Can I make Ferrofluid without evaporating Ammonium?

Question

I want to make ferrofluid, which is ferromagnetic particles suspended in a carrier fluid. Three methods I have seen involve using magnetic ink (for MICR codes), cassette tapes, or ferric chloride as the iron source. The last method is the only one that makes ferrofluid with nano-scale ferromagnetic particles. AFAIK, the process boils down to using ammonia to precipitate magnetite from a ferric/ferrous chloride mixture, coating the precipitated particles with oleic acid (acting as a surfactant), and suspending the coated particles in a carrier oil such as kerosene.

The steps (based on this):

1. Oxidize iron with ferric chloride to form ferrous chloride: $\ce{2FeCl3 + Fe -> 3FeCl2}$
2. Precipitate magnetite: $\ce{2FeCl3 + FeCl2 + 8NH4OH -> Fe3O4 + 8NH4Cl + 4H2O}$
3. Add oleic acid, evaporate the ammonia, and end up with surfactant coated magnetite nanoparticles?
4. Add kerosene to magnetite suspension => ferrofluid

There is another example here that has slightly different steps:

1. Dissolve $\ce{Fe^2+}$ and $\ce{Fe^3+}$ in $\ce{HCl}$ and mix
2. Precipitate magnetite
3. Repeatedly:
   1. Separate precipitate from liquid with a magnet
   2. Decant the liquid, leaving the precipitate
   3. Add distilled water
4. Add $\ce{N(CH3)4OH}$

It seems to me that the first process generates a 'smoother' ferrofluid. I mean to say that it looks more like a homogenous fluid, whereas the ferrofluid in the videos from the second link looks more like a sludge.

I have two questions. First, what is actually happening in the first process when the oleic acid is added? From the first article, it sounds like the oleic acid reacts to form ammonium oleate, and then adding heat decomposes the ammonium oleate, releasing ammonia gas and dissolving the oleate ions, which then coat the magnetite particles. But that sounds a bit odd. Second, would the following work (create a suspension of magnetite nanoparticles suspended in a carrier):

1. Obtain a mixture of $\ce{FeCl2}$ and $\ce{FeCl3}$ via either method.
2. Precipitate magnetite
3. Separate the magnetite precipitate from the ammonium chloride via decanting (as in the second process)
4. Add oleic acid (and heat?) to coat the magnetite particles
5. Add kerosene to magnetite suspension => ferrofluid

It seems like removing the ammonium chloride via washing would be easier than via evaporation. Less heat, less time, less nasty fumes. But I don't really know what's going on when the oleic acid is added, so I'm not sure if my proposed method would actually work.

Answer 1

To answer your first question, I believe that the ammonium oleate already being a relatively soluble salt allows its deprotonation such that the resulting oleate ions are able to bond to the magnetite in suspension (both by electrostatic attraction and hydrophobic interactions). Subsequent heating is done simply to remove dissociated $\ce{NH4Cl}$ from the water in the form of $\ce{NH3}$ and $\ce{HCl}$ gas. Might be worth noting that your initial addition of aqueous $\ce{NH3}$ (aka $\ce{NH4OH}$) for the co-precipitation is in excess. A video by NileRed suggests neutralising this excess ammonium (hydroxide) instead with the use of hydrochloric acid, instead of using high heat which would expel a large amount of nasty fumes. Cleaning of the magnetite can then be done with deionised water to remove the dissolved salts.

To answer your second question, no, adding oleic acid post-decanting would not work. Using the logic I've stated, adding pure oleic acid would not coat the magnetite particles, as oleic acid by itself will not deprotonate in water until it has reacted with ammonia to form the ammonium oleate soap.

Even if you were to add aqueous ammonium oleate, doing so post-decanting doesn't seem logical; normally, surfactants are added as soon as the magnetite has fully precipitated, without stopping the stirring, because you'd want the surfactants to coat the magnetite nano/microparticles (MNPs) while they are still dispersed (before they begin aggregating). In your case, having the intermediate decanting step will definitely introduce excessive, irreversible magnetite aggregation and the resulting MNPs will not be of high quality. Anyways, as previously mentioned, the decanting step is not necessary because the ammonium chloride salt will not contaminate your final magnetite sample, considering that it can be easily washed with deionised water, and even short-chain alcohols to remove excess oleic acid.

To summarise, your last paragraph is correct in saying that ammonium chloride can be rinsed more easily from the magnetite sample than removed through evaporation. Using an acid might be a good way to do it as proposed by NileRed.