Chemistry Everyday for Everyone
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applications & analogies
Ron DeLorenzo Middle Georgia College Cochran, GA 31014
Musk Oxen and Micelles John W. Hill University of Wisconsin–River Falls, River Falls, WI 54022 Musk oxen are large, shaggy, oxlike animals that inhabit the polar regions of North America, where their main natural enemy is wolves. When threatened, the mature male musk oxen form a protective circle; their tails are turned to the inside of the circle, and their heads face outward toward the hostile polar world. Musk oxen females and calves are safely sequestered inside the circle (1). This action of musk oxen herds when threatened by predators provides an analogy for the behavior of amphipathic substances (those having both hydrophilic and hydrophobic groups) in water. Soap is a familiar example of an amphipathic substance. When placed in water, soap anions tend to cluster in groups in a way that minimizes contact of the hydrophobic parts with surrounding water molecules. The hydrophobic nonpolar hydrocarbon chains of the soap gather together away from the polar water molecules as much as possible. The hydrophilic polar carboxylate groups, on the
other hand, tend to remain in contact with the polar water molecules. To accomplish these things, the soap anions cluster in roughly spherical micelles, with their hydrocarbon “tails” tucked to the inside and the polar carboxylate groups facing out into the water (2). As soap cleans, oily “dirt” is dissolved and held in the interior of the micelle. This “protected” greasy material is then removed in the polar wash water. In cross section, the arrangement of soap anions in a micelle is much like that of musk oxen in a protective circle: heads to the outside and tails to the inside, an arrangement that can isolate other things from a harsh polar world. Literature Cited 1. For example, see Audesirk, G.; Audesirk, T. Biology Life on Earth, 2d ed.; Macmillan: New York, 1989; pp 638–639. 2. For example, see Hill, J. W.; Petrucci, R. H. General Chemistry; Prentice Hall: Upper Saddle River, NJ, 1996; pp 302–304.
Vol. 73 No. 9 September 1996 • Journal of Chemical Education
847
