Let’s talk silica

[Ralf]

Thanks.

I assumed that adding dilute acid (e.g., 0.01M sulfuric acid or 0.1M phosphoric acid) to the dilute silicate solution (i.e., basic silicate solution into DI H2O), reducing its pH to ~1.5, would convert the silicates in the dilute silicate solution to silicic acid. And then, adding the stabilizer (carnitine HLC or sorbitol) while maintaining a pH of <2 (low alkalinity) would form hypervalent five-coordinate silicate complexes with the silicic acid. With reaction scheme showing a 1:1 silicate(silicic)-carnitine complex (or 1:1 silicate(silicic)-polyol if using sorbitol).

Maybe using a pH of 4 or 5, low enough to convert silicates to silicic acid and reduce the alkalinity, during the mixing process (before adding the complexing stabilizer) would yield a better result than a pH of <2.

In contrast, if we add the stabilizer to the basic silicate solution, which has high alkalinity and increased deprotonation, hexa-coordinated complexes should form, providing increased complexation stability. But in that case, reaction schemes show a 1:2 silicate-stabilizer complex. And I assumed once we drop pH to <2, the hexa-coordinated complexes formed under high pH and alkalinity would reduce to five-coordinate complexes due to silicic acid being fully monodeprotonated at low pH and low alkalinity.

In other words, I assumed adding acid before the complexing stabilizer (converting silicates to silicic acid) and adding acid during the addition of the stabilizer (or pre-acidify the stabilizer solution), would provide a similar result to adding acid after the stabilizer. But, instead of complexing silicates (hexa-coordinated) under high pH, which then convert to five-coordinated silicic acid complexes when we reduce pH, we would form the five-coordinated silicic acid complexes up front. And provide a better end product. Because I assumed when we reduce the pH of the hexa-coordinated silicate complexes (at a 1:2 ratio), the transition to five-coordinated silicic acid complexes (and 1:1 ratio) would result in some incomplete silicic acid complexation and/or incomplete conversion from silicate to silicic acid, along with putting free stabilizer in solution.

I don’t think it matters if we add the glycerol after the acid and stabilizer, but I could be mistaken. Because from what I read over the past few days, glycerol, PEG, PG, etc., mainly function as humectants; they don’t directly stabilize or improve the silicate complexes.

Please remember that I’m not on the same level as you, you’re a trained chemist, and I’m not. So, some of my assumptions may be comically incorrect. Try not to LOL too much

[danielfp]

@Ralf The polymerization of the silicic acid is quite fast at pH 2-8 . If you want to lower the pH, then you need to do so as fast as possible to the <2 range. The key here is that this pH reduction needs to be as fast as possible (acid addition needs to be concentrated) and the silicon needs to be at the lowest concentration possible.

About the stabilizing agent addition, I really haven’t tested the effect of adding the stabilizing agents before or after the acidification of the silicon solution. It might work, depending on the stabilizing agent. However doing the addition at basic pH really helps solubilize solid potassium silicate sources (like AgSil 16H), when you’re starting from them.

Let me know how your experiments go!

[Ibuyhemp]

Has anyone tried Front Row Si by Front Row Ag? It contains 10% monosilicic acid and is only $80 for a quart.

[danielfp]

@Ibuyhemp Remember that this means this is only 6.5% Si as SiO2. So it is basically in the same league as a product like Growtek in terms of Si concentrations. Also remember that a product can be a basic silicon product and the Si concentration reported as monosilicic acid. So you need to test the product and verify it is actually an acidic pH product to confirm it actually has monosilicic acid chemically present.

During the past few years several companies have used the above loophole to create normal potassium silicate products, where the concentration is reported in terms of monosilicic acid, and people buy thinking these are monosilicic acid products, but they are not.

[Ralf]

Thanks for your suggestions and open, friendly, can-do attitude, @danielfp. I’m glad you’re here; you exemplify what this site is all about.

I will report the results of the experiments. It will probably take a bit of time, so I can include a few weeks of stability testing and prepare a nice PDF. If/when some of the experiments fail, I will let you know immediately in case you have suggestions for improvement of the experiments.

How is Dr. Bugbee testing for bioavailable silicon? Molybdenum test strips? Is that how you would recommend testing as well?

