Nanotechnology: Ferrofluids and Liquid Crystals

Objective To synthesize an aqueous ferrofluid (magnetite) and observe its properties. To understand how nanotechnology affects everyday life. To learn about surfactants and how they work.

Grading Pre-Lab (10%) Lab Report Form (80%) TA Points (10%)

Background Information Nanotechnology is the science of controlling matter with dimensions between 1 and 100 nanometers. This includes manipulating individual molecules. It is a multidisciplinary field consisting of physics, biology, chemistry, medicine, engineering, and nearly any other applied science. The prefix nano- means ten to the minus ninth power, or one billionth. There have been great advances in nanotechnology in recent years, and scientists routinely make materials that are only a few nanometers in size, or about 1/80,000 the diameter of a human hair. See Figure 1 to notice how small a nanometer is compared to other common materials. Materials at the nanoscale exhibit interesting optical, electronic, physical, and chemical properties due to their small size. For example, catalysis chemical reactions occur at the surface of bulk material so as particles become smaller, the ratio of the surface area to the volume of the particles increases, thereby making a volume of nanoparticle catalysts more reactive than an equal volume of bulk catalyst. Optical properties of bulk materials are not size dependant, that is no matter what the size of a piece of bulk material it will have the same optical properties. This is not the case for nanomaterials. As you will see in the instructor demo, different sizes of gold nanoparticles exhibit very different colors. In the 1960s NASA Research Centers discovered fluids that could be controlled through the application of a magnetic field. These fluids were developed to confine liquids in space. These nanoparticle fluids are commonly known as ferrofluids and they are still an active area of research. Ferrofluids have many current industrial applications. They are used to dampen vibration in audio loudspeakers, they can behave as liquid O-rings in rotating shaft seals, and they are used in high-speed computer disk drives to eliminate impurities. They also have many potential applications in biomedical, environmental, and engineering fields.

Figure 1-Obtained from Office of Basic Energy Sciences, US Department of Energy A ferrofluid is a stable colloid suspension of magnetic nanoparticles in a liquid carrier. The nanoparticles are suspended throughout the liquid and have an average size of ~10 nm. It is critical that the nanoparticles are coated with surfactant to prevent the particles from aggregating together. The surfactants must be strong enough to prevent agglomeration even when a magnetic field is applied and they must overcome the intermolecular forces between the nanoparticles. For this reason, a typical ferrofluid contains 5% magnetic nanoparticles, 10% surfactant, and 85% carrier fluid by volume.

There are two basic steps in creating a ferrofluid: synthesis of the magnetic solid, magnetite ( Fe3O4 size 12{ ital "Fe" rSub { size 8{3} } O rSub { size 8{4} } } {}), and suspension in water with the aid of a surfactant. The magnetic particles must be very small on the order of 10 nm (100 Å) in diameter, so that the thermal energy of the particles is large enough to overcome the magnetic interactions between particles. If the particles are too large, magnetic interactions will dominate and the particles will agglomerate. The magnetite will be synthesized by a precipitation reaction that occurs upon mixing FeCl2 size 12{ ital "FeCl" rSub { size 8{2} } } {} and FeCl3 size 12{ ital "FeCl" rSub { size 8{3} } } {} with ammonium hydroxide (an aqueous solution of ammonia, NH3 size 12{ ital "NH" rSub { size 8{3} } } {}). The unbalanced equation for this reaction is as follows: __ FeCl 3 + __ FeCl 2 + __ NH 3 + + __ H 2 O → __ Fe 3 O 4 + __ NH 4 Cl size 12{"__" ital "FeCl" rSub { size 8{3} } +"__" ital "FeCl" rSub { size 8{2} } +"__" ital "NH" rSub { size 8{3} } rSup { size 8{+{}} } +"__"H rSub { size 8{2} } O rightarrow "__" ital "Fe" rSub { size 8{3} } O rSub { size 8{4} } +"__" ital "NH" rSub { size 8{4} } ital "Cl"} {} The surfactant used in this synthesis is tetramethylammonium hydroxide ( N(CH3)4OH size 12{N \( ital "CH" rSub { size 8{3} } \) rSub { size 8{4} } ital "OH"} {}). The hydroxide ( OH− size 12{ ital "OH" rSup { size 8{ - {}} } } {}) ions formed in solution tend to bind to the iron sites on the magnetite particles, creating a net negative charge on each particle. The positively-charged tetramethylammonium ions will then associate with the negatively-charged magnetite particles, forming a kind of shell around each magnetite particle. This charged shell raises the energy required for the particles to agglomerate, stabilizing the suspension. With the help of nanotechnology, liquid crystal displays have become very popular in recent years. Liquid crystal displays (LCD) were first produced by RCA in 1971 and are composed of two glass plates with a liquid crystal material between them. The liquid crystal material is an organic compound that is in a state between a liquid and a solid. Their viscosities are similar to those of liquids and their light scattering and reflection properties are similar to solid crystals. Liquid crystals must be geometrically highly anisotropic (having different optical properties in different directions)-usually long and narrow -but also become an isotropic liquid (same optical properties in all directions) through a stimulus such as a magnetic field, electric field, temperature, or pressure. Liquid crystals have several common phases. The simplest liquid crystal phase is called the nematic phase where the molecules spontaneously order with long axes roughly parallel. It is characterized by a high degree of long range orientational order but no translational order. An uniformly aligned nematic has a preferred direction, often described in terms of a unit vector called the director. The type of phase that a liquid crystal possesses ultimately determines its applications. Is it really necessary to describe the smetic phase? Many of the students were confused by this.l

