Effect of Apolar Chain Grafting Density on Mobile-Phase Transport Properties Revealed by Molecular Simulations

The alkyl-modified surfaces are routinely utilized in high-performance liquid chromatography (HPLC). This study investigates the effect of grafting density on the transport and structure of acetonitrile (ACN) solutions across dimethyloctadecylsilane-modified surfaces, utilizing molecular dynamics (MD) simulations as a probing tool. Simulation results reveal that as the grafting density increases, the conformation of grafted chains transitions from a relatively disordered, reclining state to a more ordered, upright configuration. As a result, the end-to-end distance of the grafted chain rises with the increase of grafting density, subsequently influencing the position and magnitude of adsorption peaks, as well as the diffusion coefficient of ACN solution on the grafted surface. Further analysis indicates that under the influence of Couette flow, increased grafting density reduces both the effective viscosity and the hydrodynamic penetration length of the ACN solution, indicating the flow phase being constrained by the grafted chains and thus inhibiting its effective penetration into their interiors. Additionally, the effective viscosity shows shear-thinning behavior with an increasing shear rate. What is more is that a slip phenomenon emerges on the ungrafted surface, whereas no such slip is observed on the grafted surface, and the slip length increases in proportion to the rise in the applied shear rate. These simulation findings underscore the subtle interplay between the structure and transport properties of the molecular liquids at the grafted interface, which provide insights for improving the design of grafted functional materials employed in advanced separation technologies.