JCE Classroom Activities are edited by Erica K. Jacobsen
Instructor Information
JCE Classroom Activity #102
Investigating Self-Assembly with Macaroni
David A. Burgan and Lane A. Baker* Department of Chemistry, Indiana University, Bloomington, IN 47405; *
[email protected] In this Activity, students learn about self-assembly, a powerful tool for creating ordered structures in chemistry and biology (1–3). A macroscale example, self-organized macaroni, is described in analogy to the self-assembly of lipid molecules (2) that make up the membranes of cells and to self-assembled materials, such as self-assembled monolayers (3). Self-assembly is a process in which molecules (usually in a solution) spontaneously organize to form a structure that displays order (see videos in Student Activity ref 2). A thiol is an example of a molecule that can make monolayers (see PowerPoint slide in online supplement). A self-assembled monolayer can be made up of molecules that have a reactive (think “sticky”) head group, and an additional portion of the molecule, often a hydrophobic tail. The molecule is first dissolved in an appropriate solvent. A surface that reacts with the molecule’s head group is then placed into the solution. The head groups stick to the surface, and the hydrophobic tails of the molecules order and “stand up”, maximizing the interactions between the tails of neighboring molecules. This organization mimics one layer of the lipid bilayer that forms cell membranes. Assembly of monolayers and lipid bilayers make use of the head and tail portions of the molecule as well as the molecule’s shape. When heated in water, macaroni can assemble spontaneously in an ordered structure to maximize the surface area that the pieces of macaroni have in contact with each other. Self-assembled monolayers can be used in a variety of applications, including molecular electronics, chemical sensors, and anticorrosion layers.
Integrating the Activity into Your Curriculum This Activity can be incorporated in discussions of noncovalent interactions such as hydrophobicity in chemistry or cell membrane structure in biochemistry. This Activity connects concepts from the molecular world, which can often be abstract, with familiar objects using materials that can be purchased at a grocery store.
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About the Activity When heated, Kraft brand EasyMac macaroni pieces dispersed in water are transformed from a random, disordered arrangement to a highly aligned, ordered arrangement. This results in a nonhazardous macroscale example of self-assembly that students can poke, prod, and investigate (see figure). Not all of the pieces will pack perfectly. This is a function of both the size of the container and the random process of self-assembly, as some macaroni can be trapped out of the layer formed. Students can also add different pasta shapes to the macaroni to observe how the shape of the pasta (“molecule”) can influence the self-assembly process. End-on view of ordered macaroni strucEasyMac can be purchased at most grocery stores for ~$1 per package. Other ture formed after heating (left). End-on brands of instant macaroni or boxed elbow macaroni did not yield as reproduc- representation of perfect hexagonal ible results. A microwave oven works best, but a Bunsen burner can be used if the close-packing (right). temperature is monitored closely. With a Bunsen burner the heat can become too high at the bottom of the beaker, disrupting the assembly process. A tall beaker or suitable microwavable container (~4–5 in. diameter) prevents overspill when cooking, as the suspension initially foams.
Answers to Questions 1. The overall shape is similar. The pieces of macaroni adhere to each other. They are different in that there is no head and no tail on the macaroni pieces. Macaroni pieces are much bigger than molecules. 2. If the macaroni assembles perfectly, each macaroni piece touches six other pieces. They are “hexagonal close-packed” (see figure). There is no other way to pack the pieces closer. In reality, each macaroni piece will likely touch fewer than six other pieces. 3. Macaroni pieces assemble in this manner because it is a more efficient way to pack the most macaroni in the smallest space. It also maximizes the contact (and adherence) between neighboring pieces of macaroni. 4. Adding oil to the solution can change the assembly process by disrupting the macaroni piece–piece adherence. Adding pasta of different sizes and shapes can reduce assembly by making it harder for macaroni pieces to line up.
