Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Cohesion-Adhesion-Tension Theory

  1. Apr 9, 2010 #1
    Hello!
    This is more about conceptual understanding, concerning cohesion-adhesion-tension theory.
    In the leaves, the water molecules adhere to the cell walls of the mesophyll cells; and a thin film of water is present over the cell walls. When the water molecules evaporate, the film curves inwards into the gaps between the microfibrils that make up the cell wall, increasing the surface area (and, as a consequence, the rate of evaporation). This increase in surface area apparently increases the surface tension, and so the water molecules forming a chain, via cohesion, are pulled up the plant to counter this curvature and decrease the surface tension.
    I've looked at a number of sources, but nothing gives a definition than I can apply here. I can understand an object that sits on the water, as the surface water molecules 'hold' togeather and the object can't break the intermolecular bonds. But how does the curvature in the plant example increase the surface tension?
    If it is energetically more favourable for the water molecules to be in contact with as many other water molecules as possible, and this was sufficiently strong, then I can understand that the flat surface reduces the surface area and therefore the number of water molecules at the surface decreases and the number 'within' (and thus in contact with a larger number of of water moleucles) increases,. Is this the explanation?
    It seems confusing because the idea of being in contact with as many molecules as possible doesn't sound like it should be called surface tension...
    I would really appreciate some help.
    Many thanks.
     
  2. jcsd
  3. Apr 10, 2010 #2

    Gokul43201

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    It might help if you cited a source for the explanation that you have paraphrased above - better to get it directly from the horse's mouth. If the source is not a common textbook (or even if it is), it would help if you could (also) find an online source providing this explanation.
     
  4. Apr 11, 2010 #3
    Thank you for the response.

    From my previous post:
    “In the leaves, the water molecules adhere to the cell walls of the mesophyll cells; and a thin film of water is present over the cell walls. When the water molecules evaporate, the film curves inwards into the gaps between the microfibrils that make up the cell wall, increasing the surface area (and, as a consequence, the rate of evaporation). This increase in surface area apparently increases the surface tension, and so the water molecules forming a chain, via cohesion, are pulled up the plant to counter this curvature and decrease the surface tension.”

    This is from Biology, 8th Edition, by Campbell and Reece. The part about the chain refers to the water molecules in the xylem, being ‘pulled up’ by negative pressure which is said to originate at the “interface” between the water and air, in the leaf, on the mesophyll cells.
    This leaves it somewhat open to interpretation. As I am unsure whether the evaporation ‘stretches’ the water surface by keeping the number of molecules at the surface the same, and stretching the intermolecular bonds between these water molecules, or if more water molecules are introduced to the surface layer. If the surface curves inwards, then the size of the surface area increases, which must be achieved by either keeping the same number of molecules and stretching the bonds between them, or increasing the number of molecules present at the surface, I cannot think of any other possible way.

    The former simply means that the water molecules will want to move closer, to reduce their energy, which results in pulling water molecules up the xylem.

    The latter suggests the following conclusion I came to previously:

    “If it is energetically more favourable for the water molecules to be in contact with as many other water molecules as possible, and this was sufficiently strong, then I can understand that the flat surface reduces the surface area and therefore the number of water molecules at the surface decreases and the number 'within' (and thus in contact with a larger number of of water molecules) increases,. Is this the explanation?”

    And the reasoning for this statement is from here:

    “A water molecule in the fluid bulk is surrounded by attractive neighbours, while a molecule at the surface is attracted by a reduced number of neighbours and so in an energetically unfavourable state. The creation of new surface is thus energetically costly, and a fluid system will act to minimize surface areas.”

    This is from an MIT source:
    http://web.mit.edu/1.63/www/Lec-notes/Surfacetension/Lecture1.pdf

    Any help is much appreciated
     
    Last edited by a moderator: Apr 25, 2017
  5. Apr 11, 2010 #4

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Curvature increases the total surface area and the total surface tension is integrated over that increased area. Usually the curvature increases to minimize surface area but in the plant...

    ... which is another way of saying that when water evaporates in the plant, underlying structures force the surface area to actually increase leading to the increase in total surface tension.
     
  6. Apr 12, 2010 #5
    Many thanks for the response chemisttree.
    There is one final point that I still do not know the answer to: Is the increased tension due to the 'strain' on the bonds a consequence of the 'stretching' of trhe intermolecular bonds or is it the introduction of more molecules to the surface layer?
    Both are not equal answers, as far as I am aware. Although I now believe both are possible: At first, if the curvature is minor, then it is merely strecthing of the intermolecular bonds; if more energy is then supplied to the system, and the curvature is further increased, the bonds cannot be stretched any more and so more water molecules are introduced to the surface. (This assumes more molecules at the surface requires more energy, and results in a higher energy state, than stretching bonds does.)
    Is this a plausible explanation?
    Any responses appreciated. This is one of those concepts that just 'pokes' you in the most annoying manner until you have some understanding.
     
  7. Apr 13, 2010 #6

    chemisttree

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member



    I don't see it in terms of stretching bonds but rather that the surface molecules of water are somewhat coordinatively unsaturated. That 'unsaturation' leads to stronger bonds with their neighbors... a tension results at the surface. The normal curvature results from the requirement to minimize total energy leading to minimized surface energy. Underlying structures can force the surface to adopt a configuration that results in a higher surface area and thus a higher total energy... manifesting itself in this case as a capillary force to counteract the increasing curvature and the higher energy state that leads to.
     
  8. Apr 16, 2010 #7
    Many thanks Chemisttree.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook