Supercritical drying (SCD) is a process by which the liquid in a substance is transformed into gas in the absence of surface tension and capillary stress and is the process most commonly used to transform gels into aerogels. Supercritical drying is performed to replace the liquid in a material with a gas isolate the solid component from the material without destroying the material’s delicate nanostructured pore network.
Supercritical drying is also called:
- Supercritical extraction (SCE)
- Critical-point drying (CPD), and
- Supercritical lyophilization
Supercritical drying/extraction processes are not only used to make aerogels but also to preserve biological specimens (aerobugs?), decaffeinate coffee, and dry clean clothes in an environmentally-friendly way.
How Supercritical Drying Works
The Critical Point
All pure substances that do not otherwise decompose possess a critical point, a specific temperature and specific pressure characteristic of that substance at which its liquid and gaseous phases become indistinguishable.
A Little Bit About Molecular Motion
Molecules/atoms in gases, liquids, and (believe it or not) even solids are constantly in motion to varying degrees. As a substance is heated, the kinetic energy of the molecules of that substance increases and the molecules move around faster. In a liquid, the molecules are close enough together that they experience attractive (sticky) forces between one another. These attractive forces weakly bind the molecules together into little packs or chains but can be easily overcome as the molecules move around and vibrate. The degree to which molecules of a substance stick to each other is called its “surface tension”. The surface tension of water is what allows water skippers to walk on the surface of water. In a gas, however, the molecules are far apart and are not close enough to other molecules to experience attractive forces. Additionally, the molecules in a gas are moving so fast that when two molecules do collide, they pretty much just bounce off each other instead of sticking.
A substance in its liquid phase is are always in equilibrium with a gaseous (or “vapor”) phase as well. For example, if you put water in a two-liter bottle and seal the bottle, the water will evaporate until a certain pressure of water vapor is reached above the liquid in the bottle. Once that pressure is reached, the number of molecules evaporating from the liquid is about the same as the number of molecules recondensing from the vapor into the liquid, a phenomenon which we call “equilibrium”. The pressure of water that will result is exclusively a function of the temperature of the liquid–the hotter the liquid, the higher the pressure. This is why it is generally more humid in the summer than in the winter (depending on how much water there is around where you live). Importantly, the pressure of the water is independent of the pressure of other gases over the water. For example, if you pressurized the bottle with two atmospheres of nitrogen gas, the pressure contributed from the water, its “partial pressure” would be the same, although now it would be a smaller percentage of the total pressure in the bottle since we’ve added more nitrogen.
So now that we recognize that the liquid phase of a substance is always in equilibrium with a vapor phase and that the partial pressure of the vapor phase is only a function of the temperature of the liquid, we can begin to understand the critical point.
When PV So Doesn’t Equal NRT
If you’ve taken high school Chemistry you might remember the famous equation
that is, the pressure of a gas times the volume of its container is equal to the number of moles of substance that make up the gas times the gas constant “R” times the temperature of the gas.
BUT what you might not remember is that this really only holds for low-pressure gases at reasonably high temperatures, in which the molecules are far apart and don’t stick to each other.
At higher pressures, and at tmperatures close to where the vapor is about to recondense into a liquid, this equation doesn’t hold. And so scientists came up with other equations that work better for these ranges, such as the Van der Waals (say “van der VAHLz”) equation.
But at the critical point, even these more complicated equations fall apart.
Imagine we have a liquid in a sealed, rigid container that isn’t going to blow up if we pressurize it. As the liquid is heated, the pressure of its vapor will increase. As this happens, more and more molecules enter the vapor phase, making the vapor more and more crowded and making the average distance between two molecules smaller. Eventually, the molecules in the gas are so close together that they are basically as close as they would be in a liquid. At the same time, as the liquid is heated, the molecules in the liquid get more kinetic energy and start moving faster and faster until they are moving so fast that they scream past one another, overwhelming the weak attractive forces that hold the liquid together. The result is a liquid which can no longer hold together and gas so dense it is as dense as the liquid and the two phases merge into a single phase with the density of liquid that expands to fill its container like a gas. The specific temperature and pressure at which this happens is the “critical point” of the substance and the single merged semi-liquid/semi-gas phase is called a “supercritical fluid”.
