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How solubility of oil in water is effected by temperature?

  1. Aug 5, 2008 #1
    I think oil will be less soluble in water in high temperature. Oil is not soluble in water because it is not favoured by entropy change, so we cannot make oil soluble just by changing temperature. Rather increasing temperature would remove energy barrier and decrease solublity.
    [Energy barrier:
    Lets talk about small portion oil that is in the solution. These oil molecules will be sorrounded by many well ordered water molecules (of high potential energy). This energy is somewhat analogous to surface energy. Any two oil molecule cannot easily access each other because they are covered by sheat of water molecules. It required energy for removing this water molecules .... only after then there can be any interaction between two oil molecule. When we provide energy this energy barrier is overcome and oil molecules coalesce.]
    What do you think?
     
  2. jcsd
  3. Aug 5, 2008 #2

    GCT

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    Consider when you have soup with a meat constituent in it you want to consume it fresh off the stove since the fat precipitates with time . Although this may be because the fat forms smaller miscelles ; I am going to need to investigate this further .
     
  4. Aug 6, 2008 #3
    There is a negative change in entropy when oil goes to solution (dissolve).
    Del G = Del H -TdelS
    For the process DelS= negative; you cannot make Del G negative anyway just by increasing temperature.
     
  5. Aug 7, 2008 #4

    GCT

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    In the cast of fatty acids molecules micelle formation is favorable in entropy ; the aggregation is not favorable however the orientation of the layers increases the entropy .

    Also consider the fact that with higher temperatures the enthalpy actually becomes more negative in the perspective of Hess .
     
  6. Aug 7, 2008 #5

    chemisttree

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    I think you have it backwards. I think that to break up the ordered structure of water at room temperature and thus make room for the oil molecule to interact, will require more enthalpy than at a higher temperature. At room temperature it would be more likely that the water would rather have an additional hydrogen bond rather than the weaker london type interaction and thus the propensity would be to reestablish the hydrogen bond network and exclude the oil molecule... thus it would coalesce with like molecules. Heating up the water breaks up the ordered structure somewhat and makes room for the oil molecule without paying so much of an entropic or energetic penalty.
    This is what is observed in mineral oil/water systems.
     
  7. Aug 7, 2008 #6

    Redbelly98

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    I thought entropy is always higher for a mixture than a non-mixed state, so for a high enough temperature the (-T delS) term will overcome a positive delH and cause mixing (dissolving). Just how high a temperature depends on the ratio (delH)/(delS)
     
  8. Aug 7, 2008 #7

    Ygggdrasil

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    The aggregation of nonpolar molecule in a polar environment (known as the hydrophobic effect) is in fact driven by entropy (at least at room temperature). It turns out that solvating a nonpolar molecule in water requires water to make a solvation shell around the nonpolar molecule. This solvation shell is fairly ordered and the loss of entropy from the ordering of water around the nonpolar molecules outweighs the gain in entropy from dispersing the nonpolar phase. Aggregation of the nonpolar molecules into droplets minimizes the area of polar/nonpolar interfaces and thus maximizes entropy.

    http://en.wikipedia.org/wiki/Hydrophobic_effect
     
  9. Aug 7, 2008 #8

    Redbelly98

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    Yggg, thank you for the link and explanation.
     
  10. Aug 9, 2008 #9
    I understand what you say. But how do you account for it in term of gibbs free energy? Free oil molecules in water is not favoured energetically and by entropy factor. Accordingly increasing temperature only worsen the scene.
     
  11. Aug 10, 2008 #10
    From the link provided by chemistree it is evident that solubility of oil increases with the temperature. The enthalpy for this change is positive (i.e not favorable) so to facilate solubility entropy factor must increase. Accordingly this may be the case that at higher temperature solvation shell around oil molecule lose some degree of order, so the overall change in entropy is less negative.
    But oil is never much soluble in water at any temperature.
     
  12. Aug 10, 2008 #11

    GCT

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    You are not understanding two points

    1) Enthalpy of a process changes with temperature

    2) Oil molecules organize in water
     
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