Real Nanoparticle Ferrofluid From Commonly-Available Materials

Intro: Real Nanoparticle Ferrofluid From Commonly-Available Materials

In the world of chemistry and material science, few substances captivate the imagination quite like ferrofluid. This mesmerizing liquid, renowned for its ability to defy gravity and morph its topology in response to magnetic fields, has fascinated scientists, artists, and enthusiasts alike since the 1960s. Its unique properties make it a versatile tool in various applications, ranging from cutting-edge technology to captivating works of art.

Unlike many of the other "ferrofluid" tutorials around the internet, over the past two or so years, I've worked on a procedure to create real ferrofluid - nanoparticles of surfactant-coated magnetite suspended in a nonpolar medium - that could be made by almost anyone, and without the need of purchasing expensive specialized tools or uncommon chemicals. The procedure is now at a point where all that can be achieved; small batches of high-quality, non-degrading ferrofluid can be made from basic lab equipment and reactants that can be readily found online or in most local hardware and grocery stores.

STEP 1: Procedure Overview

A great explanation of how a true ferrofluid works can be found in a video on the channel "Fluids and Soft Matter UvA." Essentially, a ferrofluid consists of incredibly small magnetite particles, with diameters often 1/100,000th of a millimeter. These magnetite nanoparticles, which are ferromagnetic (attracted to a magnet but not magnetic itself), are then coated in a chemical which prevents them from grouping together or settling out of their carrier fluid. In this synthesis, the magnetite nanoparticles are coated in a fatty acid (oleic acid), which has a polar (hydrophilic) head and a nonpolar (hydrophobic) tail. The polar head is attracted to the magnetite particle while the nonpolar tail sticks out. With enough of these nonpolar tails, the entire particle acts nonpolar, and so osmotic pressure and Brownian motion within a nonpolar solvent like Kerosene are able to prevent these particles from interaction or falling out of solution, as if they were "dissolved." Thus, a stable colloid is formed and ferrofluid appears as a liquid, despite a large portion of its mass consisting of what would normally be a common type of rock.

How does this procedure make these specialized nanoparticles?

The procedure in this Instructable follows two main steps in forming these nanoparticles:

1. Forming magnetite nanoparticles via chemical reduction
2. Apply a fatty acid surfactant to form nonpolar nanoparticles

As seen in figure 1 of step 1, the solution in the reaction vessel consists of a ratio of two iron salts: ferric (Iron (III)) chloride (FeCl3), and ferrous (Iron (II)) chloride (FeCl2). These form a ratio of 2:1 because magnetite, Fe3O4, is made of two Fe3+ ions and one Fe2+ ion. This ensures that all the iron will be converted to magnetite, and not other non-ferromagnetic side products. To this (rapidly mixing) solution, ammonia (NH4OH) is added through a spray bottle. This is to disperse the ammonia as much as possible and in combination with the rapid mixing, form as small of particles of magnetite as possible

As seen in figure 2 of step 1, the ammonia reacts with the iron chlorides, forming magnetite (Fe3O4) and a large amount of ammonium chloride (NH4Cl) salt. This salt is essentially inert and is left in the solution throughout the duration of the procedure. Half of the hydroxide ions (OH-) that were originally bonded to the ammonium ions (NH4+) split to become oxide ions (O2-) and free protons (positively-charged hydrogen (H+) ions). The protons bond with the other hydroxide ions to form water (H2O), and the oxide ions form ionic bonds with the Iron ions to create the magnetite.

Figure 3 of step 1 represents the now-formed magnetite particles in the solution, which must be kept mixing until they're stabilized with surfactant in the second step.

As seen in figure 1 of step 2, an ammonium oleate soap, which is water-soluble because of its negatively-charged carboxyl group, has been added to the magnetite solution, with a single magnetite nanoparticle shown. After allowing the soap to mix with the magnetite nanoparticles, hydrochloric acid (HCl) is added to the solution. There is no strong interactions between the magnetite nanoparticle and the soap, because the soap has not yet reacted with the hydrochloric acid to become a fatty acid.

As seen in figure 2 of step 2, the hydrochloric acid has reacted with the ammonium oleate soap. This has formed more ammonium chloride, which remains aqueous, and replaces the ammonium ion bonded to the charged oleate with a hydrogen. By removing the charge on the carboxyl group of the oleate, the head of the fatty acid becomes less hydrophilic, and because the tail of the oleic acid is hydrophobic, it no longer becomes water soluble. Because the head is slightly more polar, it binds to the surface of the magnetite nanoparticles, causing the tails of the oleic acid to stick out. With enough of these tails, the nanoparticles are essentially hydrophobic, or nonpolar, themselves, and the particles are isolated from the rest of solution.

