First-principles simulation study on the weakening of silane coupling to silica under alkaline conditions

Abstract

Silane coupling is commonly used to connect organic polymers to inorganic substrates for surface modification and composite material fabrication. It is known that the covalent bonds that form between the silane coupling agent (SCA) and hydroxylated SiO2 weaken under alkaline conditions. In the present work, we theoretically investigated the microscopic mechanisms of this phenomenon by combining first-principles calculations of the deprotonation free energy with density functional theory (DFT), bond-breaking transition-state barrier energy calculations, and DFT-based molecular dynamics (MD) simulations. We found that the following occurs under alkaline conditions: (i) the terminal -OH of SiO2(001) is significantly deprotonated, while the -OH group of the SCA is hardly deprotonated; (ii) the transition-state barrier energy to break the Si-O bond between the SCA and SiO2(001) becomes less than 1 eV with the assistance of H2O dissociation when 50% of the terminal -OH groups on SiO2(001) is deprotonated; and (iii) the predicted Si-O bond-breaking reactions occurred in DFT-MD simulations with realistic settings.