Adsorption and structural arrangement of cetyltrimethylammonium cations at the silica nanoparticle-water interface

Although the sorption of cetyltrimethylammonium ion (CTA(+)) on silica NOD surfaces has been studied extensively, little is known about the interactions between large surfactant molecules and nanosized colloidal particles with a high specific surface area. The aim of the study was to understand the effects of structural arrangements of sorbed CTA(+) ions on the stability and surface properties of SiO2 nanoparticles. The extent of the effect of CTA(+) sorption on the aggregation behavior of the SiO2 nanoparticle suspension was investigated with the dynamic light scattering (DLS) technique. Both the Fourier transform infrared (FTIR) and Raman spectroscopic techniques were used to probe the sorbed layers of CTA(+) on silica surfaces. Results indicate that, at a low surface coverage (less than a monolayer), CTA(+) molecules were strongly bound to the SiO2 surface via their trimethylammonium headgroups. A bilayer sorption of CTA(+) was observed at a high surface coverage, and the sorption is attributed to the hydrophobic interactions between aliphatic tails of CTA(+). Sorption of CTA(+) at a low surface coverage also caused the destabilization of the SiO2 nanoparticle dispersion as a result of surface charge neutralization, but redispersion and surface charge reversal of SiO2 colloids occurred at a high surface coverage. The present study thus confirms the adsorption mechanism of the reverse orientation model and contributes to a better understanding of the sorption and structural arrangements of sorbed surfactants at the SiO2 nanoparticle-water interface.