Figure 2 Nematic, smectic-A, and smectic-C liquid crystal phases A subclass of nematic phases that will be investigated in this lab due to its pressure and temperature sensitive properties is the cholestric phase. The distance over which the director rotates to equal 360° is referred to as the chiral pitch and is normally on the order of a few hundred nanometers, or precisely the wavelength of visible light. This allows liquid crystals to selectively reflect light of wavelengths equal to the pitch length, so that a color will be reflected when the pitch is equal to the corresponding wavelength of light in the visible spectrum. Changes in the director orientation between successive layers modifies the pitch length resulting in an alteration of the wavelength of reflected light according to the temperature. The angle at which the director changes can be made larger, and thus tighten the pitch, by increasing the temperature of the molecules, hence giving them more thermal energy. Similarly, decreasing the temperature of the molecules increases the pitch length of the chiral nematic liquid crystal. This makes it possible to build a liquid crystal thermometer that displays the temperature of its environment by the reflected color.

Experimental Synthesis

Part I. Synthesis of Gold Nanoparticles (DEMO) Chemicals 1.0 mM HAuCl4 size 12{ ital "HAuCl" rSub { size 8{4} } } {} (stored in amber bottle) 1% trisodium citrate dihydrate solution

Nanoparticle Synthesis Add 20 mL of 1.0 mM HAuCl4 size 12{ ital "HAuCl" rSub { size 8{4} } } {} to a 50 mL Erlenmeyer flask on a stirring hot plate. Add a magnetic stir bar and bring the solution to a boilThe TA’s need to be told to add the citrate as soon as the reaction boils or the solution turns colorless.. Add 2 mL of 1% trisodium citrate dihydrate solution and observe the properties.

Part II. Synthesis of the Aqueous Ferrofluid (Procedure modified from J. Chem. Edu. 1999, 76, 943-948.)

Chemicals 1 M FeCl3 size 12{ ital "FeCl" rSub { size 8{3} } } {} in 2 M HCl Solution: 2 M FeCl2 size 12{ ital "FeCl" rSub { size 8{2} } } {} in 2 M HCl Solution 25% tetramethylammonium hydroxide in water 1.0 M NH3 size 12{ ital "NH" rSub { size 8{3} } } {} Solution: Dilute at least 200 mL of concentrated ammonium hydroxide with water to 3.0 L) Open containers of ammonia are odorous and their concentration will decrease over periods of time. CAUTION: Ferrofluids can be messy. This particular ferrofluid will permanently stain almost all fabrics. Also DO NOT LET THE MAGNETITES TOUCH THE SURFACE OF THE MAGNET DIRECTLY.

Magnetite Synthesis In a hood, place 4.0 mL of 1M FeCl3 size 12{ ital "FeCl" rSub { size 8{3} } } {} and 1.0 mL of 2M FeCl2 size 12{ ital "FeCl" rSub { size 8{2} } } {} solution into a 100 mL beaker. Stir on a magnetic stir plate. While stirring, slowly add 50 mL of 1.0 M aqueous NH3 size 12{ ital "NH" rSub { size 8{3} } } {} solution over a 5 minute period using a buret. Initially a brown precipitate will form followed by a black precipitate, which is magnetite. CAUTION: Even though 1M NH3 is fairly dilute, NH3 size 12{ ital "NH" rSub { size 8{3} } } {} is a strong base. Remove from stirring and immediately use a strong magnet to work the stir bar up the walls of the beaker. Remove the stir bar with a gloved hand being careful not to let it touch the magnet. Allow the magnetite to settle, then decant off the clear liquid into a waste beaker without losing a large amount of precipitate. The settling process can be expedited by placing a strong magnet below the beaker. Transfer the solid to a plastic weighing boat. Rinse out the beaker with a wash bottle. Use a strong magnet to attract the ferrofluid to the bottom of the weigh boat. Carefully decant as much clear liquid as possible into the waste beaker. Rinse again with water from the wash bottle and decant. Repeat the rinsing process a third time. What are you removing by rinsing? Add 1-2 mL of 25% tetramethylammonium hydroxide. Gently stir the solution with a glass stir rod for at least a minute to suspend the solid in the liquid. Use a strong magnet to attract the ferrofluid to the bottom of the weigh boat. Pour off and discard the dark liquid. Move the strong magnet around and again pour off any liquid. If the ferrofluid does not produce spikes, continue to move the strong magnet around, pouring off any liquid.