Figure 3 of step 2 represents the now-stabilized magnetite particles. These can then be isolated and combined with a nonpolar liquid which, unlike water, interacts with the tails sticking out of the nanoparticles, and allows the particles to be suspended.

STEP 2: Preparation of Ferrous Chloride Magnetite Precursor

Do this procedure if you only have Ferric Chloride available:

1.   In a 50mL beaker, add 0.02 moles FeCl3 (3.24g anhydrous, or 5.40g hexahydrate, depending on the hydrate form), and add 20mL distilled water to make a 1M concentration solution.

2.   Weigh out an excess of steel wool (>0.01 moles or >0.55g). Add this to the 1M FeCl3 solution. Mix or let sit until the solution becomes a light green color and no more steel is consumed. It is recommended to stir to speed up the process. In this reaction, the FeCl3 is being reduced to a green-colored FeCl2. However, this reaction will slowly revert with exposure to oxygen in the air, forming various iron hydroxides and oxides in solution.

3.   Remove excess metal by filtration or careful decanting if needed and add more distilled water until there is 30mL of solution, now a 1M solution of FeCl2. Place a low surface area iron source, like a nail, in solution until the rest of the precursor solutions are made to keep the iron at a 2+ oxidation state.

4.   This solution will now be denoted as “Solution A.”

Alternative Preparation if you have only Ferrous Chloride:

1.   Add 3.80g anhydrous FeCl2 to 20mL of distilled water, stir until dissolved.

2.   Add more distilled water until the volume reaches 30mL.

3. Solution Should be a green-ish color. Place a low surface area iron source, like a nail, in solution until the rest of the precursor solutions are made to keep the iron at a 2+ oxidation state.

4.   This solution will now be denoted as “Solution A.”

STEP 3: Preparation of Ammonium Oleate Surfactant Precursor

1.   Add 5g of oleic acid to a 100mL beaker.

1.   Add 40mL of distilled water.

3.   Add 10mL of 10% ammonium hydroxide (by mass), or equivalent if maximum concentration is lower

4.   Stir until a thick, soapy, homogeneous mixture forms.

5.   This mixture will now be denoted as “Mixture B.”

STEP 4: Preparation of Hydrochloride Acid Surfactant Activator

1.   Prepare ~150mL of 5% hydrochloric acid (by mass) in a 200mL beaker from available concentration of hydrochloric acid and distilled water. More concentrated hydrochloric acid will need to be diluted down to the appropriate concentration. This volume is approximate because this solution is used to neutralize the reaction solution.

2.   This solution will now be denoted as “Solution C.”

STEP 5: Magnetite Nanoparticle Synthesis

1.   Add 9.73g of anhydrous FeCl3 (or 16.21g hexahydrate) to ~450mL of distilled water in a 1000mL beaker - (the greater volume, the better. Larger containers and therefore greater amounts of water lead to lower solute concentration, potentially decreasing nanoparticle size and therefore the quality of the product. Feel free to use larger container if the stirring apparatus is able to support it).

Alternative Preparation

1. If only FeCl2 is available, add 7.61g of anhydrous FeCl2 (or 11.93g tetrahydrate) to 43.8g of 5% hydrochloric acid. Slowly add 34.1g of 3% hydrogen peroxide so that the FeCl2 will oxidize into FeCl3. Alternatively, instead of the hydrogen peroxide, air can be bubbled through the solution until the solution no longer changes colors. It should go from a green color to a dark brown or orange. This solution can then be added to to ~400mL of distilled water in a 1000mL beaker - (the greater volume, the better. Larger containers and therefore greater amounts of water lead to lower solute concentration, potentially decreasing nanoparticle size and therefore the quality of the product. Feel free to use larger container if the stirring apparatus is able to support it).

2.   Add 30mL of Solution A to make a 2:1 ratio of FeCl3:FeCl2 – (effective ratios may range from 2:1 to 1.7:1, so the ratio given may not be exact for the conditions of every synthesis. It varies based on the expected amount of additional oxidation of FeCl2 that occurs while mixing. Most oxidation can be avoided by completing the magnetite synthesis quickly).

4.   Prepare ~60g (42.6g stoichiometrically, [0.24M of NH4OH] rounding up for excess) of a 10% solution of ammonium hydroxide (by mass) and dilute to about 100-200mL. Add it to a spray bottle and set nozzle to a fine mist.

