Supercritical CO2

Lots of us love a cup of coffee, but sometimes we can’t have a second cup because the caffeine will keep us awake at night.  That’s the reason the coffee industry has looked for ways to remove caffeine from coffee beans.  Various solvents have been tried, but the best method now seems to be use CO2 in its supercritical state.

Carbon dioxide is cheap and widely available, essentially nontoxic, nonflammable, and the least expensive solvent after water.  It is relatively easy to reach the supercritical state (its critical temperature of 30.99 oC and pressure of 72.8 atm are easily achieved).  The material is also easy to handle.

To decaffeinate coffee, the beans are treated with stream to bring the caffeine to the surface.  The beans are then immersed in supercritical CO2 which selectively dissolves the caffeine but leaves intact the compounds that give favour to coffee.  (Decaffeinated coffee contains 3% of the original coffee.)  The supercritical CO2 is poured off, and the CO2 is recovered by evaporation and then reused in the process.

The fact that caffeine can be so easily dissolved by supercritical CO2 suggests that it should be an ideal solvent for many purposes.  This is important because more than 3.0  1010 pounds of organic and halogenated solvents are used worldwide every year as cleaning agents.  Many have deleterious effects on the environment, so industry has long sought new methods.  Although CO2 seems an ideal solution, many materials have very low solubilities in it.

Progress is being made, however.  A U. S.-Italian team of scientists, headed by University of North Carolina chemist Joseph DeSimone, has developed a surfactant to use with supercritical CO2.  Surfactants such as soaps and detergents enhance the ability of a solvent to dissolve substances.  They allow greases and oils, non-polar materials that are insoluble in water to be taken into solutions as emulsions.  Surfactants are typically long-chain molecules that have polar and non-polar ends.  The non-polar end has an affinity for non-polar substances, and the polar end has an affinity for water.

A surfactant for supercritical CO2 would greatly enhance its solvent properties.  The problem, however, is to build a molecular substance in which one end of the molecule has an affinity for non-polar CO2.  It turns out that fluorinated hydrocarbons fit this requirement.  The surfactant developed by the DeSimone group is a polymer of styrene with a fluorinated acrylate at one end.  Among other things, supercritical CO2 with added surfactant dissolved polystyrene, a substance that would not otherwise dissolve.  This useful property could allow supercritical CO2 to be used to clean surfaces from small residues of polymer left from manufacturing processes.

(Joseph Kaiser: “Supercritical solvent comes into its own.”  Science, Vol. 274, p. 2013, 1996)