In the Classroom edited by
Erica K. Jacobsen Journal of Chemical Education
JCE Classroom Activity Connections: NaCl or CaCl2, Smart Polymer Gel Tells More Yueh-Huey Chen,* Jia-Ying Lin, and Yu-Chen Wang Department of Applied Science, National Dong Hwa University (Meilun Campus), Hualien 970 Taiwan ROC *
[email protected] Jing-Fun Yaung Department of Food Science, National Kinmen Institute of Technology, Kinmen 893 Taiwan ROC
Polymers are found everywhere in our daily lives. However, laboratory experiments involving polymers in undergraduate chemistry curricula are not in proportion to the prevalence of polymer materials in modern society (1). For the past few years, we have been working on the developments of experiments suitable for the undergraduate chemistry laboratory involving everyday polymers (2, 3). As anticipated, experiments involving common household materials strongly stimulate students' interest in doing chemistry laboratory experiments and activities. Synthetic polymers that are sensitive to environmental changes have become one important class of polymers (4). These polymers have been called “smart polymers” (4, 5), “stimuli-responsive polymers” (6), “intelligent polymers” (7), and “environmentalsensitive polymers” (8). The applications of these polymers in various areas have been increasing significantly. As a “smart polymer”, the superabsorbent polyacrylate used in diapers swells and contracts in response to changes in external conditions such as solvent composition, temperature, pH, ionic strength, and electric field, offering a wide variety of potential applications (4-10). When exposed to an aqueous solution, the polymer absorbs a large quantity of water to become a swollen gel. The water absorbed is held in the interstitial space of the polymer networks, which gives the swollen gel the solidity it has. The swollen gel is transparent, soft, and wet. It looks like a solid material, but undergoes deformation stimulated by external inputs. In the JCE Classroom Activity #80 “Ions or Molecules? Polymer Gels Can Tell”, students investigate the effect of different solid compounds on the swollen superabsorbent polymer gels,1 and further identify the compounds as ionic or covalent compounds (11). Ionic compounds make the gel collapse, while covalent compounds have no effect on the gel stability. As observed, among those ionic compounds, CaCl2 acts the most efficiently. In the presence of CaCl2, the highly swollen polymer gel quickly collapses, turns opaque, and further contracts to small white particles that stick together (Figure 1). On the basis of these observations, we present here an extended activity to demonstrate the differences between the effects of NaCl (a salt of monovalent metal ions) and CaCl2 (a salt of polyvalent metal ions) on the swollen gels by investigating the water-absorbing ability of the contracted gels. A swellable, NaCl-contracted gel versus a nonswellable, CaCl2-contracted gel is observed.
water is absorbed, it becomes a swollen polymer gel. The ions arising from the ionization of -COONa and -COOH groups in the polymer network contribute to the osmotic pressure inside the gel (12). The equilibrium state of the gel swelling in aqueous solution is where the osmotic pressure inside the gel and that of the external solution equalize. Therefore, the swelling capacity of the polymer varies when the ionic strength of the external solution is changed (13). When the highly swollen polymer gel contacts with solid NaCl, a salt of monovalent metal ions, the ionic strength outside the gel is increased. Water molecules inside the swollen gel are forced to diffuse to the outside. The Naþ and Cl- ions resulting from the dissolution of solid NaCl diffuse into the gel matrix and further interfere with water-polymer interactions. As a result, the water-absorbing capacity of the polymer gel is suppressed, leading to the collapse of the gel. However, this osmotic deswelling involves a reversible folding-unfolding process of the polymer network (14). The NaCl contracted gel reswells in deionized water and the initial swollen structure of the gel is substantially restored. In the presence of LiCl and KCl, the swollen gel appears to have the same behaviors of contracting and reswelling as that observed for NaCl. Unlike monovalent metal ions (Mþ), the polyvalent metal ions (Mnþ) tend to complex with multiple carboxylate groups on the neighboring polymer chains (15-18). This action of the polyvalent metal ions as metal-ion cross-linkers increases the cross-linking density in the polymer network, leading to constriction on the swelling capacity of the polymer. While the
Swelling and Collapsing of the Gel Superabsorbent polyacrylate is a hydrophilic ionic polymer containing functional groups of -COOH and -COONa. As
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Figure 1. Swollen superabsorbent polymer gels contracted by NaCl (left, transparent) and CaCl2 (right, opaque), respectively.
