Question

coffee is decaffeinated industrially using supercritical CO2. One advantage to using scCo2 is that the polarity of the supercritical fluid can be tuned by varying temperature, pressure and cosolvent additives. however, the need for high temperatures and pressures makes generatino of scCO2 impractical in a teaching lab setting. Based on its structure and polarity, would you expect caffeine to be soluble in liquid Co2? if we carried out this experiment with tea leaves instead of orange rind, would you expect it to work? why or why not?

Answer 1

A supercritical fluid has several advantages over the liquid form of the same substance. It has both the good properties of a liquid and a gas for extractions.

First, it a much lower viscosity than liquid CO2. This helps it move through and around the coffee beans more thoroughly without creating much back pressure. For example, if you took a syringe and filled it with water it would be much easier to squeeze out than if you filled the syringe with motor oil.

Second, it has much higher diffusivity than liquid CO2. This helps it penetrate the coffee beans on a molecular level, get into all the micro cracks and pours of the coffee bean and find the caffeine to dissolve.

Third, it has a density comparable to that of liquid CO2. This means there is as much CO2 per liter as there is in the liquid form. So this means its very efficient as well. You don't have to use massive volumes in order to extract out all the caffeine. Additionally, this helps it to act as a solvent and give it solvent like properties to dissolve the caffeine in bring it out of the coffee bean.