1. 1Degussa AG, Inorganic Chemical Products Division, Applied Research and Technical Service for Fillers and Rubber Chemicals, Koelner Strasse 122, D-5047 Wesseling, Federal Republic of Germany
2. 2Centre de Recherches sur la Physico-Chimie des Surfaces Solides, 24, Avenue du Président Kennedy, F-68200 Mulhouse, France