Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Cut the provided contact paper into 3" x 3" squares. Give each group four squares to make two sandwiches.
2. Weigh the amounts listed in Table 1 (in the Procedure) for each solution according to the group assignments in Table 2. Note: Due to the expense of the chemicals involved in this lab, limited quantities were included with the kit. It is important not to deviate from this preparation. (These compounds are from the cholesterol family and are waxy solids at room temperature. This can make them difficult to transfer to the vials).
3. Label each vial with the solution number so students know what ratio they were provided. Give each group two vials using the pairs listed in Table 2.

Safety Precautions

Cholesteryl oleyl carbonate and cholesteryl pelargonate are skin and eye irritants and may cause respiratory and digestive tract irritation. Avoid contact of all chemicals with skin and eyes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information. Please follow all laboratory safety guidelines.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Cholesteryl oleyl carbonate, cholesteryl pelargonate and cholesteryl benzoate may be disposed of according to Flinn Suggested Disposal Method #18b.

Lab Hints

• Enough materials are provided in this kit for 8 groups of students. Both parts of this laboratory activity can reasonably be completed in one 50-minute class period. The Prelaboratory Assignment may be completed before coming to lab, and the worksheet may be completed the day after the lab.
• Limited quantities of chemicals were included with this kit due to their expense. It is important not to deviate from the 

  Prelab Preparation outline.

•  

  To minimize air bubbles and difficulty in creating the sandwich, place the contact paper under a stack of text books to flatten it out. Some air bubbles are normal; it is hard to remove them completely. They will not affect the outcome of the reaction or observations.
• The observations for both mixtures will be mostly the same, with just the amount of heating differing. This is a good way to point out to students that some observations occur on a much smaller scale that we cannot always see.
• Have the students report their ranges on a master sheet so they can observe the trend between the amounts of components added and the temperature ranges.
• The vial with the liquid crystal mixture in it should be removed from the hot plate and poured immediately after it clears. If the solution becomes too hot, it can melt the contact paper.
• The liquid crystal sandwiches should not be placed on the hot plate for remelting or eliciting a heat change because the contact paper will melt. A heat gun or hair dryer is preferred for the higher transition temperature ranges. Lower temperature ranges can be heated with hands.
• Digital thermometers are strongly recommended since they will provide the students with the most accurate temperature range for their liquid crystals.

Teacher Tips

• Following is a graph of the reflected color transition temperatures for liquid crystal mixtures as a function of the percent composition of cholesteryl oleyl carbonate (COC).
  
  {13977_Tips_Figure_4}
  Cholesteryl oleyl carbonate has a lower transition temperature than cholesteryl pelargonate and cholesteryl benzoate.
• The liquid crystal sandwiches can be placed on an overhead projector for the class to view the transmitted colors. If the projector surface heats the liquid crystals above their transition temperature, place the square on top of a Petri dish to prevent it from being heated by the projector.
• The liquid crystals should stay active for a few weeks but may need remelting if they solidify. We do not recommend letting students take the chemicals home.
• This lab can also be done as a demonstration using Flinn Catalog No. AP7195.
• In addition, you can have the liquid crystals affixed to a black background similar to the aquarium thermometer. Take a piece of construction paper and place a clear sticker over the black background. Paint the liquid crystal on top of the tape, then place another clear sticker over it.

Further Extensions

• Students can perform an optional extension with aquarium thermometers. An aquarium strip can be placed on the side of a beaker and the water heated. Students can observe the change in color of the thermometer and compare it to their liquid crystal sheets.

Answers to Prelab Questions

1. Other than the products listed in the introduction, what are three additional applications of liquid crystals in everyday life?

Liquid crystal displays (LCD), optical imaging (LCTFs), liquid crystal lasers, smart glass (PDLC), battery testing strips, mucus from slugs and snails, soap, proteins and cell membranes

2. Liquid crystals can be divided into three phases: thermotropic, lyotropic and metallotropic. What is the primary characteristic of each phase?

Thermotropic—temperature-dependent liquid crystals
Lyotropic—concentration-dependent liquid crystals
Metallotropic—dependent on the ratio between organic and inorganic components

3. Most liquid crystal mixtures require a director molecule. What is the purpose of this molecule? Which of the liquid crystal components is the director in this lab?

Director molecules ensure that the other liquid crystal components line up properly. Since they are often added in small amounts, this means that cholesteryl benzoate is the director molecule in this lab.

4. A liquid crystal mixture is heated to its maximum detection temperature. Describe how this increase in temperature would affect the pitch, wavelength of visible light and color of the liquid crystals. Would the crystal helix be more tightly packed or more loosely packed?

The pitch will decrease, leading to a decrease in the wavelength of visible light. This that means the crystal helix will become more tightly packed together. At the maximum detection temperature, the crystal should appear blue.

