Solvent effects compressibility

In the sections below a brief overview of static solvent influences is given in A3.6.2, while in A3.6.3 the focus is on the effect of transport phenomena on reaction rates, i.e. diflfiision control and the influence of friction on intramolecular motion. In A3.6.4 some special topics are addressed that involve the superposition of static and transport contributions as well as some aspects of dynamic solvent effects that seem relevant to understanding the solvent influence on reaction rate coefficients observed in homologous solvent series and compressed solution. More comprehensive accounts of dynamics of condensed-phase reactions can be found in chapter A3.8. chapter A3.13. chapter B3.3. chapter C3.1. chapter C3.2 and chapter C3.5. 

With traditional solvents, the solvent power of a fluid phase is often related to its polarity. Compressed C02 has a fairly low dielectric constant under all conditions (e = 1.2-1.6), but this measure has increasingly been shown to be insufficiently accurate to define solvent effects in many cases [13], Based on this value however, there is a widespread (yet incorrect ) belief that scC02 behaves just like hexane . The Hildebrand solubility parameter (5) of C02 has been determined as a function of pressure, as demonstrated in Figure 8.3. It has been found that the solvent properties of a supercritical fluid depend most importantly on its bulk density, which depends in turn on the pressure and temperature. In general higher density of the SCF corresponds to stronger solvation power, whereas lower density results in a weaker solvent. 

J. Troe I would like to comment on the role of the solvent in the photoisomerization of frans-stilbene, as discussed by Prof. Marcus. From our extensive studies in series of nonpolar and polar solvents in compressed gases and in the liquid phase, a very detailed picture arises we now can distinguish specifically different types of solvation first, there is strong interaction with polar solvents second, we can distinguish two kinds of interaction with nonpolar solvents, one site specific of only mildly polarizable small alkanes that possibly can squeeze in between the two phenyl groups in stilbene and one non-site specific of more polarizable large alkanes that can only solvate around the outer periphery of stilbene. These different types of solvation result in characteristically different solvent effects on the kinetics. 

Supercritical fluid solvents can act in a variety of ways to affect reaction rates. Since the reaction rate is the product of the rate constant and the concentrations of the reactants, one must consider the solvent effect on the rate constant itself (discussed below), as well as changes in concentrations. It is this second possibility that has not been addressed until this study i.e., the possible influence of changes in the local concentrations of the reactants in the compressible region near the critical point... 

Solvatochromic shift data have been obtained for phenol blue in supercritical fluid carbon dioxide both with and without a co-solvent over a wide range in temperature and pressure. At 45°C, SF CO2 must be compressed to a pressure of over 2 kbar in order to obtain a transition energy, E, and likewise a polarizability per unit volume which is comparable to that of liquid n-hexane. The E,j, data can be used to predict that the solvent effect on rate constants of certain reactions is extremely pronounced in the near critical region where the magnitude of the activation volume approaches several liters/mole. 

At 20 °C the phase transition areas vary over 6 A2 per molecule. Although this range is outside the experimental error for the normalized isotherms, such small variation must be considered to be on the borderline of significance. More striking however is the approximately 10 dyne/ cm range of variation in surface pressure that results from the differences in hydrocarbon solvent effects on the work of compression. 

An improvement in aldol diastereoselection for a given boron ligand is obtained when less polar solvents are employed, presumably due to transition state compression in nonpolar solvents. This solvent effect is also significant in enolate chirality transfer in asymmetric aldol reactions. 

Several objectives motivated the extension of ACN studies to light compressible solvents [12]. Initial studies of AOT in such solvents had demonstrated the possibility of intriguing solvent effects [20,21,32], which could be clarified by additional experiments. A second objective was to test the concepts generated from the thermodynamic models that were developed for the AOT-brine-propane system [25,44]. A final objective was to study the behavior of nonionic surfactant systems as a complement to AOT systems. Nonionic systems provide an enhanced opportunity to study temperature effects on surfactant phase behavior, as nonionic surfactants are much more responsive to temperature than the anionic surfactant AOT. 

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. 

At first glance, one might consider the effect of compressed CO2 on the phase behavior of multi-component polymer systems to be a simple combination of the known effects of liquid solvents and hydrostatic pressure. Solvent effects are primarily enthapic in nature and typically manifest in upper critical solution behavior. Common solvents mitigate unfavorable interactions between dissimilar segments and enhance miscibility. In blends, the addition of highly selective solvents, e.g. a non-solvent for one component, can lead to precipitation of the unfavored species at high dilution. In block copolymers, the effect of selective solvents is less clear, but studies to date reveal a collection of the solvent at the domain interface, selective dilation of one phase, and stabilization of the disordered phase via depression of the UODT. The systems we have studied each exhibit a lower critical transition. For these specific systems, previous work indicates the hydrostatic pressure suppresses free volume differences between the components and expands the region of miscibility. 

Because the importance of local density enhancement effects depends upon the compressibility of the fluid, these effects have an unusual bulk density (pressure) dependence. Below the critical density [65] the local density enhancement effects increase with increasing bulk-density, whereas at densities greater than the critical density, these effects will decrease with further increases in the bulk-density. This is in contrast to the bulk solvent effects, which increase monatonically with increasing bulk-density as the solvent properties, e. g. the dielectric constant, vary from their gas-like to liquid-like values. Hence, the bulk-density dependences of the activation barriers, AG(T5), on... 

Cason JP, Khambaswadkar K, Roberts CB. Supercritical fluid and compressed solvent effects on metallic nanoparticle synthesis in reverse micelles. Ind Eng Chem Res 2000 39 4749-55. 

The compressibility of PEO blocks in the subphase clearly depends on the size and properties of the hydrophobic block, and the dimension of the PEO fragment itself. For example, it was shown that PEO can be effectively compressed by a factor of three [29], following the predictions for PEO behavior in a good solvent. As mentioned before, desorption of PEO from the air-water interface and the formation of a brush was also observed in this case, and seems to be a well-established phenomenon. 

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