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I Cation/cation repulsion in salt water solution

  1. Mar 31, 2016 #1
    Spherical Micelles of the surfactant DTAB, which has a cationic head, will turn into cylindrical micelles if the repulsion between the cationic heads can be reduced. I have read that adding salt (perhaps NaCl or NaBr) to this water/surfactant solution will reduce repulsion between the cations, thus triggering the change in shape. Can someone explain how adding salt would reduce coulombic forces between molecules? Salt is supposed to reduce the dielectric constant of water. Coulombic forces are inversely proportional to the dielectric constant, meaning adding salt should INCREASE repulsion. Or is the more relevant mechanism here simply that salt anions are attracted to the positive micelle surface, reducing net positive charge?
    Thanks!
     
  2. jcsd
  3. Mar 31, 2016 #2
    Back to basics here! Do ions feel more or less coulombic attraction when dissolved in a polar solvent (compared to a less polar medium)? Does higher polarity of solvent increase the forces or decrease them? In other words, does the polarity of the solvent affect its dielectric constant (permittivity) and how? I understand that ions of salt will decrease the dielectric constant of water, however the role of water's polarity in determining its dielectric constant is unclear!
     
  4. Mar 31, 2016 #3

    mfb

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    2016 Award

    Staff: Mentor

    Exactly.
    I merged your two threads as they look like the same topic.
     
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