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I'm trying to figure out how exactly does the silver 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:

nucleation

growth

Now, as you can see, i didn't draw any chemical bonds. That's because I think that the silver adherence is due to Van Der Waals forces. Am I right to think this? If this is the case, then 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!)

So, 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.

NOW, i like ranting, so i'll go on. If you make a surface treatment using silane groups, I think that you promote the presence of $\ce{OH}$ groups at your silica surface, although my understanding/knowledge is hazy at this point. I do know that it makes better silver mirroring though. I made another drawing:

silanisation

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. Second, the silver is not even near the oxygen atoms, which are supposed to be the nucleation points, according to me.

I'm confused, could you please point out to me where i'm right and wrong in my logic/knowledge? Mainly: - Is there a bond forming between the silica surface and the silver or is it Van Der Waals (Debye) attraction only? - Is my depiction of nucleation & growth correct (the first two pictures)? - Did I make the electronegativity maths correctly? - Is my logic of " higher total electronegativity difference, hence better nucleation site" correct? (assuming I'm right about the Van Der Waals thing) - Is my depiction of silanisation correct? - Am I right to say that the higher presence of surface $\ce{OH}$ groups by silanisation mean better nucleation - If this is the case, could I do the same thing 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. $\endgroup$ – Loong 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. $\endgroup$ – User5 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
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One of the comments stated that your model is naive. Your model is simply wrong. In your diagrams, you show 3 Si atoms, a couple of O atoms, an H atom and then a vertical line. What is that line supposed to represent? The surface of a solid is difficult to diagram. Charge density would be one simple way to draw a map, but that wouldn't account for the potential energy surface (the potential of various atoms in the surface to bond with atoms above the surface). Your diagram also completely (I'd say inexplicably) ignores the species absorbed onto, and hydrating the surface. I know almost nothing about Tollen's reaction kinetic mechanism, but I question your assumption that atomic silver is directly deposited. The only way that can be true is if the reaction only occurs at the surface, otherwise the silver is either solvated, which implies that it is undergoing reactions (forming clusters??) in solvated or dispersed state. I also note that NaOH attacks glass...fwiw. Precipitation is an enormously complicated process, you're not going to be able to capture it in a couple of stick model diagrams, imho. Sorry if this seems pedantic, but the process you want to diagram would require a huge amount of study and observational analysis if we wanted to be able to justify what you claim. (Otherwise it's "hand waving" and not science)

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"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.

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