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Surface tension and surfactants

  1. Jul 28, 2016 #1
    I am struggling to understand the relation between surface tension and surfactants. When surfactants are added to say water they may have charge and head groups which influence surface motion due to repulsion and their size. However often when people refer to surface tension they refer to a liquids surfaces ability or inability to be deformed.

    Let's take alcohols for example, they have different surface tensions, but are they surfactants?

    I am trying to understand how surfactants change viscosity / surface tension properties. And if alcohol is a surfactant especially here where it talks about the chain length:

    Also are gases surfactants if they change the surface tension of a liquid? but they have no head group?

    And finally does the pH of a solution relate to its surface tension?

    Apologies if the question doesnt make sense, I'm a little confused.

    Thanks for any help
    Last edited: Jul 28, 2016
  2. jcsd
  3. Jul 28, 2016 #2


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    Yes. Yes. No.
  4. Aug 2, 2016 #3
    Apologies for the delay, I had a brief holiday. Not sure what the first thing about narcolepsy is lol

    Thanks for the reply!

    You said no to the last question, but (as I understand it) this results from an equilibrium hydroxide vs hydronium balance i.e. i.e. (Γ(H+) = Γ(OH)) ---> if one is adsorbed the other compensates and the surface tension remains constant. But for more complex species they may change this equilibrium effect? that is a possible surface pH reduction causing a change in surface tension. Could you possibly confirm?

    Some of what I'm trying to understand is given in this paper:


    Any addiditonal comments would be welcome, as I'm really not a chemist.
  5. Aug 3, 2016 #4


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    Hmmm ---- pH along some nano-scale gradient vs. bulk properties? I'm not certain there are meaningful ways of discussing such things. If you'll allow me a bit to mull this over?
  6. Aug 4, 2016 #5
    Yes please, that would be helpful. Its meaningful to me because I'm dealing with the coalescence of bubbles where changes in surface tension have been seen to have an effect, especially in terms of a change in pH at the surface causing some sort of surface tension gradient. The above paper seems to be suggesting a relation between a change in pH and surface tension, while others suggest surface tension remains constant during a change in pH due to equilibrium conditions. As I understand it. So yes, any further comments / ideas would be more than welcome, thanks.
    Last edited: Aug 4, 2016
  7. Aug 5, 2016 #6


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    Coalescence of bubbles implies a reduction of surface area, and of surface free energy; methinks this is more a matter of conventions than of actual physical properties.
  8. Aug 8, 2016 #7
    yes the coalescence of bubbles is generally due to surface properties, in particular surface "drainage" and its ability to occur so rupture can then happen. but for low Bond numbers (high surface tension) it has been calculated that coalescence would occur less readily. therefore if there is a relation between pH and surface tension, this would imply a relation between pH and coalescence. But you are right in that it may not be so meaningful, as the studies on this are quite scarce :-/ Here's what I've written so far (with limited understanding) for anyone that is interested:

    If anyone has any comments on something that is incorrect, any feedback would be welcome. Also if anyone wants the references.

    Thanks Bystander for your help! again any further comments would be welcome
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