Paramagnetic fluids optimization of the chemical synthesis of magnetite ferrofluids

Ferrofluids are paramagnetic liquids. The fluid is made up from a stable suspension of magnetic nanoparticles. This study investigates the synthesis of magnetite based ferrofluids with focus on the parameters of the precipitation of magnetite particles using iron(II), iron(III) and ammonium ions. Under the hypothesis that the precipitation speed and pH value had major influence on the magnetic properties of the nanoparticles, two different experiments were designed and later analyzed using a proprietary magnetism test and X-Ray Diffraction and Scherrer\’s algorithm.
Ferrofluid is a general term for stable suspensions of ferro- or paramagnetic nanoparticles. The particles are suspended in the carrier liquid with the help of a surfactant, which coats the particles and allow them to be dissolved in certain liquids. The most noticeable property of most ferrofluids is it\’s ability to align itself in 3D according to the magnetic flux lines of an arbitrary magnet((S.W. Charles, The Preparation of Magnetic Fluids (2002))).
Ferrofluids have numerous uses, ranging from adjustable shock absorbers to future applications of precise drug delivery systems to target cancer cells or other inaccessible diseases((Ch. Alexiou, R. Schmid, R. Jurgons, M. Kreme, G. Wanner, Ch. Bergemann, E. Huenges, T. Nawroth, W. Arnold, F.G. Parak, Targeting cancer cells: magnetic nanoparticles as drug carriers (2006))).
The synthesis of ferrofluids is divided into two stages:

  1. Precipitation of nanoparticles and,
  2. Coating of the nanoparticles.

Each stages of the synthesis can be performed independently of each other.
The chemical composition of the nanoparticles, the surfactant as well as the carrier liquid can vary greatly. Magnetite (Fe 3 O 4 ) particles was synthesized in this study using Fe 2+ and Fe 3 + and NH 3 was used to increase the pH value, as it has a pKs value near the pH value slightly above that required for the precipitation of Fe 3 O 4 .
The reaction for the precipitation follows the following equation:
2F e Cl 3 + F e Cl 2 + 8NH 3 + 4H 2 OFe 3 O 4 + 8NH 4 Cl
To optimize the synthesis of a magnetite based ferrofluid I assumed that the magnetic properties of the nanoparticles were largely dependent on two parameters:

  1. The size of the particles
  2. The internal composition or structure of the particles

It was hypothesized that these factors are controlled by:

  1. The end pH value, controlled by total amount of NH 3 . My hypothesis is that the pH value will control various aspects of the precipitation, such as the crystal structure or particle size.
  2. The precipitation speed, as this will control local pH values because NH 3 drops will have less time to be mixed with the solution. My hypothesis is that a higher speed will result in larger particles.

The overall magnetic properties of the ferrofluids was assumed to be dependant on the following parameters:

  1. The magnetic permeability of the particles
  2. The concentration of the nanoparticles
  3. The surfactant
  4. The carrier liquid

Methods and materials
To determine how each parameter effected the properties of the ferrofluid an experimental design was invented to allow variations of only one parameter at the time. With the same NH 3 solution, four identical mixtures of Fe 2+ and Fe 3 + in the ratio 1:2 was precipitated using 25, 50, 100 and 200mL NH 3 at equal speed and stirring.
The examination of the precipitation speed presented a problem however. Due to limited laboratory equipment there were no way of properly controlling the flow rate of the NH 3 solution with the separation funnel that was used. The problem was solved by presuming that the total amount of H 2 O had no impact on the final result. This made it possible to set the separation funnel at a constant setting, while controlling the concentration of the NH 3 solution. Thus, the same amount of NH 3 was used in the different samples, but at different precipitation speeds. For example, 100mL of NH 3 solution would use 1 minute to finish, while 200mL solution would theoretically use 2 minutes. With the same amount of NH 3 in both solutions, the flow rate of NH 3 would be twice as fast in the first one, as in the last one.
\”Relative precipitation speeds\” of 25, 50, 100 and 200mL were used to drip 1,3g NH 3 into the ferrum solution.
Due to another unavailability of proper measuring equipment, a method was designed to analyse the paramagnetic properties of the obtained samples. A magnet and a sample were placed on a scale, while slowly moving the magnet towards the sample. The distance the sample jumped when the magnet got close enough was used as a measure for the paramagnetic properties of the sample. This was tested three times to assess the experimental error and to calculate the standard deviation on the graphs.
To determine the effect of the precipitation speed on the average particle size, an X-Ray Diffraction (XRD) analysis was performed and by using the Scherrer algorithm. This experiment was performed in collaboration with the Technical University of Denmark.

