I want to know the exact mechanism for how silver would start to grow during a mirroring with Tollen's reagent. I've made two pictures to illustrate how I see the nucleation/growth process of silver on silica:



As you can see, I didn't draw any chemical bonds, because I think that the silver adherence is due to Van Der Waals forces. Am I correct? If that is the case, then I believe a place with high permanent polarisation would be better suited for the nucleation of silver.

Now, let's follow the logic. We have a greater electronegativity difference between $\ce{O}$ and $\ce{Si}$ ($3.44 - 1.90 = 1.54$) than between $\ce{O}$ and $\ce{H}$ ($3.44 - 2.2 = 1.24$).

For the upper oxygen, assuming an angle of 109 degrees, you get a total dipole moment of: $2(1.54)\cos\left(\frac{109}{2}\right) = 1.79$

For the lower oxygen with the bonded hydrogen, assuming an angle of 120 degrees , you would get a total dipole moment of: $1.54 + 1.24 = 2.78$ (please tell me if I'm wrong to assume a straight line!)

This would mean that you have a greater accumulation of electrons at the bottom oxygen, which would favor Van Der Waals attraction (Debye interaction, I guess, since you have a permanent dipole and an induced dipole). This, in turn, would mean that it would be a better nucleation site.

If you make a surface treatment using silane groups, I believe you promote the presence of $\ce{OH}$ groups at your silica surface, though I'm unsure about this. I do know that it makes better silver mirroring though. I've made another drawing:


Following my logic, this one is wrong in many way. First of all, I drew angles on the $\ce{Si-O-H}$ bonds, which I didn't draw on my first pictures. Secondly, the silver is not even near the oxygen atoms, which I believe are the nucleation points.

I'm confused, could you please point out to me where I'm right or wrong in my logic/knowledge?


  • Is there a bond forming between the silica surface and the silver? Or is there only Van Der Waals (Debye) attraction?
  • Is my depiction of nucleation & growth correct (for the first two pictures)?
  • Is my logic regarding the electronegativity correct?
  • Is my logic of " higher total electronegativity difference, hence better nucleation site" correct? (assuming I'm right about the Van Der Waals Forces)
  • Is my depiction of silanisation correct?
  • Am I right to say that the higher abundance of surface $\ce{OH}$ groups by silanisation implies better nucleation?
  • Could I do the same when using a strong acid on my glass surface?
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    $\begingroup$ Note that the $\ce{Si-O-H}$ groups are acidic and the Tollens reagent is alkaline. Thus, there are $\ce{Si-O- }$ groups, which are missing in your drawings. $\endgroup$ – user7951 May 29 '15 at 17:16
  • $\begingroup$ Then, would you say that we get an ionic bond between the positive silver and the negative oxygen? Without the need to reduce the silver? $\endgroup$ – victorbg May 29 '15 at 17:24
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    $\begingroup$ I'd say that your representation of the surface of glass is naive. There would be all sorts of surface irregularities, and all sorts of impurities. $\endgroup$ – MaxW Oct 22 '15 at 7:00
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    $\begingroup$ I am also not very sure about it but perhaps there are only Van Der Walls forces because when we tilt the tube the silver deposited on surface leaves off the surface and suspends back into the solution. This phenomenon favors the presence of weak bonds. I have also read that some of silver forms a thin layer of a non-stoichiometric compound on glass surface which allows silver to be deposited on it but it is not confirmed. $\endgroup$ – Hamza Jun 4 '16 at 18:11
  • $\begingroup$ Maybe because the pH is quite high (you add sodium hydroxide), and you heat the actual surface of the test tube, for example, some of the Si-OH groups get deprotonated and actually coordinate to silver ions, and so they get reduced at the very surface of the glass. That would be my guess about the actual formation mechasm. I would guess that VDW forces hold silver at the glass surface, as it can very easily be scraped off. $\endgroup$ – Uros Jun 27 '16 at 1:18

"If you make a surface treatment using silane groups, I think that you promote the presence of OH groups at your silica surface". This is NOT correct. You siliconize glass to cap the free SiOH groups

2 silica_Si-OH + n SiCl2Me2 + HO-Si_silica -> silica_Si-[O(SiMe2)]$_n$-O-Si_silica. As a result you decrease the number of active OH groups and make it highly non-polar.

Before siliconization silica is surrounded by a hydration sphere. Ag$^+$ can bind it well, but we need to precipitate neutral Ag. In case of non-siliconized glass we have two effects:

(1) It is easier for silver to grow on the surface that already has a silver nano-crystal
(2) Before treatment glass have inhomogeneities, so nucliation in some places is heavily favored.

Both effects are negated by siliconization.

Note that I didn't account for surface charges which change as a result of siliconization and might play a role.


Per Wikipedia on the formation of elemental Silver in Tollens' reagent, to quote:

In a positive test, the diaminesilver(I) complex oxidizes the aldehyde to a carboxylate ion and in the process is reduced to elemental silver and aqueous ammonia. The elemental silver precipitates out of solution, occasionally onto the inner surface of the reaction vessel, giving a characteristic "silver mirror".

To quote a source,Attraction and repulsion of floating particles :

It is frequently observed that particles floating at the surface of a liquid at rest are submitted to forces which in most cases tend to produce clusters of particles. These may be either attracted to a wall or find an equilibrium position near the centre of the surface. These effects are due to the horizontal component of the capillary forces associated with the curved liquid surface. The calculation of such forces requires the determination of the equilibrium shape of the liquid surface (more generally of a fluid interface) by solving the Laplace differential equation submitted to the appropriate boundary conditions, for example, a constant contact angle of the liquid with the solid particles.

So, the short answer is the creation of elemental Silver particles, followed by clustering, and then attraction to a surface wall being attributed to capillary forces.


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