Study of the constitutive superficial water of precipitated amorphous silicas using H-1 NMR: Broad-line at 4 K and HR MAS at 300

Two silicas obtained by precipitation have been compared using two TH NMR techniques: broad-line at 4 K with simulation of the spectra and MAS at room temperature. Previous Si-29 high resolution NMR results have been used to interpret the spectra. Geminal silanols, single silanols. water molecules relatively strongly hydrogen-bonded to silanols, and water not interacting with OH groups have been characterized and quantified. Differences are found between the two precipitated silicas. Silanols, 70% of which can hydrogen-bond a water molecule, occupy all the surface of one of them. The other sample surface is not so crowded with silanols, and only 37% of them hydrogen-bond water molecules, Both geminal and single silanols hydrogen-bond water. Physisorbed water molecules not interacting with silanols can be equilibrated on both sample surfaces. Room temperature dehydration of the samples in vacuum is studied.