Role of solute-solvent interactions

Schroeder J 1996 The role of solute-solvent interactions in the dynamics of unimolecular reactions in compressed solvents J. Phys. Condens. Matters 9379... 

Solid-Fluid Equilibria The solubility of the solid is very sensitive to pressure and temperature in compressible regions, where the solvent s density and solubility parameter are highly variable. In contrast, plots of the log of the solubility versus density at constant temperature often exhibit fairly simple linear behavior (Fig. 20-19). To understand the role of solute-solvent interactions on sofubilities and selectivities, it is instructive to define an enhancement factor E as the actual solubihty divided by the solubility in an ideal gas, so that E = ysP/Pf, where P is the vapor pressure. The solubilities in CO2 are governed primarily by vapor pressures, a property of the solid... 

To understand the role of solute-solvent interactions on solubilities and selectivities, it is instructive to define an enhancement factor, E, as the actual solubility, y2, divided by the solubility in an ideal gas, so that E = where P is the vapor pressure. This factor is a normalized... 

The effects of solute-solvent interactions play a greater role in the chromatography of polymers than in conventional TLC. Normally, solubility plays a very small role in the TLC behavior of monomeric compounds. For polymers, however, the effects of solute-solvent interaction are critical. Solvents with 8 s which match that of the polymer are good thermodynamic solvents, and should displace the polymer during development. 

The variation in pK3 of a substrate on changing the solvent medium has often been used for qualitative consideration of solute-solvent interactions. Such discussions have usually emphasized the role of anion stabilization in determining the relative magnitudes of pKa values. 

Because of their sensitivities to environmental changes, wide applications for solvatochromic compounds were found in the study of solute-solvent interactions, mainly in the characterization of bulk or microenvironments. Various polarity scales employing solvatochromic dyes as solvent probes were proposed. Because these empirical scales may be used to characterize any solvent or solvent mixture, solvatochromism played an important role in the study of a wide variety of solvent-dependent processes. 

While investigating surface assemblies at the liquid-solid interface, in addition to molecule-molecule and molecule-substrate interactions, one cannot neglect the role of solute-solvent as well as solvent-substrate interactions. For most STM investigations, the solvent is chosen based on the following criteria The solvent should (i) dissolve the molecules of interest (ii) have a low vapor pressure in order... 

Numerous experimental studies have been conducted on solute-solvent interactions in supercritical fluid solutions. In particular, issues such as the role of characteristic supercritical solvent properties in solvation and the dependence of solute-solvent interactions on the bulk supercritical solvent density have been extensively investigated. Results from earlier experiments showed that the partial molar volumes 02 became very large and negative near the critical point of the solvent (4-12). The results were interpreted in terms of a collapse of the solvent about the solute under near-critical solvent conditions, which served as a precursor for the solute-solvent clustering concept. Molecular spectroscopic techniques, especially ultraviolet-visible (UV-vis) absorption and fluorescence emission, have since been applied to the investigation of solute-solvent interactions in supercritical fluid solutions. Widely used solvent environment-sensitive molecular probes include Kamlet-Taft jt scale probes for polarity/polarizability... 

One of the problems encountered when dealing with the interaction of Lewis acids and bases in a quantitative way is in evaluating the role of the solvent. Bond energies in molecules are values based on the molecule in the gas phase. However, it is not possible to study the interaction of many Lewis acids and bases in the gas phase because the adducts formed are not sufficiently stable to exist at the temperature necessary to convert the reactants to gases. For example, the reaction between pyridine and phenol takes place readily in solution as a result of hydrogen bonding ... 

The assumption of linear response played a prominent role in the derivation (given above) of the SCRF equations, and one aspect of the physics implied by this assumption is worthy of special emphasis. This aspect is the partitioning of Gp into a solute-solvent interaction part Gss and a intrasolvent part Gss The partitioning is quite general since it follows entirely from the assumption of linear response. Since classical electrostatics with a constant permittivity is a special case of linear response, it can be derived by any number of classical electrostatic arguments. The result is [114, 116-119]... 

Thus the ionization of covalent compounds cannot be interpreted by the electrostatic theory unless covalent interactions between solute and solvent are considered. It is the purpose of the present discussion to reveal the role of donor-acceptor interactions for all solution processes. 

Another type of nonideal SEC behavior, which will not be covered in this chapter, is related to the use of mixed mobile phases (multiple solvents). Because solute-solvent interactions play a critical role in controlling the hydrodynamic volume of a macromolecule, the use of mixed mobile phases may lead to deviations from ideal behavior. Depending on the solubility parameter differences of the solvents and the solubility parameter of the packing, the mobile phase composition within the pores of the packing may be different from that in the interstitial volume. As a result, the hydrodynamic volume of the polymer may change when it enters the packing leading to unexpected elution results. Preferential solvation of the polymer in mixed solvent systems may also lead to deviations from ideal behavior (11). 

In contrast to solid state crystallization, crystallization from vapor, solution, and melt phases, which correspond to ambient phases having random structures, may be further classified into condensed and dilute phases. Vapor and solution phases are dilute phases, in which the condensation process of mass transfer plays an essential role in crystal growth. In the condensed melt phase, however, heat transfer plays the essential role. In addition to heat and mass transfer, an additional factor, solute-solvent interaction, should be taken into account. 

Finally, the diffusion of a chemical may be influenced by another diffusing compound or by the solvent. The latter effect is known as solute-solvent interaction it may become important when solute and solvent form an association that diffuses intact (e.g., by hydration). This may be less relevant for neutral organic compounds, but it plays a central role for diffusing ions. But even for noncharged particles the diffusivities of different chemicals may be coupled. The above example of the glycerol diffusing in water makes this evident in order to keep the volume constant, the diffusive fluxes of water and glycerol must be coupled. 

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