Influence of the Chain Length and Surface Density on the Conformation and Mobility of n-Alkyl Ligands Chemically Immobilized onto a Silica Surface

Molecular dynamics procedures have been used to investigate the conformation and mobility of n-alkyl ligands immobilized onto an amorphous silica surface, analogous to those utilized with silica-based RP-HPLC sorbents. Molecular models were constructed for n-octyl and n-octadecyl bonded phases with ligand densities of 1.64, 2.67, and 3.69 μmol/m² for each ligand length. The hydrocarbon layer thickness, gauche-trans statistics, end-to-end distance distribution of the n-alkyl chains, and diffusion coefficients of individual atoms in the n-alkyl ligands were determined for each system. The results have been compared with data previously obtained with an n-butyl bonded phase. The conformation of the chains, their positions with respect to the surface, and the chain mobilities were characterized as a function of ligand length and surface density. The results demonstrated that die most ordered part of the chain is always the segment that is anchored closest to the silica surface. With decreasing density, the ligands become more bent, with more gauche defects occurring in the middle and at the free end of the chains. This behavior leads to a dramatic decrease in the thickness of the hydrocarbon layer at minimum surface loading. Potentially, this behavior could result in stronger surface interactions with a prospective solute. These simulation studies also indicate that both short and long n-alkyl ligands can layer over the surface at lower ligand density, thus creating additional protection of unreacted surface silanols, with this effect being more pronounced for the longer n-alkyl chains. In terms of ligand dynamics, the simulated structures indicated that the n-octyl chain exhibits the highest overall mobility at all studied densities, while the n-octadecyl ligands possess the lowest motional freedom. The degree of mobility for any length ligand decreased with increasing surface density. The mobility of individual carbon atoms within the chains also increased with distance from the silica surface for fill ligand lengths and densities. These simulated properties of n-alkyl ligands immobilized onto a silica surface correlated well with other results obtained by FITR or ¹³C-cross polarization magic angle spinning NMR experimental methods and statistical predictions of the behavior of immobilized chains.