Kinetics and mechanism of the reaction of organosiloxanes with the surface of pyrogenic silica

Abstract

Infrared spectroscopy and gravimetry were used to study chemisorption kinetics and adsorption equilibrium in the reaction of hexamethyldisiloxane (HMDS), hexamethylcyclotrisiloxane (HMTS), octamethylcyclotetrasiloxane (OMTS), and 1,3,5 trivinyl-1,3,5-trimethylcyclotrisiloxane (MVCTS) with the surface of pyrogenic silica. It was determined that the electron-donor ability of the siloxanes in the formation of hydrogen bonds with surface hydroxyl groups decreases in the series HMDS > HMTS > OMTS > MVCTS. A reaction scheme is proposed, which includes electrophilic substitution of hydrogen of the surface silanol group in the limiting stage. It was found that the chemisorption kinetics is described by an equation of an inhomogeneous-surface model with allowance for interaction between species in a neutral-field approximation. The relative reactivity of the siloxanes in these transformations is governed mainly by the difference of the energies of the reactant siloxane bonds.