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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 87 No. 12 December 2010 10.1021/ed100205h Published on Web 09/03/2010
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Scheme 1. The Ion-Exchange Cycle of Superabsorbent Polyacrylate
swollen superabsorbent polymer gel is exposed to solid CaCl2, in addition to the osmotic deswelling, the Ca2þ ions diffuse into the gel matrix, complex with carboxylate groups, and cross-link the polymer chains (Scheme 1). A rapid and abrupt contraction of the gel occurs. The resulting CaCl2 contracted gel is unable to reswell because of the formation of additional cross-links by the Ca2þ ions in the polymer network (19). The Ca2þ ions in the CaCl2-contracted gel cannot be washed out by deionized water; however, the release of Ca2þ ions is achieved through the reversal of ion exchange by immersing the contracted gel in concentrated NaCl solution for some period of time (Scheme 1). The ion-exchange cycle of the polymer gel is accomplished. Sodium ions are redistributed into the polymer network. The water-absorbing ability of the polymer gel is recovered. As ionic compounds of divalent metal ions, BaCl2 and ZnCl2 work similarly as CaCl2 on the polymer gel. Toward compounds of CuSO4, Fe(NO3)3, or Al(NO3)3, the swollen gel contracts as expected, but the reversal of ion exchange cannot be accomplished. Nevertheless, the resulting color of the contracted gels with Cu2þ and Fe3þ ions indicates that the metal ions are adsorbed by the polymer gels. Hazards The powder of superabsorbent polymer can irritate eyes, nose, and mouth. Contact with the polymers must be avoided. The metal halides or nitrates may cause skin, eye, respiratory, and gastrointestinal tract irritation. They should be handled with care and not be ingested. Students should wear goggles to prevent contact with splashed solutions. Solutions containing Cu2þ, Zn2þ, Ba2þ, and Al3þ ions should be collected and sent to an appropriate waste facility for disposal. Extension 1: Comparing Reswelling Polymers versus NonReswelling Polymers For the first extension activity, the instructor provides groups of students with these supplies: two tea bags, ∼5.5 5.5 cm (these have been emptied, washed, and dried prior to use);2 superabsorbent polymer;3 NaCl (salt) and CaCl2;4 cotton thread; beakers; Petri dishes; deionized water; or distilled water. Students are asked to follow these instructions: 1. Place ∼0.15 g of superabsorbent polymer powder in each of the two tea bags. With help from a partner, seal each tea bag by tying up the open end ∼1 cm from the open edge with a piece of long cotton thread, ∼20 cm. (See the photographs in the online supporting information.)
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2. Place both tea bags in a large beaker containing 500 mL of distilled water and leave them there until the tea bags are fully expanded.5 3. Remove the tea bags from the beaker and dry the excess water with paper towels. Place the tea bags in separate Petri dishes. 4. Spread ∼5 g of solid NaCl onto one tea bag, and ∼1 g of solid CaCl2 onto the tea bag in the other Petri dish.6,7 Record what happens to the gels contained in each of the tea bags. 5. Remove the tea bags and dry each tea bag with separate paper towels. (Avoid contaminating the NaCl-contracted gel with CaCl2.) 6. Place the tea bags in separate 250-mL beakers. Wash each tea bag by adding 100 mL of deionized water to the beaker followed by swirling the beaker for about 10 s, and then discard the water. Repeat this washing step at least 5 times. 7. Dry each washed tea bag with separate paper towels. Soak the washed tea bags in water separately to reswell the gels. Record what happens after 10 min or longer of soaking the tea bags in water. 8. Remove the tea bags from water, and dry the excess water with paper towels. Separately weigh the tea bags containing polymer gels. 9. Compare the volumes and the weights of the two tea bags.
Extension 2: Observing the Ion-Exchange Cycle For the second extension activity, using the materials the instructor provided to groups of students in Extension 1, students are asked to follow these instructions: 1. Prepare a concentrated NaCl solution by dissolving ∼20 g of NaCl in 50 mL of deionized water. 2. Immerse the tea bag containing CaCl2-contracted gels from Extension 1 in the NaCl solution for at least 1 h. 3. Remove the tea bag and wash it using the same procedures described in Extension 1, Step 6. Soak the tea bag in deionized water for at least 1 h. Remove the tea bag, dry the excess water off of it, and weigh the tea bag. Record the volume change and the weight change of the tea bag. 4. Dispose the tea bags containing swollen gels in the garbage.
