THF evaporation under partial vacuum

krobben

Long story short I'm making some nanoparticles and one of the steps require me to evaporate THF(tetrahydrofluran) out of an aqueous solution containing the NP's. This needs to be done under a partial vacuum with only one intake valve(I know, it sucks but I have no other option). I'm looking more for an equation/formula that I could maybe integrate over time because, as we all know, evaporation depends partly on the partial pressure which would increase over time as I only have a single intake valve, however, I'm considering opening the vacuum occasionally once partial pressure reaches a maximum to flush it out.

I have no exact number but here are some estimates:
Solution- 8mL H20, 2mL THF, negligible NP composition, no stirring, ~1.5cm^2 surface area.
Vacuum- ~1L, partial vacuum, surrounded by room temperature.
Hope I'm not forgetting anything...
Like I said I'm really just interested in a formula and a clear explanation on how to use it in this case.

gravenewworld

Nobody would ever waste time to come up with a formula for this just to evaporate off 2 ml of thf. For the amount of time spent doing that,you could be done. Just heat the flask your stuff is in a bit if your nps are stable, and keep adding vac slowly, by keep opening and closing the valve. As long as you don't see bumping you're fine.

krobben

Yes, I am aware this sounds completely stupid when we're just evaporating THF. However, in the process of making polythiopene nanoparticles, we go from THF to water because polythiopenes dissolve in THF and colloids in water. This begs the possibility that these NP's lie in an unstable colloid state while in an even dilute THF solution. Now you might say this is too dilute to matter but for my purposes I need these particles to be very, very uniform and precise in their size which has yet to be done to my desired point. So I would like to know exactly whats going on here to the most precision possible.

And I'd rather not heat my solution as that leans towards the mini-emulsion method whereas I'm looking to try the reprecipitation method.

MrSid

You may still be running into the same disruptive forces that heating will create whether you use a partial vacuum and "no heating", or some heating... i.e. there will be gradients at the surface of the "solution" of THF, and water that will form thermal currents. Some combination of forces may then cause aggregation. Does the idea of infinite dilution of the THF work in principle or practice?.. simply adding more H2O and then letting the solution equilibrate might let you see if the aggregation is robust or not. Ideally you could use osmosis to dilute the THF level against distilled water as long the aggregation didn't plug off the dialysis membrane pores. Might be worth a shot; you could check for the presence of plugging by some analytical technique on the washed membrane after.

MrSid

If you are really looking to "study" a phenomena of colloid aggregation with such a setup, you will have to have to remember that you are looking at a process that is not in thermodynamic equilibrium and will have an exchange of heat from the "NP solution" to perform work across the valve of gas expansion. If I surmise correctly one of the reasons that your setup is restricted to a single outlet is because the "NP solution" is in a tube that sits in your analytical apparatus. If that is not adequately thermostatically controlled, it will cool as the evaporation from the surface of the solution, and expansion of the gasses across the valve takes place; your aggregation phenomena wouldn't be a simple equation dependent on solution concentrations in a closed system. The systems involved don't close until equilibrium has been established across all of the systems.

The equations and how they are derived could be found in chemical engineering studies on flash evaporators, but as noted in other posts above why would you want to go to that extent? If you are only trying to get from point A to point B, you might want to redesign the apparatus, or just do the experiment(s).

chemisttree

Based on your method, there is absolutely no way to know how fast it would evaporate. You might use Langmuir's equation to estimate it but you have a two component solution and the molar ratios of the two components are changing continuously. You need to stir the solution or roto-vap it to maintain uniformity within the sample itself and the rate at which you apply dynamic vacuum is not given. I would apply a dynamic vacuum to your sample with stirring to assure that you approach the ppt point very slowly and that the environment in the sample at that point is uniform throughout. Temperature regulation is important as well. The rotovap connected to a dynamic, regulated vacuum source is the best application for your needs.

MrSid

Just for grins I decide to take gen chem principles for ideal gasses; Raoult's Law and the combined ideal gas Laws and applied to the proposed set-up. Sigma Aldrich had a vapor pressure for pure THF at 20C of 143 torr, and Brown Chemistry the Central Science has vapor pressure for pure H2O as 17.5 torr at 20C. (of course H2O and THF act in a strongly hydrogen bonding non-ideal fashion, so the exercise is just a back of the envelope set of calculations;-)

The partial pressures for the first iteration (have to assume that enough heat is input on the "NP solution flask" to keep it at constant 20C-more about this...) was about 24 torr after equilibrating, and the composition of THF and H2O vapors is enriched in THF as one might surmise. Ideal gas law for the 1 L volume of 24 torr "gas" gives way to determine the amount of mass removed from the "NP solution" into the vapor space. It is small (no surprise) about 0.3%... One assumes that after allowing equilibration (measure constant pressure), the system is closed off from the flask of "NP solution" at the valve and the 1 Liter flask is evacuated of the THF/H2O vapor to a hard vacuum,closed and the whole process reiterated.

And yet taking the heat of vaporization for H2O for this quantity of H2O vapor (not even counting the THF thermo) it requires 183 cals to keep the system at 20C. This is about 23C delta for 8 gms H2O (starting solution-again ignoring the THF thermo). The immediate lesson is that keeping the system at a constant temperature will be the most difficult thing. The Buchi accomplishes this with a heating bath and a rapid rotation to keep things well mixed. The good thing is that a typical water aspirator will perform well to keep a roughly 10-20 torr dynamic vacuum. Also the Buchi has a way to feed inert gas into the evaporator flask and keep things away from air.

I put it in a spreadsheet so it was easy to feed each "operation" into the next iteration. 75 iterations later, the "NP solution" lost 28% of the original mass and the concentration of THF in the H2O is about 5 wt %. Hope you have lots of patience (grin;-)

The idea I proposed of using osmosis has the advantage of having good temperature control throughout the THF exchange. If inert atmosphere is the issue, these things can also be worked out (degassed exchange DI H2O, work in a glove bag).