Atomistic models of hydroxylated, ethoxylated and methylated silica surfaces and nitrogen adsorption isotherms: A molecular dynamics approach

Abstract

In this paper, we have developed realistic models of hydroxylated, ethoxylated and methylated silica surfaces. They represent several sol-gel materials, among which a functionalized methylated silica used for chemical sensor applications. The functional groups were tuned in order to match the experimental data available. The creation of the silica models implies after the cutting of a bulk of amorphous silica, to add covalent OH, –OCH2-CH3, or -CH3 functions at the surface while keeping the ionic characteristic of the core. In total, five models of silica surface were built. The number OH groups for the hydroxylated silica were adjusted to match the Zhuravlev constant (αOH = 4.56 OH nm−2) and the repartition of the different types of silanols was found to be in good agreement with previous work with 80% of SiOH, 19% of Si(OH)2 and 1% Si(OH)3. In addition, the proportions of vicinal and isolated hydroxyls were distinguished. The non-functionalized (Silica-OEt/OHCEA) and functionnalized silica (Silica-Me2/OHCEA) contain respectively 2 OEt nm−2 and 3 OH nm−2and 3 SiMe2 nm−2 and 2 OH nm−2. Further, we have simulated nitrogen adsorption isotherms using Molecular Dynamics. Contribution of the functionalization to the surface behavior was shown not to be significative. The utilization of the CP and VSH models yields similar results. The isotherms were analyzed in terms of relaxation functions. The N2 adsorption and desorption were predicted to occur through first order mechanisms with an adsorption rate constant of 0.031 ps−1. Agreement between the simulated curves and experimental BET results is quite remarkable. This work would contribute to the development of chemical sensors as such surfaces will allow accurate investigations of the interaction of target molecules at a molecular level.