STUDY OF THE CONSTITUTIVE SUPERFICIAL WATER OF PRECIPITATED AMORPHOUSSILICAS 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 M AS at room temperature. Previous Si-29 high resolution NMR results hav e been used to interpret the spectra. Geminal silanols, single silanol s. water molecules relatively strongly hydrogen-bonded to silanols, an d water not interacting with OH groups have been characterized and qua ntified. Differences are found between the two precipitated silicas. S ilanols, 70% of which can hydrogen-bond a water molecule, occupy all t he surface of one of them. The other sample surface is not so crowded with silanols, and only 37% of them hydrogen-bond water molecules, Bot h geminal and single silanols hydrogen-bond water. Physisorbed water m olecules not interacting with silanols can be equilibrated on both sam ple surfaces. Room temperature dehydration of the samples in vacuum is studied.