Der Wassergehalt „getrockneter”︁ Siliciumdioxyd‐Oberflächen

Abstract

Mit 3 Ahhildungen InlialtsiiBersicht An Aerosil, einem femteiligen amorphen Siliciumdioxyd, o-urde die Nenge der an der Oberfliiche gebundenen Silanolgruppen in Abhangigkeit von der Temperatur durch Umsetzung mit SOC1, bestimmt. Ihre Menge fie1 zwischen 250" und 350" stark ab. Die Umsetzungen mit wasserfreiem AlCl, und BCl, und Bestimmung der gebundenen Mengen Al bzw. B nnd C1 ergab, daO die Oberfliiche des bei erhohter Temperatur im Hochvakuum entgasten Aerosils noch freies, fest adsorbiertes Wasser enthielt. Zwischen 100" und 350" war eine Wassermolekel auf zwei Silanolgruppen gebmiden. ffber 350' nahm der Wassergelialt stkrker ab, als die Zahl der Silanolgruppen. Suiuinaq-The iiumbei of surface silaiiol groups on "Aerosil". an amorphous fine particle bize silica, was determined by reaction with SOCI,. Their number decFeases sharply between 250' and 350 "C. After reaction with anhydrous AlCl, and BCl,, the"Aerosi1"xas analyzed for A1 or B and C1. The resultsshom, that free, strongly adsorbed water is bound to the surface even after high vacuum heat treatment. From 100' to 350 "C, there is one water molecule bound per two silanol groups. Above 350"C, tlic-m-ater content decreases t o a greater extent than the number of silaiiol groups. Einleil uiig uiidProblcrnstt.lluug Die Oberflache von kristallineni und amorphem Siliciuiiidioxyd ist unter iiorrnalen Bedingungen mit Hydroxylgruppen (Silanolgruppen >Si-OH) bedeckt. Die dbstittiguiig der freieii Valenzen der am Rande der Silicatscliichteii voii Toiimiiieraleii gelegeiieii Siliciuin-,4tome durch Hydrosylgruppen 1%-ar zuerst, %-on U. HOFXAXE; I) -\-ermutet worden ; sptiter nahin CAR MAN^) das gleiche beim Siliciumdioxyd an. Bei hohen 'remperaturen gehen die Silanolgrnppen miter Wasserabspaltnnp in Silovmgruppen (2Si -0 -Si<) iiber .

Nature and Estimation of Functional Groups on Solid Surfaces

“…45 % of the fixed silane was bound bipodally. These results agree roughly with earlier measurements with BCl 3 [620] where a CI:B ratio of 1.35-1.44 was observed for Aerosil outgassed at 373 to 473 K; one can calculate 56-65 % bifunctional reaction from this ratio.…”

Infrared Study of OH and NH2 Groups on the Surface of a Dry Silica Aerogel

“…Evidence from reaction of silica gels with A1CL and BC13 has, however, been interpreted to mean that molecular H20 can be strongly held even after the silica has been dried under vacuum at 350°or higher. 19 Prevailing opinion holds that most of the OH groups in silica gel are on the surface. Characteristically, however, a significant fraction of the OH content, even on silicas predried at 600°, is not removed by reaction with reagents such as diborane,3,20 CH3MgI, and CH3Li,4 suggesting that the groups are held internally rather than on the surface.…”

Characterization of the acid-base nature of metal oxides by adsorption of TCNQ

“…Actually, Stober [23] found that, even after thorough outgassing at 100°C, one molecule of physically adsorbed water is retained for every two silanol groups on the surface of amorphous silicon dioxide. This surprising result was confirmed by Boehm et al [24], using chemical reactions as well as deuterium exchange. Lee and Weller [25] confirmed the existence of surface hydroxyl groups on A'203 which had been dehydrated at 500°C.…”