Balaton Symposium '97 [antikvár]

L-01 Surface Properties of Amorphous Silicon Dioxidé Preparations. Our State of the Art. sz. Kováts E. Ecole Polytechnique Fédérale de Lausanne and üniversity of Veszprém 1015 Lausanne, Switzerland 8200 Veszprém, Hungary The surface area of powders composed of monodispersed silicon dioxidé microsphere [1] could be calculated with the diameter of the particles determined by electron microscopy. This result permitted calibration of the BET-evaluation of nitrogén isotherms. Use of a(N2) = 13.5 Á2 for the space requirement of a nitrogén molecule in the first adsorbed layer (instead of 16.2 Á2) resulted true surface areas. Densest dimethyl-alkylsilyl layers could be prepared by using (diamethyl-amino)dimethyl-alkylsilanes as silylating agents [2], Functionalized derivatives with bulky substituents gave the same result [3]. Comparison of nitrogén isotherms on silylated powders revealed that the value of the space requirement of N2 depends on the nature of the surface; on alkylsilylated surfaces a(N2) = 16.3 Á2 [4,5]. Use of true surface areas gave excellent agreeement between model calculations and experimentál results. Calculated and experimentál silanol concentration at the surface of fully hydrated silicon dioxidé powders agrees within 3 % [6,7], The space requirement of dimethyl-alkylsilyl substituents calculated with van der Waals radii agrees well with those observed in densest layers. References [1] True Surface Areas from Nitrogén Adsorption Experiments; Jelinek L., sz. Kováts E„ Langmuir, 1 0, 4225 (1994) [2] Monofuctional (Dimethylamino) silane as Silylating Agent; Szabó K, Ha N. L., Schneider Ph„ Zeltner P„ sz. Kováts E., Helv. Chim. Acta,67, 2128 (1984) [3] Tailor-Made Silylating Agents for Efficient Surface Modification; Schneider Ph., Cloux R., Fóti K, sz. Kováts E„ Synthesis, 1027 (1990)