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Homework Help: Need help reading a phase diagram for a lipid DMPC

  1. Mar 20, 2015 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations
    None required?

    3. The attempt at a solution
    I understand that Id would be a disordered phase, Io an ordered phase and the other two are regions where 2 phases exist at the same time possible due to cholestrol. im assuming so means solid ordered.

    But other than that I have no idea how to read this phase diagram, or how to make a calculation (part (ii)), if anyone could shed some light it would really be appreciated.

  2. jcsd
  3. Mar 20, 2015 #2

    Quantum Defect

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    Starting at the top, you have a single liquid phase.

    As you decrease the temperature, if your overall composition is somewhere near 0.3 chol. you will see phase separation into two liquid phases. The critical point is the point on the phase diagram, where the two liquids coalesce into a single point, the tip of the point that looks like a floppy hat.

    Inside the "lo - ld" region you have two liquid phases.

    Where do you suppose the melting point of pure DMPC is on the graph? Hint: The downward-sloping line from the kink at the left represents the freezing point depression of DMPC with added cholesterol.

    In the "so-lo" region you have a solid-liquid region. Solid has the composiiton indicated by the left edge line, liquid has the composition of the right-edge line.

    The fractions in each phase in the two phase regions are estimated by placing a point at the overall composition/temperature, and drawing horizontal lines out until you hit a phase boundary line. The lengths of the two lines tell you something about the relative amounts of each phase.
  4. Mar 21, 2015 #3
    Thats awesome cant thank you enough, we have these diagrams in our class notes but the explanation was VERY brief. Thanks again!!


    so to do this would you place the point in approximately the centre of the region? i.e. for the S0-lo region the point would be at around 17.5 degrees and half way between 0.05 and .030? i.e. where is the overall composition/temperature point?
    Last edited: Mar 21, 2015
  5. Mar 21, 2015 #4

    Quantum Defect

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    Whatever composition or temperature you are given, you place a point on the graph. If the point is in a two-phase region, you draw a horizontal line out to the two edges. The points on the edges give you the compositions for the two phases.

    You can tell the fractions of each of the two phases by solving:

    X_t = f_1 x_1 + f_2 x_2, where f's are fractions and x's are compositions.

    You can also solve for the fraction graphically. From your line above, imagine a see saw, whose fulcrum sits at the composition of the whole. The length of the left segment is proportional to the fraction of the phase whose composition is given by the right point, and the right segment is proportional to the fraction of the phase whose composition is given by the left point. It seems backwards, but if you imagine the case where your total composition sits close to either phase boundary, the majority phase will be the one closest to your point--the shortest line segment.
  6. Mar 21, 2015 #5
    Ahh i see, excellent. thanks again!
  7. Mar 23, 2015 #6
    I've been looking at past exam papers and a common related question to this are the "factors that govern the appearance of the phase separated mixture", or "the factors that control the morphology of the resulting domains".

    All we really have to go on is the following slide:


    Obviously there are many domains which eventually merge into 2 domains, but the questions are worth 8 marks and it doesnt really describe the factors, any idea of what I could put should it come up in the exam or something on the internet that explains it more thoroughly? all I can find is research papers.

    thanks for any help
  8. May 17, 2015 #7
    I recognise that slide. Doing the bionanophysics exam tomorrow?

    For "factors that govern the appearance of the phase separated mixture", or "the factors that control the morphology of the resulting domains", this is referring to the spindodal decomposition, which occurs when the temperature is lowered from a one phase system to a two phase system, and the domains are convoluted and maze like. http://www.sv.vt.edu/classes/MSE2094_NoteBook/96ClassProj/pics/spinodb.jpg [Broken]
    See the spinodal decomposition is the centre section.

    If the mixture is in the alpha1 region, and the concentration is increase, the domains are formed through the nucleation and growth model. If this happens fast, then the domains will be small in size, because they do not have time to move into their new equilibrium position.

    For factors that govern the appearance of the mixture, you could also mention that the Lo phase is thicker than the Ld phase, therefore their hydrophobic tails can be exposed. In order to reduce this, and minimise the Gibbs free energy, the Lo lipids group together, which is what a domain is.
    Last edited by a moderator: May 7, 2017
  9. May 17, 2015 #8
    lol yes. Thanks for the description, finally got it in the end.

    Good Luck!
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