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

摘要

SIR: In a recent paper, Yarovsky et al. presented molecular dynamics (MD) simulations of the stationary phases in reversed phase liquid chromatography. They analyzed both structural and dynamical properties of several chain lengths and bonding densities. In that paper, two comments were made suggesting that we had not included a van der Waals surface potential for the interaction of the chain segments with silica in a recent paper. An explicit surface potential was indeed included in those simulations, as is clearly stated in the paper. It is an integrated 3—9 potential which includes all the attractive van der Waals interactions with the underlying substrate. The parameters in our potential were obtained from experimental heat of adsorption data for alkanes on silica, so the magnitude of the attractive interaction is quite accurate. This type of potential has been widely used for the interactions of molecules with solid surfaces. In a more recent paper, we present extensive MD studies of several chain lengths and surface densities and examine further the role of the surface potential in determining the interface properties. We find that the attractive portion of the surface potential is crucial to yield a realistic interface. We computed the density profiles of the chain segments away from the solid surface and obtained very good agreement with neutron scattering experiments for the stationary phase width, which is substantially less than that of fully extended chains.