Reef Chemistry Question of the Day #267 Charge on the Surface of Your Glass Aquarium

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #267

Many surfaces carry a net electrostatic charge on them due to charged species that are part of the surface. In a solution like seawater, the surface itself has charged species, and then nearby in the water hover charges of the opposite sign (in addition to the random even mix of charged species normally present in seawater).

The charge on a surface can also change, when charged species become bound to it. For example, phosphate binding to a phosphate binder may change its charge (or the degree of its charge) when negatively charged phosphate binds to it.

Consider the inside of the glass of a typical reef aquarium. The charge on the surface will have a value when first exposed to raw seawater, and then later as more materials bind to it (for example, organics, whole bacteria, inorganic ions such as phosphate, etc.).

The expected charge is:

A. Initially positive, and positive after more materials bind
B. Initially positive, and negative after more materials bind
C. Initially negative, and negative after more materials bind
D. Initially negative, and positive after more materials bind

Good luck!

























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[Brew12]

Things that make you go hmmm.....

I'm going to go with B.

 

[dragon99]

Appears glass starts with a negative charge. http://physics.nyu.edu/grierlab/charge6c/
going to assume it gains a more negative charge as materials bind.

C

Edit: This seems to support a increase in the negative charge:
https://aslopubs.onlinelibrary.wiley.com/doi/abs/10.4319/lo.1972.17.1.0007

 

[Chuk]

Google says it starts negative and I’m guessing it gives up those electrons as things bind to it. So D

 

[Dburr1014]

I would say negative and less negative as things bind to it but that's not an option in this question.
So c is the closest.

 

[Fourstars]

B. The origanics act as an insulator?

 

[Randy Holmes-Farley]

And the answer is...
The expected charge is:

A. Initially positive, and positive after more materials bind
B. Initially positive, and negative after more materials bind
C. Initially negative, and negative after more materials bind
D. Initially negative, and positive after more materials bind

Glass has a net negative charge in seawater. The easiest way to think about that is that silica glass terminates at the surface as silanol groups, SiOH. The silanol group is somewhat acidic. Silicic acid (a fully soluble version, Si(OH)4) has a pKa around 9.5. That means that at pH 9.5, half of it is in the form of Si(OH)4 and half as Si(OH)3O-. At ph value below 9.5, there is less and less in the ionized form (the negatively charged form), but there is still some. Also, in seawater, the pKa will shift down, relative to fresh water, so there will be evenmore negative charge at typical tnak pH values.

On a glass surface, what happens is:

Glass-Si-OH ---> Glass-Si-O-

This article describes the charge on glass in water solutionsdZ:

The Charge of Glass and Silica Surfaces
http://physics.nyu.edu/grierlab/charge6c/

The article that was linked earlier, also shows it to be true in seawater:

THE SURFACE CHARGE OF PARTICULATE MATTER IN SEAWATER
https://aslopubs.onlinelibrary.wiley.com/doi/abs/10.4319/lo.1972.17.1.0007

When mineral surfaces, such as glass, are exposed to real ocean water or in a reef tank, lots of things attach to it, particularly organics and whole bacteria.

Bacteria generally have a net negative charge, so will maintain the net negative charge on glass when they bind to it.

It also turns out that the organic matter generally floating free in reef aquaria at pH 8.2 will generally have a net negative charge. While there are certainly some biomolecules that have a positive charge (e.g., the amino acids lysine and arginine), negative charges dominate in these sorts of molecules in seawater. For example, one can look through large tables of the net charge on proteins, and at pH 8.2, far more will be negatively charged than positively charged (although there are obviously some of each).

Isoelectric points and molecular weights of proteins : A table
https://www.sciencedirect.com/science/article/pii/S0021967300985376

FWIW, all of the natural particulates (bacteria, algae, and organic matter in large clumps) tested in the article below had a net negative charge in seawater:

THE SURFACE CHARGE OF PARTICULATE MATTER IN SEAWATER
https://aslopubs.onlinelibrary.wiley.com/doi/abs/10.4319/lo.1972.17.1.0007

Finally, when the above article looked at glass in raw seawater (their model 7-ion seawater) and then exposed it to organics and such in real seawater, they found it became more negatively charged:

"All the model particles in all natural seawaters exhibited negative mobilities. The mobilitics of glass and Sephadex in seawater are more negative than in 7-ion seawater."

 

[Idoc]

Well, I'm not going to argue with that explanation! But, i think my glass is purely "algae" charged cuz that stuff loves to bind to it, lol.

But awesome info... thanks! (I initially guessed wrong)