Tuesday, February 20, 2018

Supercritical Fluids: the Mosh Pit of Matter

by Rolf Schlake

As I travel around the world to various conferences giving papers or lectures on our supercritical fluid technology, we take for granted as to what a supercritical fluid is. However, I am often asked to speak to non-scientific persons, perhaps through business or civic groups such as the Chamber of Commerce or Rotary International regarding a particular application using supercritical fluids. Invariably, I must describe what a supercritical fluid is. From painful experience, as soon as I mention phase diagram, temperature, pressure, the eyes glaze over and the audience is asleep!

This explanation is quite simplistic to those of us in the know, but think of the time when you might be explaining your technology to someone at the next reception you go to. It goes like this.

We are all familiar with 3 states of matter: solid, liquid, gas; ice, water, steam. Well, there is “sort of” a 4th. I say “sort of” because it’s not really, but it works for our purposes. Let’s illustrate people as molecules, huddled and touching each other in a large group, standing cramped, like in an elevator. This is a solid.

If we toss a beach ball onto this mass, it just bounces off, not falling and being held by the mass of people. Meaning, we can’t dissolve anything in a solid. I.e. a cube of sugar, as a cube of solid sugar, will not dissolve into the sugar bowl itself.

Now if we disperse all these people widely….very, very widely, like tens of feet apart and moving around because they are not tied to anything. Then we consider this to be analogous to a gas. The molecules are widely dispersed. And if we toss a beach ball amongst this group, the ball will fall to the floor and not intermingle with the people. Also, a gas cannot dissolve compounds.  Our sugar cube does not dissolve/disappear into the air.

If we bring our people close enough together to be able to hold hands (perhaps multiple hands, like an octopus), we now have something we might describe as a liquid. Accordingly, if we toss our beach ball into this group, the ball gets caught and held in the arms of the people. This time our sugar cube is dissolved in the liquid, similar to the beach ball being contained by people’s arms.

All right, we have described the three most recognized phases of matter. But then what is a supercritical fluid?

Let’s go back to our liquid state, where we have people close to each and holding hands. Now, let’s make our persons stay close to each other but have them actively move around, where they can no longer hold each other’s hands…. constantly and quickly moving close to each other. If we toss our beach ball into this crowd, the ball does not bounce off nor does it hit the floor. It is caught up between persons because they are close to each other. However, they no longer fit the definition of a liquid because they are not holding hands. This is a supercritical fluid. It is dense like a liquid so it can dissolve compounds, but since there are no bonds, no hand holding,  it also behaves like a gas.


via GIPHY

 A SUPERCRITICAL FLUID IS THE MOSH PIT AT A ROCK CONCERT COMPLETE WITH CROWD SURFING!

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