Objective

Grading

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

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.

Figure 2

There are two basic steps in creating a ferrofluid: synthesis of the magnetic solid, magnetite ( Fe3O4Fe3O4 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 FeCl2FeCl2 size 12{ ital "FeCl" rSub { size 8{2} } } {} and FeCl3FeCl3 size 12{ ital "FeCl" rSub { size 8{3} } } {} with ammonium hydroxide (an aqueous solution of ammonia, NH3NH3 size 12{ ital "NH" rSub { size 8{3} } } {}). The unbalanced equation for this reaction is as follows:

The surfactant used in this synthesis is tetramethylammonium hydroxide ( N(CH3)4OHN(CH3)4OH size 12{N \( ital "CH" rSub { size 8{3} } \) rSub { size 8{4} } ital "OH"} {}). The hydroxide ( OH−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.

Changing the subject to liquid crystals: 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.

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.

Figure 3

Figure 4

Experimental Synthesis

Magnetite Synthesis

Ferrrofluid Properties

Part III. Synthesis of Cholesteryl Ester Liquid Crystals

Chemicals

Liquid Crystal Synthesis