5.   Cover the beaker if possible, and using a (preferably mechanical) stirrer, begin stirring the iron chloride solution as quickly as possible without causing significant surface splashing (attempting to prevent further oxidation of the Fe2+ ions into Fe3+ from dissolving oxygen gas).

6.   Using the spray bottle, quickly add the ammonium hydroxide, attempting to distribute the mist across as much surface area of the solution as possible. Color should transition from light yellow/orange to dark orange to brown, to black, becoming increasingly opaque. This step should be completed in under 2-5 minutes. Ensure pH of solution is above 9-10 and stop even if the bottle isn’t empty when the target pH is reached.

7.   Add the prepared Mixture B, ensuring it mixes well with the solution. Allow it to mix for 2-3 minutes or until the solution is visibly homogenous, whichever occurs later.

8.   Ensuring no visible changes are occurring, begin adding Solution C via spray bottle in the same way ammonium hydroxide was added. Fumes of ammonium chloride will be generated, but are harmless. As pH decreases below 7, the solution will visibly change. Black precipitate should begin coagulating and separating from solution, leaving a cloudy but colorless solution (of unbonded oleic acid and ammonium chloride). Continue adding hydrochloric acid until pH is around 5 to ensure all ammonium oleate has been destroyed, at which point turn off stirring.

9.   The nanoparticles are now stable, and synthesis is complete.

STEP 6: Nanoparticle Isolation

1.   Using a large neodymium magnet, gather all the stabilized magnetite nanoparticles to the base of the reaction beaker, and pour off any solution. Avoid pouring any magnetite if there is too much for the magnet to handle.

2.   Fill the beaker with ~200mL distilled water and mix magnetite to wash it and dilute the remaining solution.

3.   Repeat steps one and two 2-3 more times.

4.   Allow magnetite to mix with water in the beaker. Pour about a third into an appropriately sized beaker. Using the same magnet, hold it to the bottom. Pour off whatever doesn't stick in 5 seconds to another similar beaker. Repeat with that beaker but pour the rest into waste container. Wash both beakers and return the rest to the first smaller one.

5.   Repeat this process 2-3 more times, shortening the time allowed for the magnetite to stick. This is to ensure poor quality magnetite does not wind up in the final batch, if at the cost of some yield.

6.   Once clean, mix and pour the remaining magnetite into a beaker designated for the clean magnetite.

7.   Repeat steps 4-5 twice with the other two thirds of magnetite from the original beaker.

8.     Once all magnetite is in the designated beaker, allow it all to stick to the magnet, then pour off water and keep upside down over an absorbent towel for 5-10 minutes.

9.   Turn back over, and wash with a small amount 90%+ isopropanol. Solution should be slightly cloudy, as the isopropanol is removing some of the excess surfactant. Pour off into a separate waste container (if isopropanol cannot be disposed of in the same way as water, or if it is planned to be recycled). You will see that the magnetite appears to clump together less, and will form a smooth surface around the magnet

10. Repeat step 9 twice more.

11. Allow magnetite to dry completely. The ideal drying method is under vacuum but leaving it in the sun for a few hours is sufficient. Prevent exposure to high heat.

12. Magnetite should now be a slightly sticky or powdery material. It is safe for storage or use at this point, and shouldn't degrade over time.

STEP 7: Ferrofluid Synthesis

1.   To a shallow glass or sacrificial plastic dish, add a 1:1 ratio (by mass) (just an estimation, likely not ideal) of magnetite and kerosene.

2.   Use a glass stir rod to attempt to "dissolve" the magnetite. Kerosene will become black even if it hasn’t absorbed the maximum amount of magnetite. This process usually takes a few minutes.

3.   A magnet may be used below the dish to mix further, and spiking/viscosity quality may be determined at that time. Once most magnetite has been dissolved, finished ferrofluid may be transferred to a glass container.

STEP 8: Final Notes

Keep in mind Kerosene's high flammability. Kerosene can also induce expansion and damage to most rubber-based materials, can chemically damage or weaken plastic, and can create fumes, so storage must be in an appropriate container (such as a jar with metal lid or a test tube with rubber stopper kept vertical). Ferrofluid is extremely messy and will stain most materials it contacts. It will stain plastic and glass black, brown, or orange, and will be absorbed by fibrous materials, leaving them black and moderately ferromagnetic. This ferrofluid shouldn't degrade over time, nor should it need to be re-mixed if it's left undisturbed for long periods.

