Solid-state NMR characterization of the surfactant-silica interface in templated silicas: Acidic versus basic conditions

A combination of one-dimensional and two-dimensional solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) experiments has been used to investigate the hybrid organic-inorganic interfaces in surfactant templated silicas. Samples prepared with cetyltrimethylammonium bromide (CTAB) under acidic (HCl) and basic (NaOH) conditions have been compared. The use of sequences based on the Si-29-H-1 heteronuclear dipolar interactions allows us to selectively filter the NMR response of the protons close to the Si surface sites showing directly the clear difference between the two systems. The basic sample is characterized by a small amount of Si-OH groups and a short distance between the Si-O- surface groups and the surfactant polar head group, while the acidic sample exhibits a silanol-rich surface with a longer distance between the Si surface sites and the polar head groups. The nature of the interface induces consequent differences in the structure of the adsorbed water layers present at the interface, and this has been revealed by near-infrared experiments, as well as H-1 MAS NMR spectra recorded on dehydrated and partially rehydrated samples. One objective of this work was also to show that the use of standard solid-state NMR conditions (magnetic field of 7 T and magic angle spinning frequency of less than 15 kHz) can be largely sufficient to obtain extremely valuable information regarding the silica-surfactant interfaces.