Sample Data

The six different combinations of cholesteryl oleyl carbonate (COC), cholesteryl pelargonate (CP) and cholesteryl benzoate (CB) are shown in the following table with the temperature transition ranges.

Liquid Crystal Data Sheet

Assigned solution numbers: ___4___ ___8___

A. Preparation of Liquid Crystals

B. Light Reflection and Transmission

C. Liquid Crystals as Temperature Indicators

Answers to Questions

1. Based on your data, and the data collected from the rest of the class, what can you conclude about the ratios of liquid crystal components and their temperature ranges? How does molecule size (Figure 3) support your conclusion?

The higher the amount of cholesteryl pelargonate (CP) and the lower the amount of cholesteryl oleyl carbonate (COC), the higher the transition range of the liquid crystal mixture. CP is a smaller molecule than COC, this allows the liquid crystals to pack tighter together. A higher temperature is needed to force alignment.

2. Did the liquid crystals look the same when placed on the white versus dark surface? If they did not, why would the result be different?

The liquid crystals look different because the different backgrounds affect which colors of the visible spectrum can pass through. When a liquid crystal square is viewed against a black background, you will see one color. When it is then placed in front of a white light source, the complementary color is observed.

3. When the liquid crystals were heated, they exhibited a series of color changes for a few degrees then a single color persisted. Why did the liquid crystals not continue to change? Did both of your liquid crystal mixtures produce the same color scheme?

The liquid crystal mixture only reacts over a certain temperature range. It will remain blue as long as the temperature is at or above the upper range. Both liquid crystal mixtures exhibited this feature, just at different temperature ranges.

4. Other than temperature, pressure and concentration, what other property can be used to affect the orientation of liquid crystals? (Hint: Think about how ions interact in solution).

An electrical field can be applied in certain types of liquid crystals will will align them by charge.

Teacher Handouts

13977_Teacher1.pdf

References

Lisensky, G. and Boatman, E., Color in Liquid Crystals. J. Chem. Educ., 2005, 82, 1360A.

Introduction

Ever wonder how mood rings work their magic? Or how the temperature strip on the side of a fish tank always seems to know what the temperature is? This lab introduces you to a fourth state of matter: liquid crystals. The video will provide an introduction to liquid crystals.

Concepts

• States of matter
• Liquid crystals
• Nanotechnology
• Diffraction

Background

Matter includes everything in our universe that has mass and occupies space. Classification based on some specific properties can make this overwhelming concept more manageable. Matter is commonly classified based on its properties—either its state (physical properties) or how the chemical reacts (chemical properties). Solid, liquid and gas are the three most familiar states of matter, but liquid crystals and plasma are other observable states of matter we perceive in everyday life. A few additional states exist, but they can only be observed or detected under extreme circumstances. The composition of matter can further be broken down into pure substances (elements or compounds) or mixtures (heterogeneous or homogeneous). As is common in chemistry, these are not absolute categories since liquid crystals fall between the liquid and solid states with solution properties similar to colloidal suspensions. Colloids are easily mistaken as homogenous solutions since the suspended particles are generally 1–200 nm in size. Some common examples of colloids include blood, smoke, fog, mud, ink, milk, butter and cheese. As seen with these examples, colloidal suspensions can be solids, liquids or gases. They cannot be seen by the naked eye, but they can be distinguished by their ability to scatter light. This ability is known as the Tyndall effect and is a property not found in homogeneous mixtures. The idea of working on the nanoscale has led into the wide-ranging field of nanotechnology, which has a variety of applications. Nanotechnology involves the preparation, characterization and uses of nanosize particles with dimensions in the 1–100 nm range [1 nm = 1 x 10^–9 m]. Nanoparticles have unique physical and chemical properties that differ from macroscopic properties of traditional or “bulk” solids. The electronic, magnetic and optical properties of nanoparticles have proven very useful in the creation of new products that use nanotechnology. Liquid crystals consist of nano-size organic compounds that are in a state between a liquid and a solid.

Liquid crystals are partially ordered compounds that float around as if in a liquid, but align themselves to a degree as if in crystalline solid. Cholesteryl esters, found in liquid crystals, are long, cylindrical or rod-like molecules that arrange themselves in a layered helical pattern, similar to a spiral staircase (see Figure 1). Most liquid crystals require a director molecule. This molecule ensures that the other liquid crystal molecules line up properly. Typically, only a small amount of the director is needed in the mixture. Thermotropic liquid crystals, such as those used in this lab, undergo color changes in response to changes in temperature.  

The molecules in each layer line up in a parallel pattern, with each adjacent layer having this parallel pattern slightly rotated. After a certain number of layers and rotations, the molecules in the top and bottom layers are aligned in the same direction. The distance between these layers is called the pitch of the liquid crystal (see Figure 2). As the liquid crystal heats up, the rotational angle between layers increases. Since fewer layers are required to realign the top and bottom, the pitch decreases with increasing temperature.

