Room-Temperature Ionic Liquids Offer Better Viscosity

The November issue of Pharmaceutical Technology reports that university-based scientists and pharmaceutical companies are developing new ways to reduce solvent use in chemical reactions, make one-pot reactions, and reduce waste from the beginning to the end of the drug manufacturing process. Rutgers University (New Brunswick, NJ, www.rutgers.edu) reports a class of environmentally friendly chemicals called room-temperature ionic liquids (RTILs) that also can be used to achieve these goals.

RTILs are molten salts with organic cations and either inorganic or organic anions. As opposed to conventional molten salts, RTILs can act as solvents in reactions because they melt in <100 °C temperatures and can provide superior results for enzymatic catalysis. In addition, they do not contribute to air pollution because they do not evaporate or burn into the environment. According to Edward Castner, Jr., a professor in Rutgers’s chemistry and chemical biology departments, “RTILs offer great promise for the chemical and pharmaceutical industries, but will require substantially different process technologies.  The fact that many organic chemical reactions proceed more efficiently in ionic liquids, especially enantiomerically selective reactions, will make them of interest to the biopharma community.”

Despite interest in RTILs, the industry has hesitated to use them because they are much more viscous than traditional organic solvents, which could restrict flow and make manufacturing more difficult and more expensive in the long run. To address these issues, the Rutgers researchers determined that replacing silicon for carbon at a specific spot in some RTIL molecules cuts the liquid’s viscosity by almost tenfold, compared with the same liquid without the silicon substitution (1). The group finds that the interactions between the ions are much weaker for the molecule with this silicon substitution than those for the analogous molecule. “Our work has been on understanding the intermolecular interactions between the ions that comprise the ionic liquids in order to understand their physical properties,” says Castner. The next step in their project will be to develop new ionic liquids and to test ideas about what are their chemical and physical properties. This information will enable the team to move towards designing ionic liquids that are specific to an application.

According to Castner, the chemicals are already available from various manufacturers including Sigma-Aldrich, Cytech, Solvent Innovations, and others. “More significantly, both BASF and  Merck have spun off arms in Germany to develop and manufacture ionic liquids in quantities greater than one ton,” he says.

Reference
1. H. Shirota and E.W. Castner, “Why Are Viscosities Lower for Ionic Liquids with -CH₂Si(CH₃)₃ vs -CH₂C(CH₃)₃ Substitutions on the Imidazolium Cations?” J. Phys. Chem. B 109 (44), in press (2005).