Deep Insights into the Processes Occurring during Early Stages of the Formation and Room-Temperature Evolution of the Core (Amorphous SiO2)@Shell (Organocations) Nanoparticles

Sanja Bosnar, Tatjana Antonić Jelić, Josip Bronić, Maja Dutour Sikirić, Suzana Šegota, Vida Čadež, Vilko Smrečki, Ana Palčić, Boris Subotić

Index: 10.1021/acs.jpcc.8b00024

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Abstract

Following the assumption that the crucial processes governing the formation, properties, and evolution of the core (amorphous silica)@shell (organocations) nanoparticles (NPs) take place during short-time, room-temperature (rt) stirring/aging of the homogeneous reaction mixtures (HmRMs) formed by hydrolysis of tetraethyl orthosilicate (TEOS) in solutions of Org(OH)n, we investigated these processes by various experimental methods (pH, ionic conductivity, 29Si NMR, dynamic light scattering, and atomic force microscopy). The analysis of the data obtained by detailed and careful investigation of the “model” HmRMs having the starting chemical composition, xTEOS/0.25TPAOH/20H2O (TPAOH = tetrapropylammonium hydroxide; x = 0.05–1), offer some new elements for the understanding of the mechanisms of the formation and rt evolution of the core@shell silica NPs: (1) There is a resolute evidence of the formation of the stable, about 1.2 nm-sized core (amorphous SiO2)@shell (TPA+ ions) NPs below the critical aggregation concentration. (2) Because of the intensive particulate processes (growth, aggregation, disaggregation, and dissolution) which take place during the rt aging of the investigated HmRMs, the equilibrated core@shell silica NPs do not exist as individual primary ones but as the aggregates (about 2 to about 20 nm), composed of 1–2 nm-sized “primary” NPs. (3) In spite of the most frequent meaning that the NP shell is composed of the “free” TPA+ ions adsorbed on the surface of the NP core, the results of this study show that the NP shell can be formed mainly by the attachment of the polysilicate anions (silicate oligomers), associated with TPA+ ions, on the surfaces of the NP cores.