Now that you've completed the synthesis, have some fun with ferrofluid! Pour it into a glass container and watch as magnets manipulate its mesmerizing spikes and curves. Cover a small magnet with ferrofluid for an even more captivating interaction. Take your experimentation further by observing how this incredible substance behaves on different iron objects in a magnetic field or create a desktop display by following some of the various tutorials on Instructables or around the internet.

Comments

[sbaniruddh1]

How much surfactant should i use per mole magnetite, beacause i'm unable to find a stochiometric eq. for this

[nothinbutluck]

Apologies that I've taken so long to reply. This procedure is designed for non-polar surfactants, or at least ones that aren't water soluble. I do know citric acid is used in some methods as a surfactant, but I doubt it'd be a swap-in replacement for oleic acid here. Best of luck in your endeavors, though!

[nothinbutluck]

Thanks for asking! Unfortunately there's no simple way to find the perfect ratio of surfactant because it acts as a coating for a small particle made of many formula units of magnetite, and so any variation of the particle size will change the ratio of surface area to covered volume. Luckily, you can add an excess of surfactant, as it will be more than enough to coat the particles, and any extra surfactant will separate from the solution when it is reduced from the soap form back to its oil form. Excess that remains on the particles will also be able to dissolve in the carrier solution, and shouldn't significantly impact ferrofluid quality. You may experiment if you like, but I recommend scaling my quantities to the size of the reaction you're doing. You should be able to get pretty good results.

[sbaniruddh1]

Thanks! I was planning on experimenting with other surfactants like citric acid to get water soluble ferrofluid, so do the quantities change (as the molar mass changes)? Anyways, il try adding the same number of moles of surfactant for now.

[DoctorTronik]

You say using more water results in "decreasing nanoparticle size and therefore quality of the product", but then it sounds like you recommend using a larger container and more water, so here's my question: are smaller nanoparticles more desireable?

Thanks!

[nothinbutluck]

Smaller nanoparticles are indeed more desirable. The smaller the particles are, the easier it is for forces at a really small scale to keep the particles dispersed, meaning the ferrofluid won't settle over time. When working at a nanometer scale, things like Brownian motion and intermolecular forces significantly influence particles which may only contain a few thousand atoms.

[DoctorTronik]

I meant to say thank you for the clarification, but I didn't end up making the ferrofluid like I'd planned and forgot about it. I was trying to visualize some magnetic fields for a project because they were somehow interfering with each other and I just couldn't picture what was going on, I'd tried iron filings in different media, I tried those sheets of stuff that show magnetic fields, these methods and more, but nothing was precise enough. As a last resort I was going to try looking at ferrofluid, but as I was trying to figure out where to borrow some pyrex I realized that I was too far down the rabbit hole and needed to stop trying to solve my problem, and just come up with a different way to do the thing I wanted to do.

I wish I had some sort of meter in my HUD that showed me how much time/effort I'd devoted to a certain task, so I'd know when to stop.

[flash3b]

In step 6, at the end of number 12 you say "and should degrade over time". Did you mean "shouldn't degrade"?

[nothinbutluck]

Good catch. You're right, and I'll correct that real quick.

[richg99]

I admittedly know NOTHING about chemistry. However, I did read this article just prior to reading about your project. Is there any tie-in? https://phys-org.cdn.ampproject.org/v/s/phys.org/news/2023-07-environmentally-friendly-nanomaterial-revolutionize-electronic.amp?amp_gsa=1&_js_v=a9&usqp=mq331AQGsAEggAID#amp_tf=From%20%251%24s&aoh=16901011268669&csi=0&referrer=https%3A%2F%2Fwww.google.com&share=https%3A%2F%2Fphys.org%2Fnews%2F2023-07-environmentally-friendly-nanomaterial-revolutionize-electronic.html

[nothinbutluck]

Thanks for sharing that article! I hadn't read it until now, but their discovery seems to operate off the same idea of a nanoparticle suspension in some sort of liquid similar to what's used in ferrofluid. Beyond that though, the material in the article seems to have a very different composition than what I achieved, and seems to serve a very different purpose than ferrofluid. The article claims that the nanoparticles can somehow generate electricity from light like a solar panel, which I doubt you'd be able to achieve with ferrofluid. I suppose it just goes to show how widely applicable nanomaterials are!

[flash3b]

The link above would not work for me. But I found the article in question at:
https://phys.org/news/2023-07-environmentally-friendly-nanomaterial-revolutionize-electronic.html