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I have been learning about water coagulation in my chemistry textbook. Apparently, before being treated, water has clay particles, primarily silicates and aluminosilicates, which apparently have a negative charge. These particles repel each other, forming a colloidal mixture.

My question is: how exactly do the silicates and aluminosilicates have a negative charge? I understand that silicates and aluminosilicates have a negative silicate/aluminosilicate anion, but wouldn't that negative charge be counteracted by their bonding to a positive cation, causing the silicate/aluminosilicate to have an overall neutral charge?

My current conjecture is that, in water, the silicate/aluminosilicates particles dissociate, and the cations are solvated by water however the silicate/aluminosilicate anions are too large to be solvated, thus they remain in the water and repel each other. This is the only way that it currently makes sense to me because other ionic compounds like sodium chloride do not form colloids in water despite obviously having cations and anions; likely because both the cations and anions are solvated by the water particles because they are both small enough.

If anyone can correct my conjecture/explain to me how this works, I would really appreciate it.

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  • $\begingroup$ There is related argentometry indication by - I guess - eosin dye. The colloidal or finely precipitated AgCl is initially negatively charged by adsoption of excessive Cl-. After exuivalence, it gets positively charged by excessive Ag+, what is followed by adsorption of negatively charged anionic dye. (Never personally tried, I have heard the Ag2CrO4 indication is better). $\endgroup$
    – Poutnik
    Commented Sep 9, 2022 at 8:08
  • $\begingroup$ @Poutnik Maybe it’s because I’m in year 12, but I have no clue what any of this means sorry $\endgroup$
    – Scratch Cat
    Commented Sep 9, 2022 at 8:35
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    $\begingroup$ @ScratchCat Colloidal particles may cause adsorption of preferably particular ions, what may lead to slight nonzero net charge. // If you do not write in advance what knowledge level to expect, expect info at levels you may not have expected. :-) The implied level of the question seamed to me to be OK with the level of my comment. :-) $\endgroup$
    – Poutnik
    Commented Sep 9, 2022 at 9:16

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You are actually pretty close to answering your own question.

Key here is clay particles are very fine, meaning they have a very high surface area to mass ratio.

So, even though they are not dissolved, interaction with water plays a prominent role in how they act as a colloidal suspension.

The oxygen bears the negative charge in silicates. In suspension (as you said) they will repel one another and attract the positive part of water molecules. So you also have water between the clay particles as well, keeping them from clumping together (flocculating) and falling out of solution.

As particle size increases, there is less tendency to be held in suspension, even for clay because the difference in density becomes more and more a factor in separation over time.

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