Composition for hydrophobic treatment comprising one or more mixed compounds having the formula (I) in which - at least one R is a C<SUB>a</SUB> F2<SUB>a+1 </SUB>group, a being a whole number, where another R may be a Si X<SUB>3-b</SUB> R<SUP>1</SUP> <SUB>b</SUB>