[DIYGuy]

Hi Ralf and @danielfp

So i tried you method ralph.
if 54.3mL is 1M of H2SO4 (according to How do you prepare 1M sulfuric acid from concentrated sulfuric acid? - Answers)

I did the following
Made 1L of DI water with 1.5ml of 98% H2SO4 ~ (0.02M)
Used 100ml of this solution
Added 33ml of 17% KSi liquid, pH was at 11 or so
Added more of the 0.02M solution, The pH slowly came down and no gelling. After adding 500ml of this i abandoned this as it was way to diluted.

I started again with the 0.02M base of 100ml and added 33ml of 17% KSi.
This time i made a 100ml + 10ml H2S04 (2M?) solution if i did the math correctly.
It took 60ml of this solution before pH was at 1.8pH. No gelling.
I did not bother to continue and add the carnitine as again its too dilute.

So now its a matter of how many M of H2S04 can you start with before adding the KSi, and how concentrated H2S04 you can make up to slowly bring it down without it gelling.

I am going to make up the same SOP i have done previously re 40g of carnitine and 50ml of Glycerol, but i will see how concentrated I can make the carnitine, eg. 40g in 25ml of DI water not 50ml. I will also pre acidify it.

I am out of supplies so will have to wait a bit before i can continue.

Interesting when im doing my method unless i add the concentrated 98% H2S04 immediately and all at once it will gel. If the syringe jams up and i dont get it all out in one go then it gels. So im surprised adding diluted acid is not gelling…

I have also tried to increase the 33ml of 17% Ksi to 50ml and it fails. Its very hard to not gel up when adding the acid, and if i am fast enough, an hour later in the glass jar it starts to go a bit cloudy and by morning there is salts in the bottom of the jar. Is this KCL ? I will filter it out and see if the solution is stable. But prior experiments it gels up.

[Ralf]

Thanks for the info

You probably avoided gelling when adding the dilute acid to the silicate solution because the acid was so dilute. It likely diluted the silicate solution enough to reduce the acid and base silicate solution’s localized reaction (pH reduction).

Think about it like this:
The instant the acid solution hits the basic silicate solution, that interface area (surface of both solutions) is where the first reactions occur. At that point, the basic silicate solution at its container’s center, sides, and bottom haven’t come into contact with the acid. And if you’re adding 98% concentrated sulfuric acid in a small area (as a stream from a syringe), the amount of acid in that localized interface area (a few square mm, maybe) is extreme. Thereby reducing the pH of the silicate solution at the interface area below 0 pH; for reference, 98% sulfuric acid is 18.8M, with a pH of -1.27. In other words, 98% sulfuric acid is highly acidic and quite dangerous when mishandled.

Also, when you wrote “If the syringe jams up,” it sounded like you’re using a typical syringe like this for highly concentrated sulfuric acid transfer. If so, I strongly advise against doing so. The sulfuric acid will break down the plastics and rubber in typical syringes, which is why the syringe can “jam up.”

In a previous post, I described what you should use to transfer small volumes of concentrated acid:

• Class A graduated cylinder (best option), graduated glass dropper, all glass syringe, or disposable PFE pipette tips with a pipettor for small volumes.

• Another option is to pre-weigh the acid (convert mL to gram) into a small glass beaker. To add the acid, you pour it into the silicate solution.

• In either case, the silicate solution should be actively mixed (with a magnetic stirrer plate or overhead mixer) during and after the acid addition.

It should also be noted that anyone working with strong acids or bases should use full PPE. Including (ideally) a lab coat (or at least a long sleeve shirt), shoulder-length nitrile gloves, and goggles (not safety glasses). Ensure you at least have a eyewash bottles, but a hands-free faucet eyewash station, or ideally stand-alone hands-free eyewash station within an unobstructed ~50-foot distance; the closer, the better. It shouldn’t take longer than 10 seconds to begin flushing your eyes in case of a mishap, and continue flushing for at least 10 minutes. When working with caustic hydroxides like KOH, wear at least an N95 (or K95) face mask.

---

I think what @danielfp wrote to me is a great suggestion (strong acid, weak base). It’s basically the inverse of what I proposed (weak acid, strong base). It should have the same effect to avoid gelling but also avoid adding too much acid volume. The mixing process is similar to my post you tried, but with a few tweaks to reduce the silicate concentration. Once a working mix procedure is attained, increase the ingredients by 10% each time until it doesn’t work anymore, then go back to the last version that worked.

