physisorbed water), and their accessibility to water (internal On the surface of two commercial pyrogenic silicas (Degussa or external silanols). Study of the state of physisorbed water and Cabot), five resonances were identified on the basis of the itself led to the distinction between structured monolayers, chemical shift, homonuclear coupling ( T 2 ), and spin-lattice relaxclusters, and weakly bound liquid-like water molecules (14).
ation behavior (T 1 ). In accordance with previous studies we ob-Nevertheless, several difficulties remain when studying served three different types of silanol groups: (i) weakly coupled silica surfaces. First, the surface is not structured. Therefore, (long T 2 ), water inaccessible, isolated ''internal'' silanols at 1.8 although models based on small crystalline domains have ppm; (ii) weakly coupled, external ''free'' silanols revealed upon been proposed 10,, surface irregularity leads to a dehydration at 2.5 ppm; and (iii) strongly coupled external hydrogen bound silanols with an unresolved broad resonance between dispersion of the physicochemical properties of the hydroxyl 3 and 7 ppm. The resonance of water, whose position between 2.6 sites. Second, thermogravimetric dosing of structural and and 4.6 ppm depended on water content, corresponded to two adsorbed water is difficult as (de)hydration overlaps with unresolved species of slightly different T 1 . By equating this reso-(de)hydroxylation. Third, amorphous silica is a metastable nance to the weighted average of two distinct populations of water, phase, and its surface is heavily reconstructed during the we were able to distinguish the first layer of strongly hydrogen conditioning of the samples. bound water at 2.7 ppm from liquid-like water at 5 ppm. The first Among the different techniques mobilized to scrutinize layer is complete for water relative humidity as low as 3.6% and protons on the surface of silica in particular, and oxides in corresponds to a surface coverage of 4.75 H 2 O/nm 2 . If we assumed general, 1 H NMR is certainly among the most promising a cristobalite-based surface structure, this meant a 1:1 ratio between surface hydroxyls and the first layer of physisorbed water. because of its great sensitivity and dependency on hydrogen This ratio was the same for the two silicas regardless of surface bonds. Despite severe line broadening due to a combination area. ᭧ 1997 Academic Press of strong homodipolar coupling and chemical shift disper-Key Words: 1 H MAS-NMR; silica.
sion, study of precipitated and sol-gel silicas allowed identification of the resonances of water, bound silanols, and isolated silanols at circa 5, 2.5, and 1.7 ppm from tetramethylsi-