Why and how do aqueous silica systems precipitate and can its occurrence be mitigated by quantifying its speciation correctly?

**Background/Context:**


For context, I have been doing a lot of work on experimental systems in geopolymer chemistry lately with a focus on "activator" solutions which are essentially mixtures of sodium hydroxide, sodium silicate and water used to enhance the reactivity of aluminosilicate materials upon mixing all the components together (simplified explanation). When making these "activator" solutions I usually change the $\frac{Si}{Na}$ ratios around in the hope of optimising the reaction with the aluminosilicate material which I've been measuring through other means experimentally.


The Ingredients I use are the following:


* **Commercial Sodium Hydroxide (Aqueous):**
* 49.9 wt% $NaOH$;
* 50.1 wt% $H\_2O$;
* **Commercial Sodium Silicate (Aqueous):**
* 30.5 wt% $SiO\_2$;
* 16.7 wt% $NaOH$;
* 52.8 wt% $H\_2O$;
* **"Extra" (Distilled) Water (Liquid):**
* 100% wt% $H\_2O$;


Most of the time, the activator solution I make is clear and good to go. Due to the exothermicity of $NaOH$, I usually leave the solution to cool down for easier handling and so that I can be sure everything has mixed well and reacted together to a steady state. Here is a picture of what the solution looks like in this case with an electric overhead stirrer constant at 200rpm. Note also the beaker in the background which is another activator solution which has stirred a bit more and become clear and "good to go" made with the following feedstocks:


* 97.94 g Commercial Sodium Hydroxide
* 171.63 g Commercial Sodium Silicate
* 68.3 g Extra Water


This corresponds to the following mols:


* 0.87 mols $Si$
* 1.94 mols $Na$
* 11.55 mols $H\_2O$


[![enter image description here](https://i.stack.imgur.com/ttS8h.png)](https://i.stack.imgur.com/ttS8h.png)


However, I've noticed that sometimes when I make it at specified $\frac{Si}{Na}$ ratios I seem to get precipitation occurring *after* I've made the solution and allowed it to cool down as seen in the picture below. This is NOT good as it becomes hard to physically handle (like a viscous gel) and limits reaction with the aluminosilicate source; it essentially becomes useless to me and my experiments. This precipitated mixture is made with the following feedstocks:


* 97.29 g Commercial Sodium Hydroxide
* 171.67 g Commercial Sodium Silicate
* 59.47 g Extra Water


This corresponds to the following mols:


* 0.87 mols $Si$
* 1.93 mols $Na$
* 11.04 mols $H\_2O$


[![enter image description here](https://i.stack.imgur.com/98BtG.png)](https://i.stack.imgur.com/98BtG.png)


I've done some reading and consulted some colleagues and I still do not have a clear answer of what's going on and why it's precipitating. I've had a few ideas as to why this may be the case including the following which I explore in the rest of the question:


1. Atmospheric $CO\_2$ Interaction.
2. Soluble Silica Speciation.


**Atmospheric $CO\_2$ interaction?:**


I first wondered if it was $CO\_2$ getting in and causing precipitation, so I tried two identical solutions (which I knew had previously precipitated) and exposed only one to atmosphere and they both precipitated at (almost) exactly the same time (the capped one took a bit longer to cool down). So I figured that $CO\_2$ ingress is not a severely detrimental factor.


**Speciation Curves:**


Moving on and thinking it may be related to pH, I've been focusing on chemical speciation in the hope to work this problem out. To start, I reviewed some literature first [here](https://www.researchgate.net/publication/322508729_Study_of_Geochemical_Interactions_during_Chemical_EOR_processes) and [there](https://onepetro.org/spejournal/article/22/06/998/69150) to see if there were existing silica speciation curves. I then made these speciation curves based on the literature as seen below:


[![enter image description here](https://i.stack.imgur.com/EUX3U.png)](https://i.stack.imgur.com/EUX3U.png)


[![enter image description here](https://i.stack.imgur.com/or76w.png)](https://i.stack.imgur.com/or76w.png)


