surface of silicas modified with phosphonic acids.

The present invention relates to a process for modifying the silica surface, in particular for the preparation of a grafted silica with hydrophobic chains R, wherein: (A) in an aqueous medium, treated silica with divalent metal cations at least capable of binding to the surface of said silica, typically Al type aluminum cations ; then (B) in the obtained aqueous medium is introduced molecules of at least one phosphonic acid preferably having the formula R-PO (OH) 2, wherein R is a chain hydrophobe.L'invention also relates silicas hydrophobized as obtained according to this process and their uses, particularly as a filler in silicone matrices.