Teacher Notes
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Teacher Notes![]() Liquid Crystals: A Fourth State of MatterStudent Laboratory KitMaterials Included In Kit
Cholesteryl oleyl carbonate, C46H80O3, 7.5 g
Cholesteryl pelargonate, C36H62O2, 7.5 g Cholesteryl benzoate, C34H50O2, 3.0 g Contact paper, 9" x 9" square Vials, with screw tops, 16 Additional Materials Required
Water, deionized or distilled
Background surface, black Balance, 0.01-g precision Beakers, 600 mL, 2 Hot plate Hot water bath (80–90 °C) or hair dryer Permanent marker Scissors Tape, clear Thermometer, digital Wood splints (or spatulas), 30 Prelab Preparation
{13977_Preparation_Table_2_Group assignments}
Safety PrecautionsCholesteryl 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. DisposalPlease 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
Teacher Tips
Further Extensions
Correlation to Next Generation Science Standards (NGSS)†Science & Engineering PracticesDeveloping and using modelsPlanning and carrying out investigations Constructing explanations and designing solutions Obtaining, evaluation, and communicating information Disciplinary Core IdeasHS-PS1.A: Structure and Properties of MatterHS-PS1.B: Chemical Reactions HS-PS2.B: Types of Interactions HS-ETS1.C: Optimizing the Design Solution Crosscutting ConceptsPatternsStructure and function Stability and change Performance ExpectationsHS-PS1-1. Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms. Answers to Prelab Questions
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
Thermotropic—temperature-dependent liquid crystals
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.
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 DataThe 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. {13977_Data_Table_3}
Liquid Crystal Data Sheet {13977_Data_Table_4}
B. Light Reflection and Transmission {13977_Data_Table_5}
C. Liquid Crystals as Temperature Indicators {13977_Data_Table_6}
Answers to Questions
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.
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.
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.
An electrical field can be applied in certain types of liquid crystals will will align them by charge. Teacher HandoutsReferencesLisensky, G. and Boatman, E., Color in Liquid Crystals. J. Chem. Educ., 2005, 82, 1360A. Recommended Products |
Student Pages
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Student Pages![]() Liquid Crystals: A Fourth State of MatterIntroductionEver 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
BackgroundMatter 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. {13977_Background_Figure_1}
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. {13977_Background_Figure_2}
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). {13977_Background_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. {13977_Background_Figure_3}
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 OverviewIn 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
Liquid crystal mixtures, 2 (provided by teacher)
Water, deionized or distilled Background surface, black Balance, 0.01-g precision Beaker, 600-mL Contact paper, 3" x 3", 4 Hot plate Hot water bath (80–90 °C) or hair dryer Permanent marker Scissors Tape, clear Thermometer, digital Vials, with screw tops, 2 Weighing paper Wood splints or spatulas Prelab QuestionsWatch the video to see how liquid crystals are used in everyday life. Safety PrecautionsCholesteryl 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. ProcedureTable 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. {13977_Procedure_Table_1}
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.
Student Worksheet PDF |