent Polarity Scale. J.Chem.Inf.Comput.ScL, 37,756-761. 

Koppel-Palm solvent parameters Parameters to measure separately the ability of a solvent to enter into nonspecific solvent-solute interactions (permittivity, , and refractive index, nD) and specific solvent-solute interaction (solvent basicity or NUCLEOPHILICITY B and solvent acidity or ELECTROPHILICITY E) as contributing to overall solvent POLARITY. 

Mancini PME, Fortunato G, Adam C, Terenzani A, Vottero LR (2002) Specific tmd nonspecific solvent effects on aromatic nucleophilic substitutions. Kinetics of the reaction of 1-fluoro-2,6-dinitrobenzene and homopiperidine in binary solvent mixtures. J Phys Oig Chem 15 258-269... 

Semenov, S. G., and N. V. Khodureva. 1992. Quantum-chemical estimate of nonspecific solvent effect on the electronic structure and spectra of molecules modeling nucleophilic fragments of lignin. Opt. Spektrosk. 73(2) 280-290. 

Several other treatments of solvent effects on solvolysis rates have been developed. The equations typically include several terms related to (a) macroscopic nonspecific solvent properties, such as the dipole moment and dielectric constant (b) empirical polarity criteria, such as Ej.(30) (c) solvent electrophilicity and nucle-ophilicity parameters and (d) terms related to solvent cohesivily. The last term accounts for the difference in work required to disrupt structure within the solvent, when, for example, there is expansion in volume between reactants and the TS. 

At temperatures below those normally required for the thermal decomposition of coal, the yields of extract vary directly with extraction temperature. This effect is usually most pronounced with the nonspecific solvents but it has been noted that a solvent such as ethylenediamine will produce from bituminous coal almost three times as much extract at its boiling point (115°C [240°F]) as at room tanperature and also enhances the effect of extraction with other solvents such as A-methyl-2-pynoUdone (Pande and Sharma, 2002). In fact, the yields of extracts obtained with a series of primary aliphatic amines have been found to vary with the extraction temperature rather than with any other solvent property. 

The use of solvents at temperatures above their critical temperature can lead to enhanced yields of extracts due to changes in the solvent properties. Even nonspecific solvents, snch as hydrocarbons, can give yields of extracts that are ca. 20% w/w of the dry, ash-free coal (Bartle et al 1975 Whitehead and Williams, 1975 Williams, 1975). 

Extract compositions depend on the particular coal/solvent system and on extraction conditions (Raj, 1979). For example, nonspecific solvents generally extract coal selectively and dissolve primarily waxy and resinons snbstances that, althongh derived from the original plant debris, may not be integral parts of the coal snbstance. 

The amonnt of extractable waxes and/or resins decreases markedly with increase in coal rank, and since nonspecific solvents can also take small qnantities of undifferentiated coal-like material into solution, bituminons coal extracts are generally less specialized. However, even these extracts are the result of solvent selectivity and have higher hydrogen/carbon atomic ratios than the parent coal. 

Further theoretieal and computational studies of betaine(30) of the EA30) scale are reviewed by Mente and Maroncelli. Despite several differences in opinion obvious in these papers, an adequate treatment of at least the nonspeeific components of solvatoehromism would seem to be just around the comer . Finally, a suggestion should be mentioned on using the ealeulated Jt values taken from ref 55 as a descriptor of nonspecific solvent ef-feets. Flowever, this is not meaningful since these values are just a partieular blend of inductive, dispersive, and dipole-dipole forees. 

A careful analysis of molecular structures led Dunn et al. to associate the two principal components with the two molecular parameters described in Figure 7, namely, the isotropic surface area (ISA), related to the solute surface accessible to nonspecific solvent interactions, and the solvent-accessible, hydrated surface area (HSA) associated with hydration of polar functional groups. 

For nonspecific solvent-solute interactions the solvaton model provides a satisfactory account of nuclear shielding changes (36). As in the reaction field model, the strength of the solvent-solute interaction depends upon the dielectric constant, e, of the medium. 

Eor another example at the liquid/liquid interface. Steel and Walker used two different solvatochromic probe molecules, para-nitrophenol (PNP) and 2,6-dimethyl-para-nitrophenol (dmPNP), to study the polarity of the water-cyclohexane interface. These probes give spectral shifts as a function of bulk solvent polarity that are very similar because both solutes are mainly sensitive to the nonspecific solvent dipolar interactions. However, when these two dye molecules are adsorbed at the water/cyclohexane interface, they experience quite different polarities. The more polar solute (PNP) has a maximum SHG peak that is close to that of bulk water, and thus it reports a high-polarity environment. In contrast, the less polar solute (dmPNP) reports a much lower interface polarity, having a maximum SHG peak close to that of bulk cyclohexane. Clearly, the more polar solute is adsorbed on the water side of the interface, keeping most of its hydration shell, and thus reports a higher polarity than does the nonpolar solute. Other examples of the surface polarity dependence on probe molecules are discussed in Ref. 363. 

---