Gemini surfactant binding onto hydrophobically modified silica nanoparticles

Aqueous systems containing hydrophobically modified, HM, 200 nm silica nanoparticles, strictly monodisperse in size, were added with tiny amounts of a cationic gemini surfactant, 2-Quat-C-6 gemini, with two alkyl chains separated by a short, six-carbon, hydrophobic spacer. As observed, the latter surfactant strongly favors the stability of the resulting dispersions. Care was devoted to work in proper concentration ranges, where the adsorption process of gemini surfactant is strongly favored with respect to micelle formation. Binding of the above surfactant onto HM silica was monitored by a combination of different physicochemical methods, spanning from H-1 NMR chemical shift to pulsed gradient stimulated echo (PGSE) NMR and from zeta-potential to dynamic light scattering (DLS). Proper combination of results obtained from the above experimental methods allowed us quantifying the extent of gemini binding onto HM silica. Binding depends on the amount of surfactant and, to a less extent, on ionic strength. Estimates of the binding saturation conditions were made by combining results from H-1 NMR with electrophoretic mobility ones. The whole binding process has been rationalized in terms of a simplified Langmuir-like adsorption isotherm.