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Surface Tension And Polarity Of Molecules

  1. Apr 16, 2006 #1
    I have a few questions about polarity of molecules, first off i know that soaps/detergents have both a hydrophobic and a hydrophilic end which are essential to is operations but is there anything else about its shape or electronegativity that im missing, like is it polar/non-poalr cause i cant really decide which one it is and if that is even important for what im doing.

    ....And about surface tension, i know that water has a very high surface tension and many of its properties spring from this, and i know the polar ends of water is what cuases this but i was wondering if there are any other liquids that have a stronger surface tension. I also noticed that the surface tension of what is 'shut off' when i introduce any soap or detergent to the water and i am also not sure why that is, i think it is because the water is too busy being attacted to the hydrophilc ends of the soap that it cant sustain it surface tension.
     
  2. jcsd
  3. Apr 16, 2006 #2

    dav2008

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    Remember the rule of thumb that "likes dissolve likes", meaning that polar molecules will dissolve polar molecules and nonpolar molecules will dissolve nonpolar molecules.

    Since water is polar, it makes sense that hydrophilic molecules are polar and hydrophobic molecules are nonpolar.

    In the case of soaps and detergents, like you said one end is hydrophilic (polar) and the other end is hydrophobic (nonpolar).
     
  4. Apr 16, 2006 #3

    Hootenanny

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    The hydrophilic end tend to be polar (ionic) and the hydrophobic tends to be non-polar (hydrocarbon chains).

    Surface tension is a result of cohesion, the intermolecular(IM) forces holding like molecules together. Detergents belong to a group of chemicals (usually organic) which lower the surface tension of liquids. They do this by 'interfering' with the inter-molecular forces between the water molcecules; their hydrophilic 'heads' are attracted to the polar ends of the molecule, this means that their hydrophobic 'tails' project outwards, forming a sphere around each water molecule. This prevents any water molecules from forming hydrogen bonds or other IM forces with each other, thus breaking the surface tension.

    Regards,
    ~Hoot
     
  5. Apr 16, 2006 #4
    ok that confims my train of thought and also puts it into much better wording.

    but now i have another question or request, ive been lookng high and low for an explaination of how to measure surface tension, the thing is ive found a few of them its just that none of them are clicking so if someone could explain:

    Gamma(y)= Force (F) / Length (L)

    and its applications i think it would go a long way
     
  6. Apr 16, 2006 #5
    what could anyone tell me about the properties of the surface tension of 2-propanol(or any alcohol), and the alcohol and water?
     
  7. Apr 17, 2006 #6

    Gokul43201

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    Surface tension is measured using one of many types of a device known as a Tensiometer.

    http://www.kibron.com/index.html?http://www.kibron.com/Science/Science.html
     
  8. Apr 17, 2006 #7

    Gokul43201

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    1. Even if you are asking a question that may be related to a part of the discussion of an active thread, it is better to start a new thread.

    2. By the rules of this subforum, you must show your own work/thoughts when you ask a question.
     
  9. Apr 17, 2006 #8
    ok then well robyn's question does link to mine because we are also using isopropyl alcohol and its structuer looks a little like this:


    ............. H
    .......... H O H
    ...... H-C-C-C-H
    .......... H H H

    and my guess is since carbon and hydrogen have relativly the same elcetron negativity the would not contibute to the surface tension but the OH on the central carbon does have a slight charge on both the oxygen and the hydrogen atom but since it barly has half the intermolecular bonding opportunities as water it would have a considerably lower surface tension, is this assumtion correct?
     
    Last edited: Apr 17, 2006
  10. Apr 17, 2006 #9

    Gokul43201

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    That's right.
     
  11. Apr 17, 2006 #10
    ok there is one mre thing that im having trouble with and that is finding both everyday uses and boilogical uses for surface tension and its realtion with sufactants and the lowering of the force.

    what i have so far for everyday uses is that surface tension can be lowered when the temp rises and/or soap is added and this is very helpful when cleaning because water with a lower surface tension will become a better "wetting" agent which will help get in all the nooks and cranies of whatever you may be washing.

    And Biological uses would be that capillery action used in the xylem cells of trees and plants to transport water to their tallest points and for water skippers and instects like that showing how important surface tension is to them and how delicate it is too because it only takes a little bit of the right pollution to lower the surface tension of and entire lake which could seriously harm the ecosystem.

    thats all i have but i feel as if the ideas that i have come up with are not good enough so im asking if anyone had more important or interesting uses of surface tension.
     
  12. Apr 17, 2006 #11

    Gokul43201

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    Okay, so surface tension is responsible for capillarity and determines wetting ability. What else does surface tension affect ? :wink:
     
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