Cosolvent-solute interactions

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). 

Fluorescence Investigation of Cosolvent—Solute Interactions in Supercritical Fluid Solutions... 

Cosolvent effects on SCF solution behavior allow the tailoring of solvents for extractions and separations. The strong interactions in these systems currently defy prediction by popular computational methods. Only by understanding these interactions at a molecular level will we be able to guide the development of phase equilibria models successfully. One way of exploring the molecular level interactions is with spectroscopy of various kinds and we have demonstrated here an attempt to look at the cosolvent/solute interaction. 

Additional evidence for PRODAN emitting from two environments is given by the temperature dependence of emission spectra in the CO2/CH3OH binary supercritical fluid (Figure 3). As the temperature is increased (at constant density) the contribution from the red-shifted peak decreases while the relative intensity of the peak at 400 nm increases. This indicates that the less polar environment surrounding PRODAN is preferred at higher temperatures. In other words, the specific cosolvent-solute interactions responsible for the red shift in emission are disrupted at elevated temperatures. 

We have utilized the static and dynamic fluorescence characteristics of an environmentally-sensitive solute molecule, PRODAN, to investigate the local solvent composition in binary supercritical fluids. In the two solvent systems studied (C02/1.57 mol% CH3OH and C02/1.44 mol% CH3CN), specific cosolvent-solute interactions are clearly evident. Time-resolved fluorescence emission spectra indicate that the cosolvent-solute interactions become more pronounced with time after excitation. Hence, the local composition of cosolvent around the excited-state solute becomes greater than that surrounding the ground-state solute. That is, the photon-induced increase in excited-state dipole moment drives picosecond cosolvent augmentation about PRODAN. 

As mentioned previously, the physical state of a solute is susceptible to modifications by interaction with cosolvents. In principle, a cosolvent can enhance solute solubility by changing the solvency of the medium, by direct solute interaction, by adsorption, or by partitioning (Chiou et al. 1986). In a batch experiment testing the effect of humic acid on kerosene dissolution in an aqueous solution, Dror et al. (2000a) found a linear correlation between the amount of humic acid and the amount of kerosene that dissolved (Fig. 6.5). 

On the basis of excess Gibb s energy approach of VNfohl [56], Williams and Amidon [57-59] predicted solubilities in aqueous cosolvent systems. This approach is based on expressing the solubility of a compound in a binary solvent system as the sum of the solubilities of the compound in each pure solvent plus any interaction terms resulting from solvent-solvent to solvent-solute interactions. This approach predicted solubilities fairly well, but included some simplifying assumptions about solute-solvent interactions that may not be applicable to all systems. 

Williams and Amidon investigated a method that introduces estimates of solvent-solvent, and solute-solvent interactions into the basic log-linear expression. In their approach, cosolvent-water interactions are estimated from vapor pressure data of the solvent mixtures. The data are obtained from literature sources, if available, or determined experimentally. Solvent-solute interactions are estimated from experimental solubility data. An alternate approach, as described by Khossravi and Connors, divides the free energy of solubility into crystal, cavity, and solvation components. While the free energy associated with... 

It has been shown (Bartlett and McCollum, 1956) that the presence of organic impurities or cosolvents in aqueous sulfuric acid can have very powerful effects on the observed concentration ratio for nitroaniline indicators. It is not, therefore, surprising that if [eqn (1)] is present in concentrations much greater than lO-2 M, it may begin to affect its own ionization ratio through solute-solute interactions. Hammett anticipated and avoided this problem by exploiting the high molar absorptivities of his nitroanilines. in order to keep them at very low concentration where their influence on... 

This, in turn, will affect the patterns of solubilization by cosolvents. Furthermore, a high concentration of solutes may invalidate the log-linear model, which presumes negligible volume fraction of solute and no solute-solute interactions. For solid solutes, solvent induced polymorphism may also bring additional changes in their solubilization profile. 

Electron transfer reactions have also been used in the probing of solute-solute interactions in supercritical fluid solutions. For example, Takahashi and Jonah examined the electron transfer between biphenyl anion and pyrene in supercritical ethane (192). Worrall and Wilkinson studied triplet-triplet energy transfer reactions for a series of donor-acceptor pairs, including anthracene-azulene in supercritical C02-acetonitrile and supercritical C02-hexane and ben-zophenone-naphthalene in supercritical C02-acetonitrile (193). The high efficiency of the energy transfer reactions at low cosolvent concentrations was attributed to the effect of solute-solute clustering. 

Cosolvents ana Surfactants Many nonvolatile polar substances cannot be dissolved at moderate temperatures in nonpolar fluids such as CO9. Cosolvents (also called entrainers, modifiers, moderators) such as alcohols and acetone have been added to fluids to raise the solvent strength. The addition of only 2 mol % of the complexing agent tri-/i-butyl phosphate (TBP) to CO9 increases the solubility ofnydro-quinone by a factor of 250 due to Lewis acid-base interactions. Veiy recently, surfac tants have been used to form reverse micelles, microemulsions, and polymeric latexes in SCFs including CO9. These organized molecular assemblies can dissolve hydrophilic solutes and ionic species such as amino acids and even proteins. Examples of surfactant tails which interact favorably with CO9 include fluoroethers, fluoroacrylates, fluoroalkanes, propylene oxides, and siloxanes. 

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