(17a) The Silanol Content of Silica Nanoparticles (Invited)

The surface chemistry of synthetic amorphous silicas is important for their performance (e.g. in elastomers, paints or cosmetics) as well as for understanding their bio-interactions. Here, the effect of synthesis conditions on the silica surface hydroxylation is investigated. Furthermore, a correlation between silanol content/type and in vitro cytolytic activity of flame-made silicas is sought by measuring the induced membrane damage in human blood monocytes. By controlling the flame synthesis variables, silica nanoparticles with the same specific surface area but distinct surface chemistry are prepared and characterized by Raman and infrared spectroscopy, thermogravimetric analysis, and titration with lithium alanate. The surface silanol content of silicas made in relatively short and “cold” flames is rather high (up to ~8 OH/nm²) and is reduced down to ~4 OH/nm² for silicas made in long and “hot” flames. The latter is closer to that of commercial fumed silica. Moreover, “cold-flame” silicas exhibit higher proportion of hydrogen-bonded vs. isolated silanols and more physisorbed water than “hot-flame” ones of similar average primary particle diameter. Interestingly, the above surface chemistry differences translate to lower cytolytic activity for “cold-” than “hot-flame” silicas in human monocytes. Most notably, total silanol contents of up to ~17 OH/nm² are calculated by thermogravimetric analysis for “cold-flame” silicas, indicating their high internal silanol content. The latter provides new insight into the potential formation mechanism of silica particles in rapidly-quenched “cold” flames.