These pitch distances are on the order of magnitude corresponding to visible light wavelengths (300 nm to 400 nm). Visible light is selectively diffracted by the liquid crystal according to Snell’s Law (Equation 1).
Where λ is the reflected wavelength, p is the pitch, θ is the angle with respect to the surface, and n is the mean refractive index. As the temperature increases, the wavelength of visible light decreases. The reflected light changes from yellow (longer wavelength) to green to blue (shorter wavelength) as the liquid crystal is heated and from blue to green to red as it is cooled. The temperature range for these color transitions is different for each liquid crystal compound and mixture of compounds.

If a specific wavelength of light is reflected by the crystal, then all other wavelengths pass through the crystal. If blue is the reflected light, then light transmitted through the crystal is white light minus blue light, which is perceived as yellow. If orange light is reflected, then white light minus orange light, which is seen as azure (light blue) light, is transmitted. When a liquid crystal square is viewed against a black background and then in front of a white light source, the reflected color, followed by its complementary color, is observed.

Figure 3 shows the structures of the liquid crystal components for this lab. Notice the rod–like shape. This characteristic is what allows them to align as liquid crystals. All these molecules differ slightly in size and when in the liquid crystal phase and will act as a single molecule with an average size. By changing the ratio of the cholesteryl esters involved, you are affecting the composition of the mixture. This means that the average size of the liquid crystal will change and alter how the molecules pack together as the temperature is raised and lowered.

Experiment Overview

In this experiment you will investigate liquid crystals and observe how different ratios of the same chemicals can produce liquid crystals with sensitivity over different temperature ranges.

Materials

Prelab Questions

Watch the video to see how liquid crystals are used in everyday life.

Safety Precautions

Cholesteryl oleyl carbonate and cholesteryl pelargonate are skin and eye irritants and may cause respiratory and digestive tract irritation. Avoid contact of all chemicals with skin and eyes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information. Please follow all laboratory safety guidelines.

Procedure

Table 1 shows twelve different combinations of cholesteryl oleyl carbonate (COC), cholesteryl pelargonate (CP) and cholesteryl benzoate (CB). Each of these mixtures will produce liquid crystals with a different temperature range. Record the two solutions you were assigned on your data sheet.

A. Preparation of Liquid Crystals

Note: These compounds are from the cholesterol family and are waxy solids at room temperature. This can make them sticky and difficult to transfer to the vials.

1. Obtain two vials from your teacher. Record your two assigned solutions on your data sheet.
2. With the cap off, gently heat both vials on a hot plate until the mixtures completely melt and become transparent. The mixtures will be slightly thick, like syrup. Record the appearances on your data sheet.
3. Peel the backing off of one of the squares of contact paper.
4. Slowly pour the mixture, a little at a time, onto the tacky side of the clear contact paper. The liquid will be hot, so use a paper towel to hold the vial. With a wood splint (or spatula), spread the liquid crystal mixture in an approximately 5 cm diameter circle on the contact paper.
5. Peel the backing off of the other square. With the tacky side down, place the square over the tacky side of the square containing the liquid crystal mixture. Press the mixture gently to spread it out and form a thin layer. Do not press all the way to the edges or your mixture will leak out. Seal the contact paper, forming a “sandwich.” Fold a piece of tape over each edge to further seal the liquid crystals within the contact paper. Label the square as you did previously with the vial so they match.
6. Record the appearance on your data sheet.
7. Repeat steps 3–6 for the other liquid crystal mixture. Label accordingly.

B. Light Reflection and Transmission

1. While wearing gloves, hold the liquid crystal square between your palms to heat the mixture. Be careful not to apply too much pressure to prevent squeezing the mixture out of the sides or causing damage to the contact paper. After heating, place the liquid crystal sandwiches on top of a white background. Record your observations.
2. Hold the edge of the square up to the lights of your classroom or another white light source and observe the reflected colors. Gently rub your fingers over the liquid crystal to heat it up. Observe the change in color, and record this information on your data sheet.
3. Place your liquid crystals on top of a dark surface, and repeat step 2. Record observations.

C. Liquid Crystals as Temperature Indicators

1. Tape a black piece of paper to the outside of a 600 mL beaker filled with approximately 400 mL of cold water. (Note: It is important that the water is colder than room temperature to be sure you are below the temperature range of all mixtures on the table).
2. Check the sealing of all edges, then insert the liquid crystal sandwich into the beaker of water.
3. Gently heat the beaker while monitoring the temperature with a digital thermometer.
4. Observe and record any color changes of the liquid crystal mixture and at what temperatures they occur.
5. Using the data, determine the approximate temperature range of your liquid crystal sandwiches, and record this on your data sheet.
6. In addition, record the temperature ranges for your solutions on the master list for the class.
7. Repeat this process with a fresh beaker of cold water for the second liquid crystal sandwich. 

Student Worksheet PDF

13977_Student1.pdf