My hypothesis on what a SCF is

As we discussed in class supercritical fluids exist at a pressure higher than the critical pressure and a temperature above the critical temperature. Therefore, if you are looking at a phase diagram SCF are located in the area above the Pc and to the right of the Tc.

Here is a link that shows a typical phase diagram http://images.google.com/imgres?imgurl=http://www.geology.uiuc.edu/~kalinich/scf-sm.jpg&imgrefurl=http://www.geology.uiuc.edu/~kalinich/scf.html&h=219&w=300&sz=15&hl=en&start=2&tbnid=RbLNGpDmq4WwyM:&tbnh=85&tbnw=116&prev=/images%3Fq%3Dsupercritical%2Bfluids%26svnum%3D10%26hl%3Den%26sa%3DN

I think that it is important to define both critical pressure and critical temperature since it may have been awhile since the last time you considered them. The critical temperature is the temperature at which the substance exists as a gas and can not be condensed to a liquid no matter what the pressure is. The critical pressure is the least amount of pressure that has to be applied at the critical temperature to liquefy a gas. Therefore, on the phase diagram they both exist at one point called the critical point.

In class I asked Dr. Norton if she had ever seen a SCF because I did not really understand how they existed or what they looked like. After class I did a little research and found some interesting stuff that might help you better understand what a SCF looks like and what it does.

As we already pointed out, to make a SCF you would need to have a very strong container capable of withstanding large pressures. For the sake of argument I will be using water as the compound being changed into a SCF because it is an easy compound for us to identify its phases at different temperatures. As you heat the water it will begin to form water vapor. The pressure will continue to increase as more gas is created. At this point two phases can be seen, which means the Tc has not be reached and the gas is in equilibrium with the liquid phase. However, as heating continues the meniscus starts to become less discernible and the phases begin to become less distinguished, until finally, the critical temperature is reached and assuming the Pc has also been reached you have a SCF. The SCF appears as one phase, that means all of a sudden you went from having two phases (liquid and gas) to only having one phase (SCF).

Now, to answer the question of what a SCF looks like. SCFs look like gases, but they behave like both gases and liquids. Some of the liquid properties of a SCF are that it has a density very similar to a liquid (much higher than a gas) and it can act as a solvent. However, its most distinguishing gas property is that it is unable to form a distinct surface to separate it from a vapor phase.

Here is a link to what they look like. http://images.google.com/imgres?imgurl=http://www.chem.leeds.ac.uk/People/CMR/images/phase2.gif&imgrefurl=http://www.chem.leeds.ac.uk/People/CMR/whatarescf.html&h=635&w=707&sz=9&hl=en&start=11&tbnid=tg42YA6tDu6DLM:&tbnh=126&tbnw=140&prev=/images%3Fq%3Dsupercritical%2Bfluids%26svnum%3D10%26hl%3Den%26sa%3DN
T
hese fluids are useful because they offer a way to replace harmful solvents, and they are currently being used for extracting caffeine from coffee and as a means to replace CFCs.

Lastly, I have a hypothesis of what is going on at the molecular level of a SCFs and I would love to hear other opinions on this matter so please comment if you have your own hypothesis. My understanding leads me to believe that though there are not two distinguishable phases there still may be both a liquid and a gas phase present in a SCF, but they are in a homogeneous mixture. I believe that the phase, SCF, is really a state of dynamic equilibrium where intermolecular bonds are constantly being made and broken between the molecules (water in this case). As we already know, the difference between why something is a liquid versus a gas is the strength of the intermolecular bonds. Imagine if you had a sealed bottle of water, the water vapor would be in equilibrium with the liquid water, but there would be two phases that you could distinguish easily. Now shake the bottle, and as you are shaking it notice that the layers become much less discernible. Why did the layers become harder to identify? Because you input a very small amount of energy in the form of shaking, which mixed the phases, but as you stopped inputting energy that energy dissipated and the phases became distinct again.

Now imagine if you put in enough energy to pass the Tc and maintained enough pressure to be above the Pc. The example I gave of shaking would now be on the molecular level and would be much more vigorous. The energy from the supercritical temperature would constantly be breaking the intermolecular bonds sending the molecules flying apart from each other making them a gas, but the incredible pressure would be there to force other molecules with low enough energy back together in the form of a liquid. Because as soon as the heat energy breaks the molecules apart they immediately start to loose energy and will eventually comply with the forces of the pressure. This constant and extremely rapid cycling of liquid to gas phase causes the two phases to be indiscernible similar to when you shake a water bottle.

So if you follow me up to this point then congrats because I barely understand what I am writing versus what I am thinking. It's one of those things that just came to me, and you want to try and explain it before you can't understand it anymore. So thanks for reading my post and let me know what you think.