Diffusion constant

Discussion in 'General Science' started by boks, Jan 25, 2009.

  1. boks

    Thread Starter Active Member

    Oct 10, 2008
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    Phosphatidylcholine is a lipid that can be found in egg yolk. When phosphatidylcholine is mixed with water it will form micelles – spherical particles.

    Molecular weight (Mw) for the particles is 97*000 g/mole
    Density for the particles is 1.018 g/cm3
    The temperature is 25°C

    a) What is the radius for the particles?


    Assume that the particles are not hydrated.
    The viscosity of water (eta) at the given temperature is 0,9 mPa·s (milliPascal second)
    Boltzmann constant: 1,38 ´ 10-23 J/K
    (Pa=kg/ms2 and J=kgm2/s2)

    b) What is the diffusion coefficient (D) for the particles in water?


    a) One particle has the mass m = (97 000 g/mol)/6.02E23 = 1.61E-19 g.

    This gives V = 1.58E-25 m3, and since the volume of a sphere is given by (4/3)*pi*r^3, the radius for the particles is 3.35 nm.

    b) Stokes-Einstein: D = \frac{k_B T}{6 \pi \eta R_s} = \frac{1.38E-23 \cdot 298}{6 \pi \cdot 0.9E-3 \cdot 3.35E-9} = 7.24E-11

    What's wrong here?
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    That looks about right.

    I have the following table of diffusion coefficients for uncharged spheres in water at 20°C (after Shaw)

    Radius........Diffusion Coeff

    1nm............2.1E-10
    10nm...........2.1E-11
    100nm..........2.1E-12
    1μm..............2.1E-13

    Best I can do at midnight here, I'm off to bed.

    g'nite
     
  3. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    904
    Choline is a quaternary amine and has a positive charge.

    John
     
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
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    This can only happen if particles have no charge.

     
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