One of the key focuses within the production of silica has for some time been the levels of water content and its influence on how silica particles behave. In 2018 a study was undertaken to determine what these influences are and how, given certain conditions, the particles are formed and how they behave. The end goal was to provide a clearer understanding of the role particular synthesis methods would have in the establishing of reliable, reproducible silica production for industrial adoption.
What follows is an overview of that peer review by way of background to the commercialisation of SMS particles. The peer review can be found here:
The starting point was to identify compositional regimes for the formation of discrete silica particles using the Stober process; then note the influence of those compositions on particle characteristics. Finally, understanding the differences in time to completion of silica formation and establishment of particle size.
By using sol-gel methods, it allowed fabrication of silica nanoparticles that were not in the conventional pyrogenic state. This meant that the nanoparticles produced were of a purer nature, reduced energy levels in manufacture, and critically allowed both the potential to control their surface chemistry, and the development of complex structural hierarchies.
Initial experimentation aimed to identify key compositional regimes that had a significant impact on the formation of Stober spheres (under conditions of high pH levels). A secondary objective was to determine the influence of water content on the resultant silica particles.
First results produced several observations, the key one being that the increase of water content of the silica dispersion has a strong influence on the size of the synthesised particles. At a certain water content (around 33wt%), there was a particle size peak, indicating a change of stability. This was likely due to the fact that silicon alkoxides are generally incompatible with water – this is why alcohols are used as co-solvents.
Another factor in determining particle size was that of nucleic growth; particles synthesised using the Stober process could grow from two mechanisms, or a combination of both: either small nuclei aggregate to form a bigger particle or small nuclei grow over time. There was however a clear link between the level of water content and the nucleic growth of the particles, and formulations with a higher water content (above 56wt%) were highly aggregated and agglomerated, instead of discrete (or primary) particles. Achieving an optimum water content level was therefore critical to retain discrete particle integrity.
Once particle sizes had been established, surface area and porosity were analysed. As anticipated there appeared to be a relationship between water levels and porosity, in that the lower the level of water, the higher the porosity and higher surface area – and additionally, considerable residual organic content (this was as a result of incomplete hydrolosis).
Thermogravimetric analysis of the materials produced interesting results, in that the higher initial water content materials appeared to lose less weight during dihydroxylation even though they exhibited greater surface area. This could be as a result of different types of silanol present on the material’s surface, particularly if discrete particles were rich in geminal silanols (and therefore had a higher Q2/Q3 ratio).
The use of alcohols such as ethanol in sol-gel systems is well established and accepted in order to achieve chemical compatibility, as they act as the co-solvent between water and the hydrophobic alkoxides. An observation of the effect of the hydrolysis determined that as water is consumed by reaction with the alkoxide, ethanol will be simultaneously liberated. The net effect of this would be to make the composition richer in alcohol, which would increase the chemical compatibility and recue the tendency to phase separation.
In conclusion, this study determined that there is a way to produce discrete silica particles of varying sizes using the Stober process. As with most sol-gel synthesis, the resultant materials are dependent on a lot of different process parameters. The compositional approach used does however allow some design guidelines to be made clear. Specifically around the levels of water used, where uniform and discrete particles can be formed through water content control. The boundary conditions employed provide compositional guidance to the reproducible synthesis of these particles.
As a result of this research, for the first time a range of colloidal silica nanoparticles, which are discrete (primary) in nature, are porous, can be suspended in alcohol (as well as water), and can be functionalised if required to provide different characteristics (such as hydrophobicity, self-cleaning, anti-soiling and anti-icing) are available for commercial use.
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