As an armchair guess, based on the formulation you’re using, reduce all ingredients by 50% except the DI water, and increase the DI water volume by 50%. So you’re adding slightly less extra water volume than the volume you remove from the other ingredients. This means you’re making a more dilute silicate solution:

1. Purified water in the beaker
2. Place the beaker on a mag stirrer plate.
3. Insert pH probe into water (using a pH meter calibrated to pH <2.0)
4. Mix water to form a weak whirlpool
5. Add silicon solution
6. Mix for a while (2 mins?)
7. Add concentrate acid to drop pH to ~1.5 quickly
8. Mix for a while (2 mins?)
9. Slowly add carnitine HCL solution to pH 1.8-1.9
10. Mix for a while (10 mins?)
11. Slowly dropwise add 1M sulfuric acid to pH 1.3-1.5
12. Mix for a while (2 mins?)
13. Slowly add carnitine HCL solution to pH 1.8-1.9
14. Mix for a while (10 mins?)
15. Slowly dropwise add 1M sulfuric acid to pH 1.3-1.5
16. Mix for a while (2 mins?)
17. And so on until all the carnitine HLC solution has been added and pH is < 2.0
18. Mix for a while (10 mins?)
19. Add glycerol
20. Mix for a while until you have a homogenous solution (10 mins?)
21. Top up the solution with water to the target final volume
22. Mix for a while (2 mins?)
23. Slowly dropwise add 1M sulfuric acid to pH 1.5-1.75
24. Mix for a while (2 mins?)

Tips:

• Try pre-acidifying the carnitine HCL solution to ~1.75 before adding it to the silicate solution. That way, the mixing process would be simpler and much quicker. On the mag stirrer plate, slowly dropwise add 0.1M sulfuric acid to the carnitine HCL solution until reaching the desired pH.

  • It may make sense to use 0.1M hydrochloric acid rather than 0.1M sulfuric acid. And you could use higher molarity (more concentrated) acid but I wonder what effect it may have on the carintine; @danielfp what do you think?

• If the solution gels when adding glycerol after the carnitine HCL, try adding it before the acid.

• Listen to @danielfp if he advises against anything I wrote!

[DIYGuy]

Thank you, i was actually meaning to mention the safety procedure. That is really good advice and i would suggest everyone go and buy all the ppe as H2S04 is really dangerous

I will not use syringes anymore, i had previous learnt that was not safe to do, i used a new one every time to prevent it jamming but it is not the correct thing to use.

Carnitine is quite hard to acidify, even pure H2S04, 3ml in 25ml of water with 40g of carnitine only dropped it to 2.5 or so. It did however stop the ph of the mix increasing when carnitine is added.

Once i get more supplies ill continue and update.
Any quick tips on calculating the S addition from x grams/ ml of H2S04 added to the mix?

for my Si Calculation how would you calculate the total when you have liquids and salts, do i use a w/w or w/v ?

100mL of DI water
25mL of DI water (from the carnitine)
22mL of Glycerol
33mL of liquid 17.3% silica
13mL of sulfuric acid
25g of Carnitine

17.3% KSi (should i convert to Si first? 0.625x17.3 = 10.8)
10.8 Si * 33ml = 356 / Total of solution correct

Thanks !

[danielfp]

@DIYGuy

About syringes. With concentrated H2SO4, always use glass syringes and make sure they are prewet with the concentrated acid so that they do not get stuck. Never use plastic/rubber stopper syringes with sulfuric acid. For concentrated sulfuric acid it is normally preferable to just use graduated cylinders unless the addition requires precision (not the case here).

About concentration. To figure out the exact concentration you can weigh the final product. Then you can either calculate w/w% directly or measure the density using a pycnometer to then calculate the w/v% using the density and the total mass to calculate the final volume. Note that measuring the final volume is also possible but will be less precise.

If you have solids and liquids you can only calculate the w/w% composition beforehand, assuming you know the density and volume of the liquids used. To get w/v% you need experimental measurements over the final product.

About conversions, you can keep it as Si as SiO2 or convert and use, Si as elemental Si. Just make sure you keep the units consistent. To convert SiO2 to Si, multiply by 0.4660. In your case, 17.3% Si as SiO2 would be 7.95% Si as elemental Si.