I figured that the less stable silica species, like $Si(OH)\circ O\_{3(aq)}^{3-}$ are more reactive due to their unstable charge and thus likely to precipitate and used a pH meter to find that the solution was extremely alkaline (i.e. $pH \geq 14$). Note that the above speciation curves were made with the following equilibria calculations seen below in Equations 1-5 which I back calculated from the mentioned literature assuming the basis of aqueous dissolution of amorphous silica
(i.e. $SiO\_{2(s)} + 2H\_2O\_{(l)} \rightleftharpoons Si(OH)\_{4(aq)}$):


[![enter image description here](https://i.stack.imgur.com/ts7Ca.png)](https://i.stack.imgur.com/ts7Ca.png)


Where, in general, the transitioning of silica species can be defined by Equations 6 below:


[![enter image description here](https://i.stack.imgur.com/9B69B.png)](https://i.stack.imgur.com/9B69B.png)


Each of the reactions above had both their product (with subscripts $y\_1$ and $y\_2$) and reactant (with subscripts $x\_1$ and $x\_2$ for silica and a for protic species quantified by linking with their equilibrium values as per the Equation 7 below:


[![enter image description here](https://i.stack.imgur.com/SFvuO.png)](https://i.stack.imgur.com/SFvuO.png)


I then worked on [calculating the idealised carbonate system speciation](https://chemistry.stackexchange.com/questions/166780/how-do-i-quantify-the-carbonate-system-and-its-ph-speciation) to make certain I understood the maths and chemistry fundamentals. When applying the same logic to the silica system with the above equations and equilibria constants I got these curves:


[![enter image description here](https://i.stack.imgur.com/ftCxJ.png)](https://i.stack.imgur.com/ftCxJ.png)


[![enter image description here](https://i.stack.imgur.com/xpccU.png)](https://i.stack.imgur.com/xpccU.png)


These curves are entirely different to what I've seen in literature. I'm not sure what I'm doing wrong here or what other things I could try to quantify the system better so that I can mitigate precipitation occurring with the silica system. I thought understanding the pH of everything would be a good place to start, hence why I'm here.


**---UPDATE 24/11/22---**


I've been trying to understand this better from the POV of the following silicate solubility ternary plot. I've noticed that some of my solutions precipitate outside of precipitation zones (i.e. in the middle of zone 3 and not zone 2), I believe this is because of the different ways of manufacturing sodium silicate in industry. Unfortunately I have not found a link to the plot without a paywall :/ .


[![enter image description here](https://i.stack.imgur.com/BefEX.png)](https://i.stack.imgur.com/BefEX.png)




A lot of data is available online from commercial suppliers. PQ offers a handbook (free: Ref 1) which may explain some of the effects that you are observing.


Since you know the composition of the solution you are starting with, you can locate your mixture on Figure 3:


[![enter image description here](https://i.stack.imgur.com/ZRIit.png)](https://i.stack.imgur.com/ZRIit.png)


The ratio numbers are SiO2/Na2O by weight, and are for commercial liquids.


Another figure (#5, below) shows a very interesting viscosity minimum for certain concentrations and SiO2/Na2O ratios. While the data brochure does not address temperature as a variable, or precipitation, because that is undesired in a commercial product, you could very well assume that a sharp upward trend of the viscosity suggests a boundary line between two different behaviors, perhaps between clear and fluid versus much less fluid (gel-like) and perhaps also not so clear. Throw in the temperature variable, and you may be able to imagine what is happening to your mixture as you cool it.


[![enter image description here](https://i.stack.imgur.com/N9RJd.png)](https://i.stack.imgur.com/N9RJd.png)


The variability of your results indicates that you may be very close to a boundary line, or a metastable boundary region, so stirring or scratching or cooling might "precipitate" a phase change.


Ref 1: <https://www.pqcorp.com/docs/default-source/recommended-literature/pq/sodium-silicate-solids/sodiumsilicates.pdf?sfvrsn=394ebc05_3>


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