Molecular Simulation Studies On Friction Between Oligo(Ethylene Glycol)Self-assembled Monolayers

Poly(ethylene glycol)(PEG)is a typical non-fouling material which can prevent nonspecific protein adsorption effectively.Besides,PEG also exhibits low friction characteristics in water.Researches have shown that the low friction characteristics may be in close relationship with hydration layers on the surface.However,due to the limitation of current facilities,it is difficult to achieve the mechanism of the low friction characteristics for PEG in water on a micro level.Molecular simulation has been an effective assistant technique for experiment to study the microscopic molecular interaction mechanism in recent years.In this work,all-atom molecular dynamics simulation was used to analyze the mechanism and characteristics of friction of Oligo(ethylene glycol)self-assembled monolayers(OEG-SAM)by analyzing the structure and dynamics of the hydration layer.Furthermore,structure and dynamic response of water in electric fields were studied to offer basic information for the further study on controlling surface friction by electric fields.The main contents and conclusions include:1.Hydration lubrication of OEG-SAM surfaces with different separation distances was simulated.Results show that OEG-SAM evidently affects dynamic behaviors of water molecules in the hydration layer on the surface.The water in each sub-layer has different structure and dynamics characteristics compared with bulk water.Friction between OEG-SAM surfaces is in close relationship with interaction of hydration layers.While the separation distance is lower than 15A,hydration layers would directly contact,which leads to high friction.2.Hydration lubrication of OEG-SAM surfaces in different temperature was simulated.Analysis on hydration layers shows that rising temperature from 280K to 310K results in the increase of water's dynamic behaviors and the fluidity,which leads to the decrease of friction.3.Structure and dynamic behaviors of bulk water in external electric fields were simulated.Results show that exposing water to a static external electric field of 1.0V/A increases its density by 22%.When water molecules are exposed to external pulsed electric fields(PEF),the density of water fluctuates with the variation of the strength of PEF.The period of the density variation matches perfectly with the frequency of the PEF.