Note that there are many ways to report Si, Si can be reported as SiO2, as Si or as Si(OH)4 all of them have different conversion factors between them. The conversion factor between Si(OH)4 → SiO2 is 0.625 but between SiO2 → Si it is 0.4660. When a product says Si as silica, it is Si as SiO2, Si as silicic acid is Si as Si(OH)4.

Some manufacturers now favor Si as Si(OH)4 both because the number is always larger and because people prefer silicic acid based products (although as I mentioned, reporting the concentration as silicic acid has nothing to do with the form actually present in the product).

[danielfp]

@Ralf Great summary! Adding the highly concentrated acid to the carnitine will be fine. However it will take a lot of acid to bring the amino acid to a very low pH, so I don’t know if this is necessary.

From what I have been seeing in experiments, perhaps the carnitine is not really necessary.

I did some experiments at final Si concentrations of 1-2% with glycerol and sulfuric acid and the results seem to be quite stable.

I have also done an experiment with Ribose and phosphoric acid, to see if using Ribose - another molecule that leads to stable complexes with silicic acid - we can get a more stable result without the need for strong sulfuric acid.

[emdub27]

Is there any reason to use glycerol over sorbitol? The sorbitol and sulfuric formulation is still completely stable.

I choose sulfuric at most facilities because there have been multiple instances in the past where they decide to add an additive after I leave that is basic and corrected the pH with phosphoric acid leading to highly imbalanced solutions. The plus side is I get hired again, but I feel sorry for the plants. It’s hard to add enough sulfuric to get to a point that it’s detrimental for the plants.

[danielfp]

@emdub27 I also agree on the use of sulfuric, just that at home it is much easier to get phosphoric due to the less strict regulations (it is also substantially less dangerous), so I want to have a DIY route that is viable with phosphoric acid as a pH down in any case. Right now however we only have long term stable routes using sulfuric acid.

About glycerol vs sorbitol. The sorbitol should be more stable long term (per the chemistry theory itself), but it is substantially more expensive, so a glycerol option might be more viable for the larger scale, especially for less expensive crops. I however, lack any experimental data to back one over the other. So far it seems that sulfuric acid solutions prepared both with sorbitol and glycerol are stable at least in a 2 months timeframe.

[Ralf]

@danielfp Thanks for the info! I’m looking forward to your findings with Ribose.

I’m surprised glycerol without a complexing agent has shown stability. Are you testing stability as the concentrated product or diluted in a nutrient solution at a typical pH range?

How are you and Dr. Bugbee testing for bioavailable Si? Mo test strips?

@emdub27

Is there any reason to use glycerol over sorbitol? The sorbitol and sulfuric formulation is still completely stable.

I choose sulfuric at most facilities because there have been multiple instances in the past where they decide to add an additive after I leave that is basic and corrected the pH with phosphoric acid leading to highly imbalanced solutions. The plus side is I get hired again, but I feel sorry for the plants. It’s hard to add enough sulfuric to get to a point that it’s detrimental for the plants.

@danielfp

@emdub27 I also agree on the use of sulfuric, just that at home it is much easier to get phosphoric due to the less strict regulations (it is also substantially less dangerous), so I want to have a DIY route that is viable with phosphoric acid as a pH down in any case. Right now however we only have long term stable routes using sulfuric acid.

About glycerol vs sorbitol. The sorbitol should be more stable long term (per the chemistry theory itself), but it is substantially more expensive, so a glycerol option might be more viable for the larger scale, especially for less expensive crops. I however, lack any experimental data to back one over the other. So far it seems that sulfuric acid solutions prepared both with sorbitol and glycerol are stable at least in a 2 months timeframe.

From my understanding, sorbitol is dual acting as a complexing agent of silicic acid and humectant, while glycerol functions as a humectant and biostimulant but not a complexing agent. So, when diluted into a nutrient solution, only the version where silicic acid is complexed (with sorbitol or carnitine) would provide silicic acid for a limited time (a few days?) at higher pH and under high EC. Please correct me if that’s not your understanding.

It seems like the greatest stability and effect may be achieved with carnitine as the primary complexing agent and a mix of sorbitol and glycerol as adjuvents.