This Classroom Activity may be reproduced for use in the subscriber’s classroom.
fold here and tear out
Background
References, Additional Related Activities, and Demonstrations 1. Campbell, D. J.; Freidinger, E. R.; Hastings, J. M.; Querns, M. K. Spontaneous Assembly of Soda Straws. J. Chem. Educ. 2002, 79, 201–202 (and references therein). 2. Bell, G. M.; Combs, L. L.; Dunne, L. J. Theory of Cooperative Phenomena in Lipid Systems. Chem. Rev. 1981, 81, 15–48. 3. Ulman, A. Formation and Structure of Self-Assembled Monolayers. Chem. Rev. 1996, 96, 1533–1554. Supporting JCE Online Material at http://www.jce.divched.org/Journal/Issues/2009/Jun/abs704A.html
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 6 June 2009 • Journal of Chemical Education
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JCE Classroom Activity #102
Student Activity
Investigating Self-Assembly with Macaroni Nature uses a number of strategies to coordinate and control matter at the molecular and even atomic level. One of these strategies, self-assembly, organizes matter by making use of molecular properties such as hydrophobicity, polarity, and size. An example of self-assembly observed in nature is the membrane of a living cell. The cell membrane, which acts as a barrier between the inside and outside of the cell, is composed of two alternating layers, called a bilayer. Assembly of the molecules of this bilayer involves hydrophobic–hydrophilic effects between molecules and the shape of the molecules. This bilayer is composed of molecules called lipids that have a hydrophilic (waterloving) head group and a hydrophobic (water-fearing) tail. The roughly cylindrical shape of this tail is a key factor in forming a self-assembled structure. When these lipid molecules are dissolved in water, they spontaneously organize in an effort to minimize exposure of the hydrophobic tail to water, and to maximize the interaction between neighboring tails. An ordered structure forms (see figure), in which the hydrophobic “tails” of the molecules line up parallel to one another and the hydrophilic “heads” of the molecules are exposed to water. All living organisms, from bacteria to humans, rely on the self-assembly of lipids to build cell membranes. In this Activity, you will investigate self-assembly using macaroni Schematic representation of lipid molecules in random order (center) and in a self-assembled bilayer structure (right). as an analog for self-assembly at the scale of molecules.
Try This
Be Safe! The micro-
You will need: 1000-mL beaker or similar microwaveable container (~4–5 in. diameter), water, a graduated waved container and cylinder or measuring cup, microwave, pot holders, paper towels, and a package of Kraft-brand EasyMac its contents may be hot. macaroni. Use pot holders and __1. Pour 250 mL (~1 cup) of water into a 1000 mL beaker or similar microwaveable container. handle with caution. __2. Open a package of EasyMac macaroni. Remove and discard the cheese packet. Add the rest of the Do not eat the resulting product. package’s contents to the water, making sure that each macaroni piece is wet. __3. Observe the behavior of the uncooked macaroni. Do the pieces sink or float? Do they exhibit any kind of pattern? How are they oriented? __4. Place the beaker or other microwaveable container in a microwave and cook on high power for ~6 min. __5. Carefully remove the beaker or container using pot holders. __6. Observe the macaroni now. How has it changed? Do the pieces exhibit any kind of pattern? How are they oriented? __7. Let the macaroni cool in the beaker until it can be handled easily. __8. Place several layers of paper towels on a flat surface. Gently tip the beaker and carefully turn out the macaroni onto the towels. __9. Observe the macaroni now. Do the pieces exhibit any kind of pattern? Draw the most common patterns you observe. (Note Question 2 below.) __10. Gently pick up the mass of macaroni. (It will typically be disc-shaped.) Separate the mass across its diameter to form two semicircles. Look at the newly exposed surface. Do the pieces exhibit any kind of pattern? How are they oriented? When finished making observations, dispose of the macaroni in the trash.
More Things To Try __1. Add cooking oil to the macaroni before cooking. Does this have an effect on the assembly process? Why or why not? __2. Add other pasta of different shapes and sizes to the macaroni before cooking. Does this have an effect on the extent of assembly observed? Why or why not?
Questions 1. In this Activity, how is a piece of macaroni similar to a lipid molecule? How is it different? 2. What is the greatest number of macaroni pieces that an individual piece touches? (Count and view the pieces from above.) Is this the most efficient arrangement or is there a different way to pack more macaroni around each individual piece? 3. Why do you think that the macaroni arranges itself the way it does? 4. Suggest a way to prevent the organization of the macaroni in the microwave.
Information from the World Wide Web (accessed Mar 2009) 1. Nanooze! – Self Assembly. http://www.nanooze.org/english/articles/article14_selfassembly.html 2. MRSEC Nanostructured Interfaces: Self Assembly Videos. http://www.mrsec.wisc.edu/Edetc/cineplex/self/ 3. DiscoverNANO. http://www.discovernano.northwestern.edu/index_html This Classroom Activity may be reproduced for use in the subscriber’s classroom.
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Journal of Chemical Education • Vol. 86 No. 6 June 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