Ferrrofluid Properties Hold a magnet underneath the weigh boat that contains the ferrofluid. Move the magnet around the underside of the weigh boat. Move the magnet close to and far from the weigh boat. Record your observations Add a couple of drops of ferrofluid to a small piece of clean paper. Let the solution dry. Once it is dry, bring a strong magnet close to the paper. What happens? Use several different magnets to observe the properties of the ferrofluid and record your observations in your notebook.

Part III. Synthesis of Cholesteryl Ester Liquid Crystals

Chemicals Cholesteryl oleyl carbonate Cholesteryl pelargonate Cholesteryl benzoate Vials Heat gun

Liquid Crystal Synthesis Place 0.65 g cholesteryl oleyl carbonate, 0.25 g cholesteryl pelargonate, and 0.10 g cholesteryl benzoate in a vial. In a hood, with the cap off, melt the solid in with a heat gun. While the mixture is still a liquid, divide it into separate vials using a disposable pipette. Put the caps back on the vials. Allow the vials to cool and observe their properties. Clean up your bench area. Listed below is a chart of the different ratios that produce liquid crystals with different transition temperatures. Placing liquid crystals with different transition temperatures next to each other on a clear piece of contact paper makes it possible to make a thermometer. Cholesteryl oleyl carbonate (g) Cholesteryl pelargonate (g) Cholesteryl benzoate (g) Transition range, degrees C 0.65 0.25 0.10 17-23 0.70 0.10 0.20 20-25 0.45 0.45 0.10 26.5-30.5 0.43 0.47 0.10 29-32 0.44 0.46 0.10 30-33 0.42 0.48 0.10 31-34 0.40 0.50 0.10 32-35 0.38 0.52 0.10 33-36 0.36 0.54 0.10 34-37 0.34 0.56 0.10 35-38 0.32 0.58 0.10 36-39 0.30 0.60 0.10 37-40

Pre-Lab: Nanotechnology

(Total 10 Points) Hopefully here for the Pre-Lab Note: In preparing this report you are free to use references and consult with others. However, you may not copy from other students’ work (including your laboratory partner) or misrepresent your own data (see honor code). Name(Print then sign): ___________________________________________________ Lab Day: ___________________Section: ________TA__________________________

Part I. Demo of Gold Nanoparticles In a 3 nm gold particle, what percentage of the atoms are on the surface? Hint: You need to find the radius of a single gold atom and the equation for the volume of a sphere is also helpful.Remember that the volume of a shell is Vshell=4/3πr3 (r3-(r-2r)3).I am giving them this because all the students had trouble with this. The other things are easier to find but this isn’t.

Part II. Synthesis of an Aqueous Ferrofluid How would you make a 100 mL solution of 1.0 M FeCl3 size 12{ ital "FeCl" rSub { size 8{3} } } {} in 2.0 M HCl from FeCl3⋅6H2O size 12{ ital "FeCl" rSub { size 8{3} } cdot 6H rSub { size 8{2} } O} {} and concentrated HCl solution? Show all calculations. Hint: First find volume of water needed to make concentrated HCl ~12.1 M

Part III. Liquid Crystal Synthesis In your own words, explain what a liquid crystal is.

Report: Nanotechnology On my honor, in preparing this report, I know that I am free to use references and consult with others. Hopefully here for the Report Form Note: In preparing this report you are free to use references and consult with others. However, you may not copy from other students’ work (including your laboratory partner) or misrepresent your own data (see honor code). Name(Print then sign): ___________________________________________________ Lab Day: ___________________Section: ________TA__________________________

Part I. Demo of Gold Nanoparticles What was the starting color of the HAuCl4 size 12{ ital "HAuCl" rSub { size 8{4} } } {} solution? What was the beginning oxidation state of Au? What was the ending oxidation state of Au? What is the purpose of the sodium citrate solution? What is the total surface area of 3 g of 5 nm gold particles assuming gold has a density of 9.3g/cm3 size 12{9 "." 3g/ ital "cm" rSup { size 8{3} } } {}? What is the total surface area of 3 g of 5 micrometer gold particles assuming gold has a density of 9.3g/cm3 size 12{9 "." 3g/ ital "cm" rSup { size 8{3} } } {}?

Part II. Synthesis of an Aqueous Ferrofluid Write out the over-all balanced equation for the ferrofluid synthesis. Why did we rinse the nanoparticles three times with water (what are you specifically removing)? What is the purpose of the surfactant? Chemically explain the process. Other than those listed, what are three uses for ferrofluids.

Part III. Liquid Crystal Synthesis As the color of the liquid crystal changes, what is happening to the pitch length? If the color is green and changes to red, chemically/physically explain what has happened to the liquid crystal and be specific..Hint: Look at the wavenmber values assigned to each color. Does this correspond to a temperature increase or decrease? Why? A student measures 0.66 g cholesteryl oleyl carbonate, 0.69 g cholesteryl pelargonate, 0.15 g cholesteryl benzoate and combines them to make a liquid crystal. What is the most likely transition temperature?