For acids, has anyone tried nitric? I plan to test nitric, sulfuric, phosphoric, and hydrochloric. Considering the use rate for stabilized silicic, at least for PowerSi (at 0.665 ppm silicon as Si), is greater than an order of magnitude less than silicon as Si and two orders of magnitude less than silicon as SiO2, using phosphoric or hydrochloric acids shouldn’t add enough P or Cl to affect plant growth or throw off a nutrient profile, assuming P is < 10 ppm and Cl is < 5 ppm.

@danielfp, have you found published research on stabilized silicic acid concentration for hydroponics or soilless cultivation?

[danielfp]

@Ralf

I’m surprised glycerol without a complexing agent has shown stability. Are you testing stability as the concentrated product or diluted in a nutrient solution at a typical pH range?

With Bruce’s group we are testing the stability in a pH 6 buffer, at the concentrations typically used in hydroponics. However we are also testing the stability of the concentrated solutions (not quantitatively, but just verifying they don’t gel or precipitate).

How are you and Dr. Bugbee testing for bioavailable Si? Mo test strips?

We are using a colorimetric method to measure molybdenum reactive silicon. This is akin to the method used by the Mo test strips, but contains additional reagent steps to improve accuracy and uses a colorimeter to receive a quantitative answer. His lab has also verified the accuracy of these readings Vs ICP analysis.

From my understanding, sorbitol is dual acting as a complexing agent of silicic acid and humectant, while glycerol functions as a humectant and biostimulant but not a complexing agent. So, when diluted into a nutrient solution, only the version where silicic acid is complexed (with sorbitol or carnitine) would provide silicic acid for a limited time (a few days?) at higher pH and under high EC. Please correct me if that’s not your understanding.

All of these compounds with OH groups in treo configurations can potentially form complexes with silicic acid. However sorbitol will preferably form them in the presence of glycerol, so whether or not glycerol acts as a stabilizing agent depends on whether there is something better or not in solution. I don’t see a reason why glycerol cannot complex silicon. We are planning on doing some Si NMR to verify this though.

We have some evidence that these treo OH complexes can exist in solution, even at relatively low Si and complexing agent concentrations and neutral pH (see this paper, https://pubs.acs.org/doi/10.1021/ja411822s, where complexes with PEG are studied using Si NMR).

@danielfp, have you found published research on stabilized silicic acid concentration for hydroponics or soilless cultivation?

You can review the papers that exist on stabilized silicic acids. This paper (The Effects of Foliar Sprays with Different Silicon Compounds - PMC) is a review that cites many papers where the researchers carried out studies using stabilized silicic acids. You can see their application rates there. There are currently only few studies comparing silicate vs silicic acid applications.

I am also unaware of any root application studies of stabilized silicic acid derived silicon.

[Ralf]

Thanks for such a great reply! It’s chalk full of useful information.

@danielfp

We are using a colorimetric method to measure molybdenum reactive silicon. This is akin to the method used by the Mo test strips, but contains additional reagent steps to improve accuracy and uses a colorimeter to receive a quantitative answer. His lab has also verified the accuracy of these readings Vs ICP analysis.

I’m familiar with a colorimeter. If you have resources you could point me towards to read about the procedure, I would be grateful.

You can review the papers that exist on stabilized silicic acids. This paper (The Effects of Foliar Sprays with Different Silicon Compounds - PMC) is a review that cites many papers where the researchers carried out studies using stabilized silicic acids. You can see their application rates there. There are currently only few studies comparing silicate vs silicic acid applications.

I am also unaware of any root application studies of stabilized silicic acid derived silicon.

I have read that study previously, but I will reread it, focusing on silicic acid stability. I was hoping you knew of research on root application (mainly relating to pH ~6 under high EC).

[AgTonik]

When I worked at a beer company, they used a nitric/sulfuric acid to clean stainless because it would take care of organic material and minerals. They never used PPE more than gloves.

It looks like sulfuric acid has more special handling requirements as far as heat, moisture, barrel ullage, etc. I really like the fact that it won’t cause modification of a nutrient formulation.

[Ralf]

Did they maybe instead use nitric acid and phosphoric acid? Nitric is used for SS passivation, and phosphorus is used for mineral deposits removal from SS. It doesn’t make sense if they used nitric and sulfuric because the former is better, safer (at 10-15% concentration), easier to use, and cheaper. If they used sulfuric acid for passivation, it would have to be concentrated AFAIK, around 98%, and hot.