Properties of Supercritical Fluids
Supercritical fluids are strange substances. Since they are dense like a liquid, they conduct heat like liquids (if you could touch a supercritical fluid, it would feel kind of like a liquid). But at the same time, supercritical fluids expand and compress like gases. Supercritical fluids often also possess the ability to dissolve stuff that their liquid version cannot. For example, supercritical carbon dioxide exhibits the uncanny ability to dissolve fluorinated hydrocarbons, like some types of Teflon® used in lubricants. Supercritical fluids are infinitely compressible at the critical point and as a result exhibit a significant density gradient due to the weight of the fluid pushing down on itself (they are denser at the bottom than they are at the top).
Surface Tension and Capillary Stress
When the liquid in a porous material such as a gel or a dead bug is removed by evaporation or boiling, capillary stress in the pores of the material cause the struts of the pore network to collapse and the material shrinks. Think of the liquid molecules in a pore as being a bunch of little magnets all stuck together and think of the pore as being made out of flimsy strips of sheet metal. Now imagine pulling on one of the magnets and all of the other magnets getting pulled up with it until you pluck the magnet out from the bunch. The pile then collapses back to fill in the gap from where you pulled out the magnet and pulls the flimsy sheet metal walls of the pores inwards. This is sort of what happens when a liquid molecule is evaporated from a pore–all of the other molecules get pulled in and tug on the struts that carve out the volume of the pore. The more molecules evaporate, the more the struts get tugged on. This is called “capillary stress”. If the liquid has a high surface tension, the molecules in the liquid are strongly attracted and the capillary stress exerted on the pores of the material will be higher.
Supercritical drying entails heating/pressurizing the liquid in a material past its critical point, at which it is transformed into a supercritical fluid. As a liquid approaches its critical point, the molecules in the liquid move past each other faster and faster and stick together less, meaning the surface tension of the liquid decreases. Thus the capillary stress that the liquid can exert decreases as well. Finally at the critical point the supercritical fluid loses all surface tension and can no longer exert capillary stress.
At this point, the supercritical fluid can be removed from the pores of the material by depressuriziing the fluid while keeping it above its critical temperature (isothermal depressurization). If the temperature of the fluid drops below the critical temperature, liquid will start to rain out of the fluid. As pressure is released from the vessel containing all of this, molecules are removed from the fluid to the surroundings as a gas and the fluid becomes less dense. After a while, enough of the fluid will have been removed from the pressure vessel so that when it is cooled below its critical point there just isn’t enough substance to recondense to a liquid (the fluid is too low in density) and instead reverts to a gas.
We have now just sneakily converted what was liquid in the material into a gas without causing any capillary stress, leaving behind the delicate solid component of the material with its pore structure in tact.
The Critical Neighborhood
Technically, any substance above its critical temperature and critical pressure is a supercritical fluid, but if it is much hotter than its critical temperature, the fluid really just behaves like a gas. So actually, only within a range of temperature and pressure near the critical point is the term supercritical fluid useful.
Gas-Expanded Liquids (GXLs)
Liquids that are below but near their critical point will be much lower in density than their normal phase and, when mixed with a small amount of another liquid, can make a “puffed out” liquid called a gas-expanded liquid or GXL. Since they are near their critical point, GXLs have almost zero (but non-zero) surface tension and work a lot like supercritical fluids. They are amazing solvents and can dissolve all sorts of things normal liquids cannot.
