Supercritical Fluids

Last week, worked picked up nicely and I got a lot of good stuff done, so I thought I would write a bit more about my science.

I talked a bit last about how I am working with supercritical fluids and what they are. I don't think I talked too much about why they are so useful though. First, they have the properties of both a liquid and a gas: they can dissolve solids like a liquid, but can also diffuse through solids like a gas. Additionally, the most commonly used SC fluids are water, ethanol, and carbon dioxide, which are a lot less harmful than the organic solvents used in lots of typical liquid reactions. Finally, synthesis performed with SC fluids are very "tunable" in that very small changes in pressure or temperature change the density of the fluid a lot, which can really affect the products of your synthesis. This is mainly because the solubility of the materials in the SC fluid depends on the density of the fluid. If the SC fluid is more dense, the material will be more soluble and thus will have a better chance of interacting (undergoing a chemical reaction) with any other chemicals you put in there. Obviously, if you increase the pressure on the system, the density is going to go up. Temperature is a bit trickier, but if you increase the temperature the solubility will generally increase (this is not true right by the critical point, but it's pretty true everywhere else). There are also other synthesis properties you can vary, but I'll talk about those a bit more later.

Some of the more interesting uses of SC fluids are decaffination of coffee beans, extraction of chemicals from hops for beer production, dyeing, and biodiesel prodution. Of course, there is also nanoparticle formation, which is what I am investigating (it's also used a lot in the pharmaceutical industry). SC fluid synthesis is very good at creating very small particles in a very narrow size range. This is because of the way crystals grow. The starting step of crystal growth is called "nucleation", which is the very first bond that is formed between the atoms. After nucleation, more and more bonds and atoms are added to this first growth site, to form a larger crystal. In this case, in order for crystals to form, the SC solution needs to be what is called "supersaturated" with the crystal material. This means that the amount of material dissolved in the solution is more than the solubility limit. Most of the time, this is accomplished by suddenly dropping the pressure or temperature to reduce the solubility limit. By supersaturating the solution at the same time as nucleation occurs, we can cause a whole bunch of nucleation events to occur in a very short period of time. This uses up all of the crystal material, so not much crystal growth occurs, only a lot of nucleation. This results in teeny-tiny crystals that are all about the same size.

My PtBi particles are made by dissolving Pt and Bi compounds in a solvent (water or ethanol), then reacting them at about 320C and 250 bars (about 250 atmospheres; you're feeling 1 atm right now, unless you're reading this while scuba diving or something equally ridiculous). However, there are a lot of things I can vary to create different compounds and different size particles. First of all, I can vary the solvents that I use, which will change the solubility of the Pt and Bi compounds and how well they react with each other. I can of course change the temperature and pressure, but they don't seem to react too well below or above 320C. The Pt and Bi compounds I'm using aren't super soluble in water in the first place, so it also helps to keep the pressure up high. Another thing I can vary is the residence time of the chemicals in the reactor, by controlling the flow rate of the solutions into the reactor. This basically changes how long they are held at that high temperature, which could change the crystal growth rates or even the reaction products.

So, now that you're convinced that I at least know what I'm doing, am I actually doing it? Well, I've had some luck making at least some sort of PtBi compounds, but I'm not sure that I've got the straight 1:1 Pt:Bi ratio I'm looking for. I also know, for sure, that I have some unreacted Pt in there. Multiphase compounds are super annoying; getting your reaction product to be "phase pure" is like the holy grail of materials science. Lately I've been playing around with some of the more subtle variables, like flow rate and solvent ratios, so we'll see what I get out of that.

Coming up next time: characterization! Just how do I figure out what I have made? Stay tuned for the answer...

Wikipedia fun fact: supercritical fluids exist in nature also. Two examples are undersea volcanoes, which are very hot but also under a lot of pressure, and the atmosphere of Venus, which is again very hot but at very high pressures.