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The rational design of silica particles – an overview

Wed, 05 June, 2019

One of the key focuses within the production of silica using sol-gel methods has for some time been the levels of water content and its influence on how silica particles evolve. 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.

A paper was published covering this work in a peer review journal (Journal of Sol-Gel Science and Technology). What follows is an overview of that peer review paper by way of background to the commercialisation of SMS particles. The peer review paper can be found here.

The starting point was to identify compositional regimes for the formation of discrete silica particles using the Stöber 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 are very different to those produced using conventional pyrogenic methods. This meant that the nanoparticles can be produced in a far wider range of sizes, are primary particles rather than aggregates, have lower manufacturing energy demands, and are available in solvents that allows selective and specific surface functionalisation. This last point enables the potential to control their surface chemistry, and the development of complex structural hierarchies.

The key findings reported in the paper are that the water content of the initial formulation has a determining effect on the resultant size of the synthesised particles. The researchers found both upper and lower composition boundaries beyond which the particles produced were no longer discrete but aggegrated. The lower boundary was related to the availability of reactants, whilst the upper boundary was due to solubility of the silicon precursor and its polymerised derivatives.

Achieving an optimum water content level was therefore critical to retain discrete particle integrity. 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. The researchers observed that the effect of the ethanol liberation as function of hydrolysis was to make the composition richer in alcohol, which increased the range over which chemical compatibility was seen. Once the compositional range to generate discrete particles over a range of diameters was established, surface area and porosity were analysed. As anticipated there appeared to be a relationship between the initial formulation (water content) and porosity, in that the lower the level of water, the higher the porosity and higher surface area.

Thermogravimetric analysis of the materials produced interesting results, in that the higher initial water content materials appeared to lose less weight during dehydroxylation. 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).

In conclusion, this study determined that there is a way to produce discrete silica particles of varying sizes using the Stöber process. As with most sol-gel synthesis, the resultant materials are dependent on many process parameters. The compositional approach used does however allow some design guidelines to be determined. 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 and can be suspended in alcohol (as well as water) are available for commercial use. This enables selective and specific functionalisation to be achieved to provide different characteristics (such as hydrophobicity, self-cleaning, anti-soiling and anti-icing).

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