Summary This activity successfully demonstrates how differently CaCl2 and NaCl affect the behaviors of the superabsorbent polymer gel. It can be adapted as a follow up experiment to the featured classroom activity. This extension explores the effect related to the charges on metal ions, monovalent versus polyvalent, on the gel stability. The chemistry involved in the gel contraction and swelling and the applications of the polymer gel in water softening and deionizing are revealed. The students can also experience an example of Le Ch^atelier's effect through the performance of the reverse ion exchange of the contracted gel. Other compound pairs of monovalent versus polyvalent metal ions involving LiCl, NaCl, KCl, CaCl2, BaCl2, ZnCl2, CuSO4, Fe(NO3)3, and Al(NO3)3 give similar results in the activity of reswelling versus non-reswelling. Students could be encouraged to examine all these compounds, look for similarities in the results, and organize the compounds into groups of monovalent or polyvalent metal ions. Moreover, the formation of colored gels from polymers contracted with Cu2þ and Fe3þ
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In the Classroom
ions provides evidence that the polymer gel is capable of adsorbing polyvalent metal ions. Acknowledgment We would like to extend our thanks to the National Science Council of Taiwan for support under grant NSC-98-2511-S-259-001. Notes 1. To reveal the aspect of real-world connection, we substituted small pieces of diaper (approximately 5 5 cm) cut from unused diapers containing superabsorbent polymer for the polymer powder when the featured activity is performed. Once the swollen gels collapsed in the presence of CaCl2, the students can see the polymer particles easily. 2. The tea bags used in this experiment are the pyramid-shaped tea bags from Lipton tea products, which can be purchased from most grocery stores. The bags are emptied, washed with deionized water, and oven-dried before the activity. 3. Superabsorbent polymers can be purchased from chemical companies, science suppliers, or extracted from unused diapers following the procedures described in the online supporting information of an experiment published in this Journal (3). 4. CaCl2 is also available in stores as a product of moistureabsorber such as DampRid. 5. Leaving the tea bag under the water level, one can pull the long end of the cotton thread up and down to accelerate the swelling of the polymer. 6. Because CaCl2 works so efficiently, the amount of CaCl2 used to deswell the gels is reduced to ∼1 g. 7. Turn the tea bags around to ensure thorough contact with the solid compounds, and press the bags with a plastic spatula to speed up the contraction.
Literature Cited 1. Hodgson, S. C.; Bigger, S. W.; Billingham, N. C. J. Chem. Educ. 2001, 78, 555–556. 2. Chen, Y.-H.; Yaung, J.-F. J. Chem. Educ. 2006, 83, 1534–1536. 3. Yaung, J.-F.; Chen, Y.-H. J. Chem. Educ. 2009, 86, 347–349. 4. Kumar, A.; Srivastava, A.; Galaev, I. Y.; Mattiasson, B. Prog. Polym. Sci. 2007, 32, 1205–1237.
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5. Smart Polymers;Applications I Biotechnology and Biomedicine, 2nd ed.; Galaev, I., Mattiasson, B., Eds.; CRC Press, Taylor & Francis Group: New York, 2008. 6. Gil, E. S.; Hudson, S. M. Prog. Polym. Sci. 2004, 29, 1173–1222. 7. Kikuchi, A.; Okano, T. Prog. Polym. Sci. 2002, 27, 1165–1193. 8. Qiu, Y.; Park, K. Adv. Drug Delivery Rev. 2001, 53, 321–329. 9. Osada, Y.; Gong, J.-P. Adv. Mater. 1998, 10, 827–837. 10. Polymer Gels;Fundamentals and Biomedical Applications; DeRossi, D., Kajiwara, Y., Osada, Y., Yamaruchi, A., Eds.; Plenum Press: New York, 1991. 11. Criswell, B. J. Chem. Educ. 2006, 83, 576A–576B. 12. Silberberg, A. Gelled Aqueous Systems. In Polymers in Aqueous Media Performance through Association; Glass, J. E., Eds.; Advances in Chemistry Series 223, American Chemical Society: Washington, DC, 1989; pp 1-14. 13. Kulicke, W.-M.; Nottelmann, H. Structure and Swelling of Some Synthetic, Semisynthetic, and Biopolymer Hydrogels. In Polymers in Aqueous Media Performance through Association, Glass, J. E., Ed.; Advances in Chemistry Series 223, American Chemical Society: Washington, DC, 1989; pp 15-44. 14. Tanaka, T. Phase Transition of Gels. In Polyelectrolyte Gels Properties, Preparations, and Applications; Harland, R. S., Prud'homme, R. K., Eds.; ACS Symposium Series 480, American Chemical Society: Washington, DC, 1992; pp 1-21. 15. Mouginot, Y.; Morlay, C.; Cromer, M.; Vittori, O. Anal. Chim. Acta 2000, 407, 337–345. 16. Morlay, C.; Cromer, M.; Mouginot, Y.; Vittori, O. Can. J. Chem. 2001, 79, 370–376. 17. Roma-Luciow, R.; Sarraf, L.; Morcellet, M. Eur. Polym. J. 2001, 37, 1741–1745. 18. De Stefano, C.; Gianguzza, A.; Piazzese, D.; Sammartano, S. Talanta 2003, 61, 181–194. 19. Shimomura, T.; Namba, T. Preparation and Application of HighPerformance Superabsorbent Polymers. In Superabsorbent Polymers Science and Technology; Buchholz, F. L., Peppas, N. A., Eds.; ACS Symposium Series 573, American Chemical Society: Washington, DC, 1994; pp 112-127.
Supporting Information Available Photographs; examples; data table. This material is available via the Internet at http://pubs.acs.org.
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