If they were using hot 98% sulfuric acid without full PPE, they are lucky they didn’t win the Darwin award. And they are a good argument for restricting dangerous acids from the general population, lol.

[AgTonik]

Never underestimate my ability to mis-remember something. Now that I’m looking at the different chemical actions of each acid, you’re totally right.

Cleaning Acid Comparisons

Adjusting Alkalinity with Acids

[danielfp]

This is an interesting data point. We prepared two 0.2% stock solutions of Si, one was a potassium silicate stock and the other was an acid stabilized stock using sulfuric acid, glycerol and potassium silicate.

Preparing a dilution to 40 ppm of Si as SiO2 in a pH 6 buffer, we got basically the same reading for dilutions from both stocks one week ago but today new dilutions were prepared and measured and while the solution from potassium silicate yielded basically the same, the acid stabilized one yielded only ~6ppm of Si as SiO2. The solution looks optically transparent and is not gelled or has anything precipitate in it.

We are repeating this entire experiment (new stocks from scratch), but this begs the question of whether we can actually trust the stability of acid stabilized Si solutions just by optical evaluations of them.

[Ralf]

Interesting!

Based on the following studies, it seems that the glycerol (or its concentration) isn’t sufficiently complexing the monosilicic acid to prevent its polymerization at pH < 2.0. But, the glycerol (or its concentration) is slowing the monomer polymerization into particles, which otherwise would rapidly occur at pH < 2.0 without glycerol. Plus, the glycerol (or its concentration) and the low silicon concentration may be slowing (or preventing) the subsequent flocculation of the particles after polymerization.

Please correct me if I’m off base or way out in left field, lol. Also, I assume I may have made errors in identification (e.g., monomeric, polymeric, etc.); if so, I would be grateful to receive corrections.

I based most of this information on these resources (in order of significance):

1. Gorrepati et al. (2010)
2. Choppin, et al. (2008)
3. Iler (1979); page 174, “General Theory of Polymerization”
4. Alexander (1954)
5. Laane (2017)

In other words, it seems the acidic potassium silicate stock lost its monomeric silicic acid concentration (as monosilicic acid) to condensation polymerization. But, the stock remained optically transparent after one week, without precipitation, by limiting (or preventing) flocculation of the particles after monomer polymerization.

Considering the stock is less than pH 2.0 and only 0.2% Si as SiO2, the monosilicic acid was probably lost to polymeric silicate particle formation during polymerization rather than oligomer formation - resulting in a colloidal dispersion of ~3-5 nm silica particles. And it remained optically transparent because the particle sizes were well below the optical transparency threshold of ≈40 nm. In time, flocculation of the colloids should occur, forming an optically translucent colloidal dispersion in the range of ≈50-200 nm. Further particle aggregation into visible precipitation would probably happen.

When you replicate the experiment, measuring the monosilicic acid concentration [per ASTM D859-05 and Currie & Perry (2008)?] may be informative after 24, 48, and 72 hours, then again on day 7. I bet you’ll find most of the monosilicic acid has disappeared after 24 hours.

And it may be fruitful to include a third and fourth stock solution in your experiment:

• #3: Where either sorbitol or, preferably, carnitine is the complexing agent, and glycerol is an ancillary complexing agent/humectant. After seven days, I bet the third solution would probably retain most of its monosilicic acid, or at least significantly more than the glycerol-only solution.

• #4: Power-Si, OSAB, or another commercial stabilized monosilicic acid product like Grow-Genius (label claims 19% silicon):
  

  

If the lab has a DLS instrument, it would be interesting to track the silica particle size growth at the same time intervals as the monosilicic acid measurements.

Alaxander (1954) The Polymerization of Monosilicic Acid

Choppin, et al. (2008) Polymerization and Complexation Behavior of Silicic Acid: A review

Currie & Perry (2008) Silica in Plants: Biological, Biochemical and Chemical Studies

Gorrepati, et al. (2010) Silica Precipitation in Acidic Solutions: Mechanism, pH Effect, and Salt Effect

Iler (1979) The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry (full text link here)

Laane (2018) The Effects of Foliar Sprays with Different Silicon Compounds

Lanne (2017) The Effects of the Application of Foliar Sprays with Stabilized Silicic Acid: An Overview of the Results From 2003-2014

Iler (1979)

Choppin, et al. (2008)