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.

  • $\begingroup$ So, I've been trying to make Ferrofluid with powdered Iron (III) Oxide (Synthetically created, 1 micron) using Citric Acid, Heat, and Water. It didn't work. I'm trying the same thing with Ammonia today, if it works I'll let you know! $\endgroup$ – Daniel Sep 25 '17 at 17:24

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