[Figure 1] shows a reduction of magnetic permeability, as more NH 3 is added.

[Figure 2] shows a link between magnetic permeability and particle size, and the particle size descending the slower the particles are precipitated.
The analysis of the end pH value shows a decrease in the magnetic momentum as more NH 3 is added. The could be the particles oxidizing at a higher pH value, which would also explain the plateau from 2 to 4g NH 3 , where all the particles have oxidized.
The analysis of the precipitation speed confirms my hypothesis that a slower precipitation speed will lead to smaller particles having a smaller magnetic momentum.
The particle size seems to keep on rising the faster the precipitation speed is, but at a certain point the particles will be so big, that they will fail to suspend in the carrier liquid resulting in a ferrofluid with less or no paramagnetic properties.
In my experiments I have shown that higher local pH values causes a larger particle size, and I have shown that a lower pH value is better to optimize the crystal structure and particle size.
Future perspectives
In the future I plan to conduct more tests to find the best end pH value. There are also a lot of different coating and precipitation methods to try out to compare and optimize to get an even better ferrofluid. I also plan to use the particles for drug targeting.[8]
About the author
The project started at a special theme day at school. The theme was nanotechnology and we were shown videos of this amazing fluid making brilliant sculptures with the flick of a switch. It clearly made my day when I heard that we were going to make this liquid.
A few – or 7 – hours later we had stained just about every labware in both school labs with this annoying rusty thing, and all we were left with was this brown/grey slush which did in fact not look like the one on the videos at all. All this made me wonder what went wrong, and I started to research the subject more elaborate. A few days later when we had to choose a subject for our project, I knew it had to be ferrofluids. This allowed to make this awesome fluid, and playing around with magnets, all while there were a lot of great perspectives within the project, ranging from day to day technology in Ferraris to space technology and curing cancer. Everything accessible with the lab equipment available at the school.
A few weeks in the project though, I wanted to analyze my particles in detail. This did not present a very large obstacle however, due to the fact that my school is located on the campus of the Technical University of Denmark, where students or even professors gladly help high school students with their projects. I first set out to find a TEM machine to take an \”actual\” picture of my nanoparticles, but after a talk with a lecturer we decided on X-Ray Diffraction and Mossbauer spectroscopy if the time would allow.
When I was told that I should attend the national contest for Danish young scientists, my heart skipped several beats. As the competition neared I prepared my presentation with poster, slides and so on, while also doing some more work on the project.
The resulting 2 nd place in the contest unlocked 8000DKR and a trip to China to attend the national Chinese contest for young scientists, CASTIC. Again, I had to prepare a presentation, only this time in Chinese. Unfortunately, my Chinese were a bit rusty, so I made it in English and had my abstract translated by Young Scientists Denmark.
I held summer vacation in New York this year (wonderful city), and due to poor planning it somehow overlapped with the trip to China, so I ended up flying 9 hours back to Copenhagen, waiting 9 hours in a dead boring lounge, meet my fellow Danish participants (there were two projects that won this trip), and then flying 9 hours to Beijing only to find out it\’s morning and I was some 40 hours behind my sleep schedule. I slept rather well that night.
Beijing was awesome, and fortunately we had arrived just before the city was tainted with tourists visiting for the Olympics, but close enough to the Games, that the Chinese had adapted to western culture. A bit, Luckily we had some resident students to guide us around and show us the most important stuff.
A few days later we flew 4 hours to north-west China, to Urumqi in the Xinjiang province. There were around Chinese 500 participants and 39 international participants. In the end i brought home an award for outstanding international research, and a whole lot of wonderful memories.

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