Impact of Silica Surface Nanoconfinement on the Microstructure of Alkoxysilane Layers Grafted by Supercritical Carbon Dioxide

The impact of nanoconfinement on the microstructure of alkoxysilane layers grafted by supercritical CO2 was determined using model system made of silica nanochannels, i.e., planar silica surface spaced by few nanometers. Two types of silica nanochannels of 3 and 5 nm gap size were grafted with alkoxysilanes having different head groups (thiol, amine, and iodo) using the same protocol as in our previous study on open flat silica surface (materials, T, P). Using the same characterization technique, X-ray reflectivity, we have directly compared the results obtained on open planar silica surface with confined ones. We show that the microstructure of grafted layers obtained on nonconfined silica flat surface is not directly transposable on a confined silica planar surface. In our experimental conditions, on a flat surface, the microstructure of the grafted layer is only driven by the alkoxysilane molecule, whereas in the confined planar silica surface, the microstructure is driven by both the confinement size and the nature of the molecule. This may be explained by the modification of the molecules transport and the change of supercritical CO2 properties in nanoconfined media.