In low-temperature supercritical drying of aerogels, gels containing a solvent such as ethanol or acetone in their pores are soaked in liquid carbon dioxide. The liquid carbon dioxide then replaces the solvent and can be supercritically extracted at relatively benign conditions. During the solvent exchange process, however, a GXL mixture of part liquid carbon dioxide, part solvent will result. GXLs look like “gooey” or “puffy” liquids and can dissolve Teflon seals in your pressure vessel, so be aware.
Supercritical Drying of Aerogels
Now that you know how supercritical drying works, here’s how it applies to aerogels:
- A gel is prepared using sol-gel chemistry. The gel contains a mixture of organic solvent and water in its pores.
- The gel is soaked in a pure organic solvent several times over the course of several days to remove the water from its pores.
- Finally, the gel is supercritically dried in a pressure vessel one of two ways:
- The gel is placed in a pressure vessel filled about half-way with the same liquid held in its pores (so if your gel has ethanol in its pores you would fill the pressure vessel with ethanol). The vessel is then sealed, heated past its critical temperature and pressure, and then isothermally depressurized to give aerogel.
- The gel is placed in a pressure vessel and the pressure vessel is filled with liquid carbon dioxide. The liquid carbon dioxide is essentially another solvent that can displace the liquid in the pores in the gel. The gel is soaked in liquid carbon dioxide over the course of several days, flushing new carbon dioxide through every 12-24 h to replace the solvent in the pores of the gel with liquid carbon dioxide. After the solvent in the pores of the gel has been replaced by liquid carbon dioxide, the carbon dioxide is heated past its critical temperature and pressure. The vessel is then isothermally depressurized to give aerogel.
High-Temperature Supercritical Drying
Most organic solvents have relatively high critical temperatures of 300-600°C with critical pressures of 50-100 atm, and are dangerously flammable and potentially explosive at these conditions. As a result, high-temperature supercritical drying needs special safety precautions.
In the case of supercritical drying of silica aerogels, methanol is frequently used as the solvent. At its critical point, methanol can react with hydroxyl groups on the surface of the gel’s solid framework to form methoxy groups. These methoxy groups end up making the aerogels partially hydrophobic (water-resistant) and is the reason why high-temperature supercritically dried silica aerogels are generally higher quality.
High-temperature supercritical drying from organic solvents is also the best way to minimize shrinkage of the gel, allowing for supercritical drying of much lower-density gels than from carbon dioxide.
Certain aerogels, such as organic polymer aerogels, can’t be made using high-temperature supercritical drying because they decompose or react at the temperatures at which most organic solvents are supercritical.
Low-Temperature Supercritical Drying from Carbon Dioxide (the Hunt Process)
Instead of using flammable, explosive solvents, a safer, non-flammable solvent can be used instead–carbon dioxide. In this process, the organic solvent in a gel is replaced with liquid carbon dioxide by soaking. The liquid carbon dioxide can supercritically extracted at a much lower temperature (31.1°C) than an organic solvent and without the risk of combustion.
Certain aerogels can’t be made with liquid carbon dioxide, such as certain metal oxides, because they react with carbon dioxide to form metal carbonates.
When the original solvent in the gel is exchanged for liquid carbon dioxide, the gel may shrink slightly. This is caused by favorable interactions between the two liquids resulting in a mixture with molecules closer together than molecules in either of the liquids separate. This is called ΔV of mixing. Solvents with low ΔV’s of mixing with liquid carbon dioxide include acetone and amyl acetate, although ethanol and methanol are pretty good. Importantly, it is the solvent exchange into liquid carbon dioxide, which has a relatively low density of 0.59 g cm-3 that causes the gel to shrink, no the supercritical drying process. In general, aerogels made using supercritical drying from carbon dioxide may shrink up to 5%.
Critical Points of Various Substances
Source: CRC Handbook of Chemistry and Physics, 85th ed., CRC Press, 2004 <http://www.hbcpnetbase.com>
In English units:
|Formula||Name||Tc (deg F)||Pc (psi)|
In Metric units:
|Formula||Name||Tc (deg C)||Pc (atm)|