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Determining the size of solute

  1. Oct 11, 2012 #1
    In reverse osmosis of salt water, the mesh size in filter determines the size of particles dissolved in the solution than can pass through. Here does the size of particles refer to molecular size? If so, how can I possibly determine the size of say Eg. NaCl dissolved in water.

    I have been searching in google for access to molecular size of various chemicals. Are there any links or sources where I can get access to molecular size of various solutes. Would be grateful for any such links or other possible suggestions.

  2. jcsd
  3. Oct 11, 2012 #2
    Firstly, in the reverse osmosis of salt water, an ionic solution, there is an effective mesh size that is much finer than the physical mesh size, because what stops ionic material from passing the filter is the electrostatic repulsion between the surface charge due to adsorbed ions on the mesh, and the counter-ions in the solution.

    Secondly, in aqueous solutions the "size" of any particular molecule or ion is often quite different from the size that you might infer from the van der Waals radii if its atoms. Water is a polar solvent that "solvates" things very strongly. When a molecule (or ion) of any sort moves in water, it usually takes with it at least a single layer, and often more than a single layer, of adsorbed water molecules with it. There is a recognised "surface of shear" which usually amounts to roughly 1 layer of water molecules all around the solute molecule or ion -- sometimes more, sometimes less.

    Check out terms like "salvation" "solvation shell" "surface of shear" in Google or similar.
  4. Oct 12, 2012 #3
    Thanks for the reply. Just going through the topics.

    btw, I have yet another doubt (may be childish). Just curious. I have been reading that RO membranes can purify water even free of virus and bacteria (hope so!). my doubt is, if that is the case can the RO membrane seperate any salt that can be dissolved in the water?

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