Solvent pressure effect

We have also investigated the kinetics of free radical initiation using azobisisobutyronitrile (AIBN) as the initiator [24]. Using high pressure ultraviolet spectroscopy, it was shown that AIBN decomposes slower in C02 than in a traditional hydrocarbon liquid solvent such as benzene, but with much greater efficiency due to the decreased solvent cage effect in the low viscosity supercritical medium. The conclusion of this work was that C02 is inert to free radicals and therefore represents an excellent solvent for conducting free radical polymerizations. 

Besides temperature and addition of non-solvent, pressure can also be expected to affect the solvency of the dispersion medium for the solvated steric stabilizer. A previous analysis (3) of the effect of an applied pressure indicated that the UCFT should increase as the applied pressure increases, while the LCFT should be relatively insensitive to applied pressure. The purpose of this communication is to examine the UCFT of a nonaqueous dispersion as a function of applied pressure. For dispersions of polymer particles stabilized by polyisobutylene (PIB) and dispersed in 2-methylbutane, it was observed that the UCFT moves to higher temperatures with increasing applied pressure. These results can qualitatively be rationalized by considering the effect of pressure on the free volume dissimilarity contribution to the free energy of close approach of the interacting particles. 

Whilst vapour pressure may be the major solvent factor involved in the degradation process, there could also be a contribution from solvent viscosity or even, yet less likely, from surface tension. It has already been argued (see Section 2.6.2) that although an increase in viscosity raises the cavitation threshold, (i. e. makes cavitation more difficult), provided cavitation occurs, the pressure effects resulting from bubble collapse... 

In catalysis applications, the tunable solvent properties result in a variety of effects, such as controllable component and catalyst solubilities. Moreover, it is possible that kinetic rates are affected by both temperature and pressure effects, equilibrium constants are shifted in favor of the desired products, and selectivity and yields are increased by manipulating the solvent s dielectric constant or by controlling the temperature in highly exothermic reactions through an adjustment of the solvent s heat capacity [18-23]. 

Reactions of (ii)-l-decenyl(phenyl)iodonium salt (6a) with halide ions have been examined under various conditions. The products are those of substitution and elimination, usually (Z)-l-halodec-l-ene (6b) and dec-l-yne (6c), as well as iodobenzene (6d), but F gives exclusively elimination. In kinetic studies of secondary kinetic isotope effects, leaving-group substituent effects, and pressure effects on the rate, the results are compatible with the in-plane vinylic mechanism for substitution with inversion. The reactions of four ( )-jS-alkylvinyl(phenyl)iodonium salts with CP in MeCN and other solvents at 25 °C have been examined. Substitution with inversion is usually in competition with elimination to form the alk-l-yne. 

The effect of pressure on the measured bimolecular rate constant of the Diels-Alder reaction between maleic anhydride and isoprene was investigated in supercritical CO2 and subcritical propane. The reaction was carried out at 35°C in CO2 and 80°C in propane. The rate constants in supercritical CO2 agreed closely with the thermodynamic pressure effect predictions over the entire pressure range. The rate constants in the subcritical propane solvent significantly diverged from the thermodynamic pressure effect predictions and were found to deviate from this linear density dependence at the lower pressures studied. The results show solvent-solute and cosolvent-solute interaction (Reaves and Roberts, 1999). 

Other patents illustrate the use of a solvent extraction process to separate the alcohol products from the catalyst (130, 131). When a catalyst solution containing alcohol products is mixed with water and a water-immiscible solvent, the alcohol products are extracted into the aqueous phase and the rhodium species enter the water-immiscible solvent. The effectiveness of the extraction and the stability of the rhodium catalyst can be greatly increased by carrying out the process under CO pressure (131). 

Supercritical solvents can be used to adjust reaction rate constants (k) by as much as two orders of magnitude by small changes in the system pressure. Activation volumes (slopes of In k vs P) as low as —6000 cm3/mol were observed for a homogeneous reaction (97). Pressure effects can also be pronounced on reversible reactions (17). In one example the equilibrium constant was increased from two- to sixfold by increasing the solvent pressure. The choice of supercritical solvent can also dramatically affect an equilibrium constant. An obvious advantage of using supercritical fluid solvents as a media for chemical reactions is the adjustability of the reaction kinetics and equilibria owing to solvent effects. 

Many questions in the analysis of solvent dynamics effects for isomer-izations in solution have arisen, such as (1) when is a frequency-dependent friction needed (2) when does a change of solvent, of pressure, or of temperature change the barrier height (i.e., the threshold energy), and (3) when is the vibrational assistance model needed, instead of one based on Eq. (1.1) or its extensions ... 

Becaus-e of the similarity in the relations for osmotic pressure in dilute solutions and the equation for an ideal gas, van t Hoff proposed his bombardment theory in which osmotic pressure is considered in terms of collisions of solute molecules oil the semipeniieable membrane. This theoiy has a number of objections and has now been discarded. Other theories have also been put forward involving solvent bombardment on the semipermeable membrane, and vapor pressure effects. For example, osmotic pressure has been considered as the negative pressure which must be applied to the solvent to reduce its vapor pressure to that of the solution. It is, however, more profitable to interpret osmotic pressures using thermodynamic relations, such as the entropy of dilution,... 

Table II. Temperature and Oxygen Pressure Effects in Oxidation of Cyclohexanone to Adipic Acid (Reaction Conditions Cyclohexanone, 5 mL Re2(CO)10, 50 mg Solvent, 10 mL)...

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