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\nmagnetic resonance (MAS NMR) experiments has been used to investigate the hybrid organic−inorganic\ninterfaces in surfactant templated silicas. Samples prepared with cetyltrimethylammonium bromide (CTAB)\nunder acidic (HCl) and basic (NaOH) conditions have been compared. The use of sequences based on\nthe <sup>29</sup>Si-<sup>1</sup>H heteronuclear dipolar interactions allows us to selectively filter the NMR response of the\nprotons close to the Si surface sites showing directly the clear difference between the two systems. The\nbasic sample is characterized by a small amount of Si−OH groups and a short distance between the\nSi−O<sup>-</sup> 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\nthe interface induces consequent differences in the structure of the adsorbed water layers present at the\ninterface, and this has been revealed by near-infrared experiments, as well as <sup>1</sup>H MAS NMR spectra\nrecorded on dehydrated and partially rehydrated samples. One objective of this work was also to show\nthat the use of standard solid-state NMR conditions (magnetic field of 7 T and magic angle spinning\nfrequency of less than 15 kHz) can be largely sufficient to obtain extremely valuable information regarding\nthe silica−surfactant interfaces.