Molecular dynamics of tethered alkanes: Temperature-dependent behavior in a high-density chromatographic system

Molecular dynamics simulations of the temperature dependence of physical properties of alkanes chemically tethered to a solid surface are presented. The system was modeled after the C₈ stationary phases of chromatographic interfacial systems. The chains were placed randomly on the surface with excluded volume interactions at a density of 4 μmol/m² or 41.5 Ų/chain, the highest density obtainable by current synthetic methods on porous silica substrates. The simulations were carried out in the absence of solvent at temperatures ranging from 96 to 391 K. Various equilibrium and dynamical properties of the chains were computed including z profiles, torsional angle populations, order parameters, mean-square displacements, and power spectra of the velocity autocorrelation function (VAF) averaged over all atoms and for selected atoms along the chains. The chains undergo a slow transition from glassy to fluidlike behavior over a broad temperature range in this model, consistent with experimental findings.