Materials Included In Kit

Additional Materials Required

Safety Precautions

Ethylenediamine is a strong irritant to skin and eyes and is moderately toxic by inhalation and skin absorption. Charcoal powder is highly flammable. Acacia is found to be an allergen by a small portion of the population. Ethanol is flammable, and the addition of chemical denaturants makes it poisonous. Avoid contact of all chemicals with skin and eyes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron or laboratory coat. 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. Excess acacia, charcoal powder, and citric acid may be disposed of according to Flinn Suggested Disposal Method #26a. Dispose of excess ethylenediamine solution, ethanol and the carbon quantum dot solutions according to Flinn Suggested Disposal Method #26b.

Lab Hints

• A fun extension activity is to examine the ingredient labels of various consumer products and discuss why each is there.
• One way to distinguish between a solution and a colloid or suspension is by light scattering. If light is shone through a solution the beam path is not visible; however, when shone through a colloid or fine suspension the beam path is visible.

Further Extensions

Online Educational Resources

Cabot Microelectronics slurry settling studies
https://www.youtube.com/watch?time_continue=14&v=f8y6-iYX_rw

Synthesis of cadmium selenide quantum dots
https://www.youtube.com/watch?v=s3H0_8TLs-A

Shampoo ingredients explained
https://collegeofcuriosity.com/25-ingredients-of-shampoo-explained/

Answers to Questions

1. What did you observe when charcoal was mixed with water?

   The charcoal formed a suspension and, if left to stand, would settle to the bottom of the water.

2. What did you observe when the acacia was added to the charcoal-water mix?

   The addition of the acacia made the solution much thicker and gooier. It was a little lumpy.

3. What did you observe when the ethanol was added to the ink mixture?

   The ethanol improved the consistency of the solution and it became much more uniform in appearance.

4. What color was the solution of citric acid and ethylenediamine before heating?

   Prior to heating, the solution was colorless.

5. What color are the carbon quantum dots when viewed under standard conditions?

   After heating, the carbon quantum dot colloid was a pale yellow color.

6. What color was the light emitted by the carbon quantum dots when irradiated with UV light?

   The carbon quantum dots emitted blue light when irradiated with UV light.

References

Xiaoyou Xu, Robert Ray, Yunlong Gu, Harry J. Ploehn, Latha Gearheart, Kyle Raker, and Walter A. Scrivens, J. Am. Chem. Soc. 2004, 126(40), 12736–12737.

Yujin Choi, Seongho Jo, Ari Chae, Young Kwang Kim, Jeong Eun Park Donggun Lim Sung Young Park , and Insik In, ACS Appl. Mater. Interfaces 2017, 9(33), 27883–27893.

Md Palashuddin Sk, Amit Jaiswal, Anumita Paul, Siddhartha Sankar Ghosh, and Arun Chattopadhyay, Scientific Reports  2012, 2, 383

http://www.plasmachem.com/shop/en/226-zncdses-alloyed-quantum-dots

Introduction

Every day we encounter various consumer products in which a mixture of chemicals has been used to optimize the product’s effectiveness. By combining different chemicals, chemical engineers can promote desirable properties while minimizing undesirable ones. In this lab, you will be examining various types of liquid-based mixtures, some of which directly relate to consumer products and others that are the topic of current research efforts.

Watch the introductory video.

Concepts

• Solutions
• Intermolecular forces
• Consumer chemistry
• Chemical engineering

Background

When chemicals become dispersed in a liquid, the result is either a solution, colloid, or suspension. In a solution, small particles are dispersed uniformly throughout the liquid, forming a homogeneous solution. The dissolved material, referred to as the solute, is kept in solution through intermolecular forces, such as London dispersion forces, dipole-dipole interactions, hydrogen bonding, and ion-dipole interactions. Solubility predictions can be made by considering the types of intermolecular forces that the solvent and solute are able to undertake. If both can engage in similar interactions, then dissolution will readily occur. For example, both water and sugar have hydrogen bonding, dipole-dipole and London dispersion force interactions, and sugar is soluble in water. Conversely naphthalene, which does not engage in hydrogen bonding or dipole-dipole interactions, does not dissolve in water.

If large particles are dispersed throughout a solvent, then over time they will begin to settle out. This is because the particles are too large for successful solvation. All suspensions are unstable from a thermodynamic point of view; however, some suspensions are stable over a reasonable period of time. This stability can be due to the particles needing time to aggregate before they settle out of solution. Mud is an example of a suspension, where soil is suspended in water. Because the components of a suspension will physically separate, it is considered a heterogenous mixture.

