Phenomena solvent polarity

Solvent polarity also affects the rate of peroxide decomposition. Most peroxides decompose faster in more polar or polari2able solvents. This is tme even if the peroxide is not generally susceptible to higher order decomposition reactions. This phenomenon is illustrated by various half-life data for tert-huty peroxypivalate [927-07-1]. The 10-h half-life temperature for tert-huty peroxypivalate varies from 62°C in decane (nonpolar) to 55°C in ben2ene (polari2able) and 53°C in methanol (polar). 

Some of these model-dependent quantities were formulated as measures of a particular phenomenon, such as electron-pair donor ability but many of them have been proposed as empirical measures of solvent polarity, with the goal, or hope, that they may embody a useful blend of solvent properties that quantitatively accounts for the solvent effect on reactivity. This section describes many, although not all, of these empirical measures. Reichardt has reviewed this subject. 

Kosower made the first use of this phenomenon for measuring solvent polarity. The model process is the absorption transition of l-ethyl-4-carbomethoxy-pyridinium iodide, 7 ... 

This analysis shows that in order to account properly for solvent polarity effects, a solvation model has to be characterized by a larger flexibility with respect to the same model for ground state phenomena. In particular, it should be possible to shift easily from an equilibrium to a nonequilibrium regime according to the specific phenomenon under scrutiny. In the following section, we will show that such a flexibility can be obtained in continuum models and generalized to QM descriptions of the electronic excitations. 

The solvent polarization can be formally decomposed into different contributions each related to the various degrees of freedom of the solvent molecules. In common practice such contributions are grouped into two terms only [41,52] one term accounts for all the motions which are slower than those involved in the physical phenomenon under examination (the slow polarization), the other includes the faster contributions (the fast polarization). The next assumption usually exploited is that only the slow motions are instantaneously equilibrated to the momentary molecule charge distribution whereas the fast cannot readjust, giving rise to a nonequilibrium solvent-solute system. 

It should be noticed that, in many theoretical works, the term solvent polarity is defined by the values of the relative electric permittivity, also called dielectric constant. However, such a definition is by no means precise. The existence of hydrogen bonds (H-bonds) between solute and solvent molecules is one of the important limitations of the use of the continuum models based on the theory of dielectrics. In modern physical chemistry of solutions in order to quantitatively describe the solvatochromism phenomenon various empirical scales of the polarity are used. The exhaustive reviews on this topic have been presented by Reichardt [1, 2],... 

The same phenomenon is observed for other fluorophores such as TNS or PROD AN. Figure 3.40 displays the position of the emission maximum of TNS as a function of solvent polarity (Z). 

The analysis procedure developed in the previous section for gas permeation forms the basis for analyzing RO. However, the RO analysis is more complicated because of 1) osmotic pressure, which is included in Eq. fl7-12T and 2) mass transfer rates are much lower in liquid systems. Since the mass transfer rates are relatively low, the wt frac of solute at the membrane wall x will be greater than the wt frac of solute in the bulk of the retentate x, . This buildup of solute at the membrane surface occurs because the movement of solvent through the membrane carries solute with it to the membrane wall. Since the solute does not pass through the semipermeable membrane, its concentration will build up at the wall and it must back diffuse from the wall to the bulk solution. This phenomenon, concentration polarization, is illustrated in Figure 17-10. Concentration polarization has a major effect on the separations obtained in RO and UF (see next section). Since concentration polarization causes x > Xp the osmotic pressure becomes higher on the retentate side and, following Eq. fl7-12). the flux declines. Concentration polarization will also increase Ax in Eq. tl7-13 and flux of solute may increase, which is also undesirable. In addition, since concentration polarization increases solute concentration, precipitation becomes more likely. 

With preferential sorption of one component of the binary solvent on the polymer coil, an increase or decrease of the polarity of the polymer microenvironment occurs depending on whether the more polar (water) or less polar (organic solvent) component is sorbed. Preferential sorption occurs for PHEMA in 1-propanol/water, dioxane/water, and acetone/water mixtures (Figures 4 and 5). When the more polar component (water) is preferentially sorbed from mixtures in which its concentration is low, then the apolar contribution of the polymer may be compensated to that extent, since the polarity of the polymer chain microenvironment is even higher than the bulk solvent polarity. As a result, the curves of the dependence of Ej for the polymer on the solvent composition intersect the same dependence for mixed solvents. This phenomenon was observed for PHEMA in 1-propanol/water (Figure 4), dioxane/water, and acetone/water (Figure 5). Preferential sorption is also indicated by the results for PMMA and PBMA in methanol/toluene mixtures. Preferential sorption was previously found in this system by dialysis equilibria. ... 

---