Local solvent compression

Local solvent compression. The next application of the solvato-chromic data will be to determine the magnitude of the local compression of a supercritical fluid solvent in the immediate environment of the solute. The of a dye such as phenol blue can be predicted in liquids where no specific interactions are present by treating the solvent as a homogeneous polarizable dielectric (22,29). The intrinsic "solvent strength", E, °, describes dispersion, Induction, and dipole-dipole forces and is given by (22). 

In the framework of solvent shift theories, using a Lennard-Jones type potential for the interaction of a nonpolar dye in a nonpolar matrix, Sesselman et al. [34] developed a simple theory for the interpretation of hole-burning data in the low pressure range (<20 MPa). In this regime, die pressure shift Av varies linearly with the solvent shift Avs, i.e. the difference between the molecular absorption frequency in the matrix and in vacuum, the local hydrostatic compressibility k and the pressure change Ap ... 

This local solvent density enhancement raises the solvent quality in the thin film between the surfaces and resists flocculation even at bulk solvent densities below the UCSD. The enhancement is larger at pg = 0.35 where the solvent free volume and compressibility are larger than at pg = 0.45. Flocculation occurs, however, when the density of solvent in the thin film between surfaces equals the UCSD for the stabilizer chains. This result suggests that shorter stabilizers, with UCSDs close to may be able to stabilize latexes and emulsions near and p. Of course the stabilizer must be large enough to screen the Hamaker forces. These issues warrant further experimental studies. 

The origin of local solvent density enhancements in a compressible SCF can be understood from two different viewpoints. The more recently proposed viewpoint [10,12] ties the existence of local density enhancements directly to the presence of the solvent s critical, correlated density fluctuations, while the more common viewpoint bases the existence of local density enhancements upon the attractiveness of the solute-solvent interaction potential and the compressibility of the fluid [2,10,17,22,23,29-32]. These two viewpoints are described below. (Note that the effect of local density enhancements can also be understood within a purely thermodynamic framework through the Krichevskii parameter, lirrix- Q dP/dx)v r where x is the solute mole fraction. See Refs. [27], [28] and [33].)... 

The large maximum in v LR) at pc is expected, because it is here that kj and are maximized. The observation that the maximum in v (SR) occurs at bulk densities below the critical density is more subtle but can be understood as follows. When p < Pc, local compression of the solvent brings the local solvent density closer to critical density, thus increasing the effective local compressibility and facilitating further compression. In contrast, if p > Pc. the local compression reduces the effective local compressibility, hindering further compression. Consequently, the maximum compression occurs when the bulk density is less than the critical density, and the local compression effects tend to be compounded. 

Following earlier work by Wood et al., Luo and Tucker have relaxed the constant density restriction, and developed a continuum model in which the dielectric constant may be position-dependent. This dielectric function, s(7), is defined, at each point r in the fluid, in terms of the local density of the fluid at , pi(f), which is itself determined by the local values of the electric field and the compressibility. However, the local value of the electric field at 7 must be found from electrostatic equations (see Poisson-Boltzmann Type Equations Numerical Methods) which depend upon the dielectric function s(r) everywhere. Hence, all of the relevant equations must be solved self-consistently, and this is done using a numerical grid algorithm (see Poisson-Boltzmann Type Equations Numerical Methods). The result of such calculations are the density profile of the fluid around the solute and the position-dependent electric field, from which the free energy of solvation may be evaluated. The effects of solvent compression on solvation energetics can be quite substantial. Compression-induced enhancements to the solvation free energy of nearly 15 kcal mol" have been calculated for molecular ions in SC water at Tt = 1.01 and pr = 0.8. ... 

At the same time, since there has been, in each co-sphere, an increment in the density of the solvent, we must expect some modification in other properties of the solvent, such as its compressibility. In a very dilute solution it may be difficult to detect such a change by measuring the compressibility of the solution. At higher concentrations, however, when a sufficient fraction of the total solvent lies within the ionic co-spheres, the sum of these local modifications can be detected by measuring the compressibility of the whole solution. 

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... 

The local density of solvent about the solute may be determined by comparing the experimental and calculated curves. Consider points A and B in Figure 5 at a constant value of E, i.e., 55 kcal/mol. A hypothetical homogeneous fluid at point B gives the same "solvent strength" as he actual fluid at point A. The local density about the solute exceeds the bulk density due to compression, such that... 

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