[galpinj]

Hello everyone,

I have a complicated question regarding acid etching. In dentistry, acid etching is used on tooth enamel, where the acid causes demineralization. This also occurs in dentin tissues, where we see hydroxyapatite demineralized via acid etching. However, in both cases (but more prominently in dentin), the organic collagen matrix is left undamaged. Why does and acid (32% phosphoric acid in this case) only affect the hard crystalline material and not the organic proteins?

Likewise, we can use acid etch on glassy-ceramic, where it eats away at the amorphous glassy matrix (SiO2). Conversely, it doesn't seem to eat away at the crystalline fillers (leucite, feldspar, etc.) inside the glassy matrix. Why? Unlike on dentin, where crystalline structures are quickly destroyed, here we see only the amorphous structure destroyed. The problem is further compounded when I think of zirconia oxide crowns, which are in a rigid crystalline structure but are said to be etch-resistant!

This is a very specific series of questions, but I haven't found any textbook or instructor that can adequately answer the question. I would love any and all input! Thank you!

[Borek]

I wouldn't be so sure the organic matrix isn't damaged - it is not dissolved, but it is definitely changing in the low pH.

In low pH PO₄^3- groups get protonated, which shifts the dissolution right.

Generally speaking acid etching can eat away only those things that somehow react with the acid - and not everything does. It is more about chemical properties of the substance than about the form (crystalline/amorphous).

[galpinj]

Thanks for the prompt reply, Borek!

It's great to see you are still active on this forum. I remember you answering quite a few of my questions back when I was trying to prepare for entrance exams a couple years ago.

I definitely had tunnel vision regarding acid etching and substrate structure.  That would explain why the acid is reacting with materials of differing solid structures. May I ask what you mean by "shifting the dissolution to the right"? Does this refer to increasing the pH (via removal of H+) or that the protonated phosphates will precipitate out of the hydroxyapatite crystal?

What do you think influences how the acid reacts with a substrate? Hydroxyapatite has predominantly ionic bonds whereas SiO2 is relatively covalent. What's more, the collagen proteins have varying degrees of polarity. Trying to work it out chemically, a strong acid would dissociate completely (e.g. HCl  H⁺ + Cl^-), and the resulting H+ ions would interact with the substrate. I could see how this might pull the phosphate out of the hydroxyapatite, but I don't get why this would affect the covalent SiO2 bonds. What's more, though you alluded to it, I would expect a more breakdown of collagen (with its ionic and polar bonds), but we don't really seem to get that.

Your consideration is, as always, much appreciated!

[galpinj]

P.S. I just wanted to add the equation for acid etching of the glassy matrix (source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5874387/). I'm a little puzzled on how the hydrogen ion is able to pull away the oxygen atoms in SiO₂ (or even the phosphate ion from hydroxyapatite, for that matter). The article also mentions that, in glassy ceramics, the crystal phase (leucite) gets affected by etching, but it doesn't specify how. This is counter to what I've learned in school, where only the glassy matrix is affected.

[Borek]

Does this refer to increasing the pH (via removal of H+) or that the protonated phosphates will precipitate out of the hydroxyapatite crystal?

It doesn't precipitate, it gets protonated. That's equivalent for its "removal" from the solution - it is no longer available for the hydroxyapatite precipitation. And PO₄^3- is a quite strong base, it gets protonated very easily.

I wouldn't try to explain the reaction with SiO₂ in terms of simple protonation. After all, it is reaction in which flurodide replaces oxygen, so the mechanism can be quite different.