Macroscopic solvation behavior

While the phase structures of the pure mesogens employed in studies of this type may be well-characterized, it is always necessary to determine the effects of solute incorporation on bulk medium structure. The effects of the medium on solute reactivity are microscopic ones, and can only be completely understood (or at least meaningfully interpreted) when the structure of the bulk solute/mesogen system - i.e. the macroscopic solvation behavior - is known. 

Instead of arguing about the validity of the above conjectures, here we invoke the solvation formalism. Section 8.2, to rationalize some experimental findings and their interpretations by drawing explicit links between the (macroscopic) thermodynamic pressure effect on the kinetic rate constant and the (microscopic) species solvation behavior in a highly compressible medium. To that end, we study the solvent effect (or, more precisely, the solvation effect) on the kinetic rate constant within the framework of the TST (Hynes 1985 Steinfeld, Francisco, and Hase 1989), and its thermodynamic formulation that allows us to link it to changes of Gibbs free energy of activation. 

Now we make contact between the macroscopic coefficients ky T,p), defined by Equation 8.39, and the microstructural details of the reference system, that is, the solvation behavior of the species in the infinitely dilute system, by invoking the solvation formalism discussed in Section 8.2 (and references therein) from which we have that... 

The solvation of polyatomic ions or polar neutral molecules is even more difficult to describe. There are two sources of additional problems first of all, the symmetry of the system under investigation is drastically reduced and hence the number of different configurations increases tremendously. Furthermore, the strength of the electric field is much smaller than in the case of monatomic ions with spherical symmetry and therefore the dynamic behavior of the solvation shell is even more important for a priori calculations of macroscopic properties. 

As discussed below, ionic liquids often behave comparably to conventional polar organic solvents [6, 8, 10]. But the physics underlying solvation are entirely different. As noted above, ILs are characterized by considerable structural and dynamic inhomogeneity, and even simple concepts, such as the dipole moment, cannot be productively applied. We are therefore in the unusual position of needing to explain how an exotic microscopic environment produces conventional macroscopic behavior. To this end, we will review empirical characterizations of the ionic liquid environment, and then turn our attention to the underlying physics of solute-solvent interactions. 

At least two points should be especially emphasized, (i) From the solvent part, the parent radical cations exist in a non polar surrounding. Hence, the cations have practically no solvation shell which makes the electron jump easier in respect to more polar solvents. In a rough approximation the kinetic conditions of FET stand between those of gas phase and liquid state reactions, exhibiting critical properties such as collision kinetics, no solvation shell, relaxed species, etc. (ii) The primary species derived from the donor molecules are two types of radical cations with very different spin and charge distribution. One of the donor radical cations is dissociative, i.e. it dissociates within some femtoseconds, before relaxing to a stable species. The other one is metastable and overcomes to the nanosecond time range. This is the typical behavior needed for (macroscopic) identification of FET ... 

The study of dilute fluid solutions has been essential to the foundation of solvation thermodynamics and the development of macroscopic modeling for the description of observed behavior and the correlation of experimental data. Typical studies of dilute solutions have dealt with the determination of limiting values for the solute activity coefficients and the corresponding slope of their composition dependence at infinite dilution (Jonah 1983, 1986), where these quantities played a crucial role in constraining the parameterization of excess Gibbs free-energy models (Van Ness and Abbott 1982 Lupis 1983 Wallas 1985 Chialvo 1990a). 

We have discussed some formal developments around the fluctuation theory of mixtures from Kirkwood and Buff, with special emphasis on the behavior of dilute species in highly compressible media. These formal results were then used to interpret a few specific solvation phenomena, to provide support to the selection of macroscopic quantities for the successful regression of experimental data, and to facilitate the development of truly molecular-based modeling of these challenging systems. 

Until very recently, the theoretical evaluation of the effect of the medium on the behavior and properties of molecules was beyond the reach of quantum mechanical computations, essentially due to the prohibitive size of the systems which would have to be considered. Thus the habit evolved to treat essentially the isolated molecule and be satisfied by qualitative considerations or, at best, approximate evaluations of the bulk effect of the medium following early models [l]-[3]. The last few years have seen a number of attempts at a refinement of these continuum or macroscopic representations [4, 5, 6, 7]. One constant and essential inconvenience of these models is, however, the absence of precision concerning the arrangement of the solvation layer(s) around the solute, which is considered as residing in a cavity (generally spherical) embedded in a polarizable dielectric. No information is obtainable in this way about the details of the short-range solute-medium interactions. 

Although these are preliminary results that need further experimental work, they are a good indication that a very different behavior should be expected when metal evaporation is carried out on a conjugated oligomer as opposed to alkanethiol SAMs. The formation of a macroscopic metallic lead on the organic surface might lead to the solvation of part of the metal within the layer and/or the introduction of conformational disorder in the layer. 

While up to a certain degree solute-induced effects occur in all types of (nonideal) solutions, its manifestation in electrolyte systems deserves special attention. The presence of charged species in a dielectric solvent adds an important ingredient to the solvation phenomenon, i.e., the possible formation of neutral ion pairs. In fact, an outstanding property of water as a solvent at normal conditions is its intrinsic ability to solvate, and consequently dissolve, ionic and polar species, owing to its unusually large dielectric constant. This solvation process is typically described in terms of ion-solvent interactions, ion-induced solvent microstructural changes, solvent dielectric behavior, and their effects on the macroscopic properties of the solution. ... 

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