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The water molecules at the surface of water are surrounded partially by air and partially by water. These surface molecules would be much more stable if they could be in the interior of the liquid where all their hydrogen bonds could be fulfilled (cohesion). Therefore, water normally tends to have the smallest surface possible, i.e. it has a high surface tension, in order to achieve the lowest possible energetic state.
If a solid material more dense than water is placed on the surface of water, what happens next depends on the nature of the material. If the material is hydrophilic ("water loving") it has a surface to which water is attracted. The adhesion of water to the surface of this material coats the surface of the object with water, reduces the surface tension, and causes the object to sink.
If the solid object is hydrophobic ("water fearing"),the unfavorable interactions between the water surface and the object make it difficult to wet the surface. Two forces now come into play -- the energy it would take to overcome this repulsion and the force of gravity. If the force of gravity is strong enough, it will prevail and the object will sink (assuming that the object has a density greater than water). If the gravitational force is less than the surface tension then the object will float on the surface of the water.
Surface tension is what permits water striders and other insects to walk across the surface of water and what enables a needle to float. Of course, the critical feature here is the amount of force per unit area -- put a needle into water end-on instead sideways and the needle will immediately sink.
When one drop of liquid detergent is added to the beaker without stirring, the sulfur suddenly sinks to the bottom of the beaker.
If you have Apple's (free) QuickTime installed, you can watch a color movie of the demonstration. This movie is 1.05 Mb in size, so it may take a while to download if you have a slow Internet connection.
To view the movie, simply click on the picture below:
A typical example of a detergent molecule is sodium lauryl sulfate (read that shampoo bottle of yours!). The structure can be represented in several different ways. Notice that in the models the Na ion has been left off because the anion and cation completely dissociate in water:
If you have the MDL Chime plug-in [not available for Mac OS X, sorry...be sure to complain to MDL about this] installed, you can play with this interactive 3-D model of a sodium lauryl sulfate molecule. You can rotate it, change the display features, enlarge/shrink, display solvent accessible surfaces and more...click and play:
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This page was last updated Friday, March 27, 2015 and is copyright 1999-2016 by Rob Toreki. All rights reserved.