Average diameters of colloids

Discussion in 'General Science' started by boks, Jan 22, 2009.

  1. boks

    Thread Starter Active Member

    Oct 10, 2008
    A dispersion of spherical polymer particles contains Np = 4•1013 particles/dm3. The total surface of the particles in 1 dm3 is Asp = 820 m2.

    a) What kind of average diameter can we calculate. Calculate this average diameter.

    b) If we know that the dispersion contains 60 vol% water, we can calculate another type of average diameter. Which? Find it!

    a) Surface average.

    \bar{d_s} = \sqrt{\frac{\Sigma n_i d_i^2}{\Sigma n_i}}

    b) Volume average.

    \bar{d_s} = \sqrt{\frac{\Sigma n_i d_i^3}{\Sigma n_i}}

    I don't know how to incorporate the data into the formulas.
    Last edited: Jan 22, 2009
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
    That's because these formulae are too complicated.

    In both a and b consider unit volume = 1 cubic dm.


    Total area = average area x number of particles


    Since 60% is disperse medium 40% is disperse phase

    Total Volume x 0.40 = average volume x number of particles

    I presume you can substitute suitable formulae for area and volume of a sphere.
  3. boks

    Thread Starter Active Member

    Oct 10, 2008
    OK, thanks! I found the answers 2.55 μm and 2.67 μm, which means that this is a polydisperse system.