Understanding the organization of the hydration layer at functionalized
silica surfaces is relevant for a large range of biosensing applications or surface phenomena such as biomolecule adsorption.
Silane monolayers are widely used to functionalize
silica surfaces. Using molecular dynamics simulations, we have investigated the role of
silane molecule head-group charge, alkyl chain length, and surface coverage in the structural organization and dynamic properties of Na+
ions, Cl-
ions, and water molecules at the interface. The
silane molecules studied are 3-aminopropyldimethylethoxysilane, n-propyldimethylmethoxysilane, octadecyldimethylmethoxysilane, and (dimethylamino)dimethylsilylundecanoate. Our results suggest that the distribution of interfacial
ions is sensitive to the 2D dispersion of the
silane-charged head groups. Also, while charged
silane monolayers show a strong orientation of interfacial water molecules, which leads to a
rupture in the hydrogen bond network and disturbs their tetrahedral organization, the arrangement of water molecules at the interface with uncharged
silane monolayers seems to be related to the surface roughness and to alkyl chain length. In line with these results, the diffusion of
ions and water molecules is higher at the CH3 long monolayer interface than at the CH3 short monolayer interface and at the charged monolayer interfaces. Also, whatever the
silane molecules studied, bulk properties are recovered around 0.7 nm above the interface. The interfacial water organization is known to impact biomolecule adsorption. Therefore, these results could further help in optimizing the functionalization layers to capture analytes.