Colloids can be thought of as being halfway between a solution and a suspension. They are large particles, usually in the range of 1 to 1000 nanometers, that are dispersed throughout the solvent. What sets a colloid apart from a suspension is that the dispersed particles do not aggregate or settle out over time. Even though a colloidal solution may look quite uniform, it contains two distinct phases, one belonging to the dispersed material and the other to the solvent. For this reason, only solutions are considered to be true homogeneous mixtures.

In determining if a large particle will form a colloid or a suspension, a range of interactions need to be considered. Van der Walls forces will attract particles together and, if unchecked, will result in aggregation and sedimentation. Electrostatic forces can help keep the particles apart; the colloidal micelles formed by soap have a negatively charged outer surface, which causes them to repel each other and helps prevent aggregation (see Figure 1). Polymers are often attached to the surface of particles to introduce a steric barrier to aggregation. When these particles approach each other, the long polymer arms prevent them from getting close enough to aggregate. Due to the many factors affecting colloidal stability, it is easy for changes in the solution to destabilize the particles. This can be due to the addition of a surfactant, charged particles or other large particles. Chemicals that are added to colloids to bring them out of solution are called flocculants.

Nanoparticles are particles with dimensions between 1 and 100 nanometers. They are of particular interest because their behavior and properties are different to their bulk material. For example, gold nanoparticles are red in color rather than the usual metallic yellow and have been used for centuries to produce red stained glass. The size of a nanoparticle can also greatly influence its properties. Quantum dots are nanoparticles made from semiconductor particles and their properties are between those of a bulk semiconductor and an atom. Cadmium-based quantum dots emit light when irradiated with UV light, with smaller dots (2–3 nm) emitting blue light and larger dots (6–7 nm) emitting red light (see Figure 2). Unfortunately, the toxicity of cadmium has limited the potential applications of these nanomaterials.

The different colors come not from a change in composition, but rather through changes in the particle size.

In 2004, carbon quantum dots were accidentally discovered by Xu et al. These carbon-based nanoparticles have the advantage of being much less toxic. In fact, one study found that carbon quantum dots naturally occur during the baking process, meaning that people had been producing and consuming these particles long before they were discovered. The biggest challenge associated with researching carbon quantum dots is that their mechanism for light emission is not well understood. Some groups have reported size-based properties whereas others report that oxygen and nitrogen surface impurities are responsible for the different colors. This has resulted in much more trial and error-based development. The use of microwave assisted synthesis has been extremely useful as it has enabled rapid and repeatable synthetic methods to be developed.

Watch the accompanying video.

Experiment Overview

This laboratory is divided into two sections. In the first section, you will be making your own black ink while observing how the different components change the ink’s properties. In the second section, you will synthesize a carbon quantum dot that emits blue light when irradiated.

Materials

Prelab Questions

Research the following products and categorize them as either solutions, suspensions or colloids.

Safety Precautions

Ethylenediamine is a strong irritant to skin and eyes and is moderately toxic by inhalation and skin absorption. Charcoal powder is highly flammable. Acacia is found to be an allergen by a small portion of the population. Ethanol is flammable, and the addition of chemical denaturants makes it poisonous. Avoid contact of all chemicals with skin and eyes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron or laboratory coat. 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

Part 1. Making Ink

1. Place 1 g of charcoal into a mortar.
2. To the mortar, add 5 mL of distilled water.
3. Mix with a pestle and record your observations.
4. Next add 3 g of acacia to the mortar.
5. Grind with the pestle and record your observations.
6. Finally add 6–8 drops of ethanol to the mortar.
7. Grind with the pestle and record your observations.

Part 2. Synthesis of Carbon Quantum Dots

1. Place 1.5 g of citric acid in a 25 mL Erlenmeyer flask.
2. Add 5 mL of the ethylenediamine solution to the flask.
3. Cover the mouth of the flask with a small square of aluminum foil.
4. Pierce the aluminum foil.
5. Place the Erlenmeyer flask on a hot plate and bring it to the boil.
6. Carefully swirl the flask every five minutes until only approximately 1 mL of solution remains.
7. Remove the flask from the hot plate, remove the aluminum foil and leave to cool.
8. Once cooled, add 5 mL of distilled water to flask.
9. Shine a UV light on this solution.
10. Remove 1 mL of solution from the flask and dilute it to 10 mL.
11. Shine a UV light on this final solution.
12. Consult your instructor for appropriate disposal procedures.

Questions

1. What did you observe when charcoal was mixed with water?
2. What did you observe when the acacia was added to the charcoal-water mix?
3. What did you observe when the ethanol was added to the ink mixture?
4. What color was the solution of citric acid and ethylenediamine before heating?
5. What color are the carbon quantum dots when viewed under standard conditions?
6. What color was the light emitted by the carbon quantum dots when irradiated with UV light?