Cosolvent tuning

AK DiUow, KP Hafner, SLJ Yun, F Deng, SG Kazarian, CL Liotta, CA Eckert. Cosolvent tuning of tautomeric equilibrium in supercritical fluids. AIChE J 43 515, 1997. 

The benefits from tuning the solvent system can be tremendous. Again, remarkable opportunities exist for the fruitful exploitation of the special properties of supercritical and near-critical fluids as solvents for chemical reactions. Solution properties may be tuned, with thermodynamic conditions or cosolvents, to modify rates, yields, and selectivities, and supercritical fluids offer greatly enhanced mass transfer for heterogeneous reactions. Also, both supercritical fluids and near-critical water can often replace environmentally undesirable solvents or catalysts, or avoid undesirable byproducts. Furthermore, rational design of solvent systems can also modify reactions to facilitate process separations (Eckert and Chandler, 1998). 

Remarkable tuning of reaction rates has been achieved for the isomerization of several dye molecules in supercritical fluid solvents using both small pressure changes and small additions of cosolvents. Rates of the thermal cis-trans relaxation were measured spectroscopically following irradiation for three dyes in supercritical carbon dioxide and ethane, pure and with several polar and protic cosolvents. These results demonstrate the versatility of supercritical fluid solvents, both to examine reaction mechanisms and as a means to tune rates (DiUow et al., 1998). 

One of the most fascinating features of Sml2 is the ability to modify its behaviour through the use of cosolvents or additives. For example, cosolvents or additives can be used to control the rate of reduction or the chemo- or stereoselectivity of reactions. Additives commonly utilised to fine-tune the reactivity of Sml2 can be classified into three major groups ... 

It is possible to fine tune the solvating strength of the SF from an ideal gas to nearly that of a pure liquid. It is even possible, by adding small quantities of cosolvents (modifiers) to the SF, to custom design a SF for a specific application. 

Liquid carbon dioxide is a non-polar (non-ionizing) solvent, in many ways like hexane. However, in the supercritical phase the dielectric constant of CO2 increases with increasing pressure. ( ) The dielectric constant is an indicator of the polarity of the SCFCO2 solvent [Figure 4]. Thus, by controlling pressure (or the addition of a polar cosolvent), the selectivity of SCF can be fine tuned for the preferential extraction of the compounds of interest based on their polarity. 

Ozin and coworkers recently extended the supra-molecular I S+ assembly into the synthesis of binary mesoporous yttrium oxide-stabilized-zirconium oxide materials.These materials were synthesized by a modified sol-gel method under basic conditions, where zirconium ethoxide and yttrium acetate were used as the precursors for the transition metal oxides, and CTAB was used to form the supramolecular templates. The use of ethylene glycol with coordinating capability as a cosolvent may play a role in controlling the hydrolysis rate and solubility of zirconium(IV) and yttrium(III). This synthesis strategy is similar to that of so-called polymerizable-complex method, which was widely used to prepare multicomponent single-phase oxides. The yttrium content in these binary materials can be tuned from 12-56 wt%, and no phase segregation of yttrium and zirconium oxides was observed. These materials could be applied in designing new solid oxide fuel-cell electrode materials. 

Parameters that may be used to tune the phase behavior of microemulsions include salinity, surfactant type and concentration, cosolvent type and concentration, pH, oil composition, temperature, and pressure. As salinity increases, there is a steady progression from lower phase to middle phase to upper phase microemulsions. This reflects a continuous evolution of the preferred curvature of the surfactant film and corresponds to an increase in hydrophobicity with added electrolyte such as NaCl. At low salinity the droplet size in the water-continuous lower phase increases with increasing salinity. This corresponds to an increase in the solubilization of oil and is reflected in increased light scattering. As salinity increases further, the middle phase appears and is initially water-continuous. 

Another important requirement for the development of new polymer processes based on SCCO2 is knowledge about the phase behavior of the mixture involved, which enables the process variables to be tuned properly to achieve maximum process efficiency. Determining parameters in the phase behavior of a system are the solvent quality, the molecular weight, chain branching, and chemical architecture of the polymer, as well as the effect of endgroups and the addition of a cosolvent or an antisolvent An overview of the available literature on the phase behavior of polymers in supercritical fluids has been published by Kirby and McHugh [50]. 

With nanoparticles, the architectnre of the particle shell or matrix can be formulated and fine-tuned to offer controlled release of its contents, ranging from constant but prolonged release to zero release. The particles can be formulated with desired characteristics via key structural features, such as CToss-tmking density, hydrophilic-hydrophobic balance of the copolymer units, and stiffness of the polymeric network. Furthermore, erodible or biodegradable particles can be used to combine the release mechanisms of diffusion and erosion. In addition, the particles can be designed to respond to different stimuh from the external environment by adjusting a number of parameters, such as pH, temperature, ionic strength, cosolvent composition, pressure, or electric field. 

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