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Table This table from my textbook (p. 20), shows nature of charge on particles of listed sols, in their original or natural form.
What reasons can be attributed to oxides (sols) being positively charged? Any general conclusions that can be made for their preparation or occurence?



(Supplementary)
Reasons I could find out by myself to some of these:

Positively charged sols

  • Hydrated metallic oxides are formed by hydrolysing metal halides, the oxides so formed preferentially adsorb metal ions present in the solution making the sol positively charged, for example $$\ce{FeCl3 + 3H2O ->[Hydrolysis] Fe(OH)3(sol) + 3HCl}$$ Here, $\ce{Fe^3+}$ ions present in the solution are preferentially adsorbed by the colloidal particles.
  • Basic dyes are cationic dyes, so their particles are positively charged.
  • Oxides - ?

Negatively charged sols

  • Colloidal sols of metals such as gold, silver, platinum etc. can be prepared by Bredig's arc method. The electrons disperesed in the medium are captured by the sol particles making the sol negatively charged.
  • Metallic sulfides are prepared by treating Metallic oxides with hydrogen sulfide, for example $$\ce{As2O3 + 3H2S ->[Double decomposition] As2S3(sol) + 3H2O}$$ The $\ce{S^2-}$ ions present in the solution are preferentially adsorbed by the sol particles making it negatively charged.
  • Acidic dyes are anionic dyes, so their particles are negatively charged.


(Additionally) It will be appreciated if anyone can give idea on the case of gum, clay and charcoal too.

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  • $\begingroup$ The net charge of sol particles depends on ion being chemically released or captured( like for weak organic acids or bases) or ions selectively adsorbed due chemical affinity. E.g. sol particles of AgCl would be positively charged with Ag+ excess and negatively charged with Cl- excess. $\endgroup$
    – Poutnik
    Sep 23, 2022 at 9:10
  • $\begingroup$ @Poutnik yes I am aware of that. But how could we here judge what kind of charge a sol will acquire on its natural occurence or production? For metal sols I know they are made from Bredig arc method, hence I can conclude from there (due to electron capture) that they are negatively charged sols. But I could not shape such a general idea for other kind of sols. For example, how would one justify that naturally occuring oxides are positively charged sols? (Please suggest how I can make the question more clear if it's missing the point.) $\endgroup$
    – Harshil
    Sep 23, 2022 at 17:11
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    $\begingroup$ It may be a too broad and complex question, with proper answer in a scope of a long article or a short book. Consider also removal of some cases, not representing a single or well defined sol particle type, like blood. $\endgroup$
    – Poutnik
    Sep 28, 2022 at 8:54
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    $\begingroup$ "The surface of red cells carry a negative charge due to the ionization of the carboxyl group of NeuNac (N-acetyl neuraminic acid), also called NANA or sialic acid. In saline, red cells will attract positively charged Na+, and an ionic cloud will form around each cell." in page on zeta potential. $\endgroup$
    – Karsten
    Sep 28, 2022 at 11:00
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    $\begingroup$ More on the zeta potential, its origin and consequences. $\endgroup$
    – Karsten
    Sep 28, 2022 at 11:05

1 Answer 1

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The original, and fairly limited question, is: "What reasons can be attributed to oxides (sols) being positively charged?"

Disregarding the method of preparation, one answer is that these metal oxides are more basic than H2O, and therefore will adsorb H$^+$ ions from a neutral (pH =7) H2O medium. The pH will rise, but it doesn't take many protons to saturate a surface, so the rise may be only about 1 or possibly 2 pH units.

The example given in the question:

FeCl3 + 3H2O −→ Fe(OH)3(sol) + 3HCl

could well result in adsorption of metal ions, but surely the production of hydrogen ions will result in a great excess of H$^+$ and adsorption of H$^+$also. FeCl3 solution is quite acidic.

On the other hand, metals, metallic sulfides, clays, etc. could be more Lewis acidic, i.e., attractive toward the oxygen atom in water. Once adsorbed, the water molecule would be more likely to release a hydrogen ion than water would, resulting in a negative charge on the solid and an increase in acidity - again, perhaps by only 1 or 2 pH units.

The bottom line is that the solvent (water) and its initial and final pH are a serious factor in determining the charge and its magnitude on suspended particles. This is not to minimize the effect of adsorption of metal ions on suspended particles when the metal ions are in significant concentrations, but these ions also have the much more plentiful H2O molecules to interact with.

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