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Surface tension of ethanol solution?

  1. Jan 3, 2008 #1
    A query about, I think, extinguishing surface tension in 75% EtOH:H2O.
    I need to examine microscopically (~50x) specimens preserved and stored in the above medium, which I do by immersing them in dishes filled with the above, illuminating w fiber optics, and examining with a stereomicroscope.
    I notice that when filling the dish for the first time, inevitably a floating "island" of crud appears (technical term for dust, grit, fibers, skin particles, who knows). Would like to get rid of it by a more elegant means than dragging a bit of lens paper across the surface of the alcohol:water mixture. So,
    1. Can you add something (surfactant?) to 75 EtOH: 25 H2O to cut surface tension enough that said island sinks. Have tried several commercially available surfactants, "Cascade," Kodak PhotoFlo, detergent, none work. Is the surface tension of Ethanol all that different from water?
    2. Out of curiosity, why does the island always come to rest centered on (and thus ruining) the field of view? I assume it has something to do with having fiber optic lights trained on the specimen (width ca. 2mm), and the heat convection patterns of a fluid in a round dish, but would like a more exact explanation.
    thanks in advance,
  2. jcsd
  3. Jan 4, 2008 #2


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    The surface tension is a result of the O-H bonds of the molecules interacting with each other. The O-H in ethanol is similar to an O-H in H20 but ethanol only has one and the molecule is also larger in size so the cohesion is not as efficient. Water has considerably greater surface tension.

    I don't know what you're trying to view but one idea might be to increase the concentration of ethanol vs water if possible.
  4. Jan 4, 2008 #3
    Thanks! That makes sense, but if so, you'd think surfactants in 75% EtOH would have about the same effects they do in water. But, as near as I can tell, they don't. The solution has to stay at about 75% because these are museum specimens, and anything over say 90% will dehydrate the tissues and lead to damage.
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