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Recently I did a reaction in the lab (the reaction itself is not important) whose product was colloidal sulfur. As I watched the sulfur particles form I noticed something strange: At first the sulfur particles appeared white, but after a while the solution turned yellowish (probably when the size of the sulfur particles grew a little). I know that sulfur's colour is yellow by nature, because of its absortion soectrum, and I understand that the white colour is probably due to Mie scattering of light by the colloidal particles.

My question is: Why aren't the particles yellow? if the absortion spectrum has as a consequence a yellow colour I would expect that spectrum to remain valid at all dimensions of the particle, the particle can't just randomly decide to stop absorbing light of some wavelenght when it reaches a threshold size... can it?

I know that Mie scattering predicts that a particle will scatter all wavelenghts equally if its size is comparable or bigger than the wavelenght of light, but I would have thought that if the particle absorbed a specific wavelenght that wouldn't change with size.

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  • $\begingroup$ What about oxidation states of sulfur? Could that or a ligand be responsible? $\endgroup$
    – Dale
    May 29, 2015 at 9:41

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There are a number of different effects:

  1. Rayleigh scattering is the predominant effect for the tiniest particles, much smaller than the wavelength of light, i.e. before they've coalesced. It is dependent on the fourth (!) power of the frequency of the radiation, so short wavelength light is scattered toward the eye. Looking through the colloidal mixture, it should look redder.

  2. Tyndall scattering applies to larger colloidal particles, and it too is dependent on the fourth power of the frequency of the radiation and preferentially scatter bluer light, so the yellow of the sulfur would be less apparent.

  3. Finally, particles coalesce and scattering becomes less important than reflection from the larger particles.

  4. Perhaps sulfur nano-particles behave a bit differently from bulk material due either to quantum effects or to surface effects. See Elemental Sulfur Nanoparticle Coarsening... If someone has experience with nanoscale sulfur, please feel free to comment or edit this.

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  • $\begingroup$ I think that what I call Mie scattering is what you call Tyndall scattering. Anyway, I hadn't considered the possibility that sulfur was actually scattering more light of the wavelenght that it tends to absorb (blue or violet) and thus looks more white. This sounds quite reasonable and maybe kind of obvious in hindsight, I'm still not totally convinced but if none has a much better answer I'll accept that. Aren't colloids usually whiteish irrespective of the color of the bulk material? that's what bugs me. $\endgroup$
    – Ignacio
    May 29, 2015 at 4:09
  • $\begingroup$ According to the Journal of ACS, 1921 (books.google.com/…), "The color produced by the action of alkaline sulfides on compounds of iron and certain ... from brown to green but that the color was due to colloidal iron sulfide." Thanks for the question, as it encouraged me to research more (I would have expected black or brown). $\endgroup$
    – DrMoishe Pippik
    May 29, 2015 at 21:10

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