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Name: Stephanie P.
Status: student
Age: 9
Location: N/A
Country: N/A
Date: 6/9/2003
Question:
What else besides soap can change water tension?

Replies:


Replies:
Almost ANY substance you add to water will change the surface tension of water (which is about 72 ergs/cm^2 at room temperature). Some more than others. Most simple salts, like NaCl or NaOH have little effect, or can actually increase the surface tension slightly. Substances that reduce the surface tension significantly (say reduce it to 20-30 ergs/cm^2) are called surfactants (an acronym for SURface ACTive AgeNT). The subject of how and what makes a "good" surfactant is rather complicated and advanced, but there are some trends and rules that can be guides. A "good" surfactant must have a water loving (polar or ionic) and a water hating (hydrocarbon or fluorocarbon) part in the same molecule. The "good" surfactant will usually be rather water insoluble (soap is a good example). The molecules of a "good" surfactant will tend to collect at the surface of the water, so that there is a slightly higher concentration of the surfactant near the surface compared to the concentration in the bulk of the water. These "good" surfactants bury their water-loving part in the water and the water-hating part is pointed toward the air above. Because the water-hating parts of the molecule do not bond to one another very well it is easier to "stretch" the surface of the water. In pure water the bonds formed between the hydrogens and oxygens (so called hydrogen bonds) at the surface make is difficult to "stretch" the surface. Detailed and quantitative treatment of the subject of surfactant behavior is and active area of research, and is call by one author RIP (which stands for Research In Progress).

Vince Calder


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