Computational study of acidic and basic functionalized crystalline silica surfaces as a model for biomaterial interfaces

Auteurs

M. Corno, M. le Piane, S. Monti, M. Moreno-Couranjou, P. Choquet, and P. Ugliengo

Référence

Langmuir, vol. 31, no. 23, pp. 6321-6331, 2015

Description

In silico modeling of acidic (CH₂COOH) or basic (CH₂NH₂) functionalized silica surfaces has been carried out by means of a density functional approach based on a gradient-corrected functional to provide insight into the characterization of experimentally functionalized surfaces via a plasma method. Hydroxylated surfaces of crystalline cristobalite (sporting 4.8 OH/nm²) mimic an amorphous silica interface as unsubstituted material. To functionalize the silica surface we transformed the surface Si–OH groups into Si–CH₂COOH and Si–CH₂NH₂ moieties to represent acidic/basic chemical character for the substitution. Structures, energetics, electronic, and vibrational properties were computed and compared as a function of the increasing loading of the functional groups (from 1 to 4 per surface unit cell). Classical molecular dynamics simulations of selected cases have been performed through a Reax-FF reactive force field to assess the mobility of the surface added chains. Both DFT and force field calculations identify the CH₂NH₂ moderate surface loading (1 group per unit cell) as the most stable functionalization, at variance with the case of the CH₂COOH group, where higher loadings are preferred (2 groups per unit cell). The vibrational fingerprints of the surface functionalities, which are the ν(C═O) stretching and δ(NH₂) bending modes for acidic/basic cases, have been characterized as a function of substitution percentage in order to guide the assignment of the experimental data. The final results highlighted the different behavior of the two types of functionalization. On the one hand, the frequency associated with the ν(C═O) mode shifts to lower wavenumbers as a function of the H-bond strength between the surface functionalities (both COOH and SiOH groups), and on the other hand, the δ(NH₂) frequency shift seems to be caused by a subtle balance between the H-bond donor and acceptor abilities of the NH₂ moiety. Both sets of data are in general agreement with experimental measurements on the corresponding silica-functionalized materials and provide finer details for a deeper interpretation of experimental spectra.

Lien

doi:10.1021/acs.langmuir.5b01828