# How do I determine which liquids are immiscible with one another?

I am trying to work out which type of ferrofluid I can use to form a weak magnetic seal for a container of liquid photopolymer resin. Is there a mathematical approach to determine what substances will form a solution, and which will remain immiscible, or will I have to buy each ferrofluid and try each one?

• The mathematical approach requires insanely hardcore quantum chemistry and is not all that reliable anyway. You'd better stick to the old good "polar mixes with polar" rule. Jan 16, 2016 at 17:44
• Ah yeah I was afraid that would be the case. All good though, I'll stick with that. Cheers. Jan 16, 2016 at 17:48
• generally, there are three major classes of (mostly) mutually unmixable liquids: nonpolar hydrocarbons, polars like water and lower alcohols and metals (like mercury). I heard that perfluorocarbons are unmixable with hydrocarbons, but can't support the claim on spot. Jan 16, 2016 at 18:11
• That helps heaps, thanks. Now I just need to work out which to make my ferrofluid, and which to make my resin. I have a choice of water or oil for both so I guess I need to weigh up the advantages and disadvantages of each. Jan 16, 2016 at 18:17
• @permeakra Yeah, perfluorated stuff is another "class". Jan 16, 2016 at 18:19

## 1 Answer

If you want high accuracy for solubility, UNIFAC is a good way to go. The basic idea behind it is to break the molecules down into groups then generate correlations for how each of the groups interact. The weighted sum of all the group contributions gives you the total molecular interactions. From there you can get solubility.

The Dortmund Data Bank maintains a comprehensive list of all the group coefficients and offers a program to get the various properties, like solubility. Unfortunately, the free version is pretty limited. You an only access a handful of fluids.

If you want to put in a bit more work, someone has put an Excel version on the internet. It doesn't explicitly do solubility, but basically if the activity coefficients are very far from 1, you can assume they are insoluble.

Furthermore, I'm not sure what the ferrofluids will do. I would guess that the concentrations would be low enough that you could just calculate solubility using the surfactant. Otherwise, you might have to search the literature for how to handle magnetic nanoparticles with UNIFAC.