Molecular Dynamics Simulation Of Asphaltene Adsorption On Hydrophilic/Hydrophobic Surfaces

Petroleum is an important non-renewable fossil energy source,mainly used as fuel oil and gasoline.In addition,petroleum is also a raw material for many chemical industry products,such as plastics,lubricants,fertilizers and pesticides.It can be seen that our production and life are inseparable from petroleum resources.However,with the rapid development of society,the demand for petroleum energy continues to increase.After conventional crude oil is gradually mined,its reserves and output decrease year by year.At the same time,with the development of exploration technology,more and more heavy oil resources are discovered,and their reserves account for an increasing proportion of petroleum resources.The exploitation of heavy oil can greatly ease the human demand for petroleum resources.Therefore,it has extremely high mining potential.As the focus of petroleum resource extraction gradually shifts to heavy oil resources,more and more researches on heavy oil are made,and the properties of heavy oil are gradually known.Heavy oil has the characteristics of high viscosity,poor fluidity and complicated composition.The study found that the high content of asphaltene and resin polyaromatic components is the main reason for the viscosity of heavy oil.Asphaltene is the molecule with the largest structure,the largest density,the strongest polarity and the highest surface activity in crude oil,while the surface activity of resin is second only to asphaltene.So,resins and asphaltenes are prone to aggregation and adsorption on complex solid surfaces,which in turn causes heavy oil to block oil wells and pipelines.Therefore,it is of great significance for the development of heavy oil to fully understand the interaction between polyaromatic components(asphaltene and resin)and the structure and role at the adsorption interface.By theoretical computational chemistry,we can study the motion state of molecules at the molecular level and have a visualization function,which can help us analyze the aggregation and adsorption behavior of heavy oil molecules at the interface.In this thesis,molecular dynamics simulation was used to study the aggregation and adsorption of heavy oil molecules on the silica surface.The main results are shown as follows:The molecular dynamics simulation system was used to study the adsorption mechanism of C5Pe molecules on the surface of silica with different hydrophobicity,and the adsorption differences were analyzed at the molecular level.The only variable controlled by the simulation process is the proportion of hydrophilic and hydrophobic groups on the surface of the silica.Five systems were generated:completely hydrophilic system,25%hydrophobic system,50%hydrophobic system,75%hydrophobic system and complete hydrophobic system.The visualization and analysis software were used to conduct a comparative study on the two adsorption structures of aggregates and single molecules.The interaction of C5Pe with the silica surface,hydrogen bonding,and changes in adsorption structure were further analyzed.The results prove that there are tiled adsorptions and tilted adsorptions structures in each system and C5Pe at the interface of 25%hydrophobic silica tends to adsorb in the form of tiles and has stronger adsorption stability.In addition,the study found that tiled adsorption of C5Pe molecules has better adsorption stability and higher adsorption strength than tilted adsorption of C5Pe molecules.In the study of the change of C5Pe tiled adsorption formation,we found that C5Pe molecule tiled adsorption on the silica surface must overcome two energy barriers.Adsorption of oil droplets on the different hydrophilic and hydrophobic silica surface was studied by molecular dynamics simulation.The composition of heavy oil is complex,and it is often adsorbed in the form of oil droplets on a solid surface.To simulate the composition of oil droplets,a heavy oil droplet containing three asphaltene molecules,six resin molecules,and eight solvent light oil molecules is first constructed.The same oil droplets were placed on the surface of fully hydrophilic,50%hydrophobic and fully hydrophobic silica,respectively.The oil droplets adsorption process and the mechanism of adsorption and aggregation of asphaltenes and resins in oil droplets were studied.The results of the oil droplet adsorption process show that the fully hydrophobic system moves faster at the COM(center of mass)and occupies the silica interface relative to the oil droplets of other systems.After the system reached stability,the asphaltene and resin molecules in the oil droplets showed different adsorption and aggregation states in different systems.In the fully hydrophilic system,molecules aggregate through face-to-face stacking interaction and are adsorbed vertically on the silica surface.Such a structure is conducive to the formation of more hydrogen bonding interactions between the model molecule(asphaltenes and resins)and the silica surface.This result indicates that the fully hydrophilic system has a stronger interaction with the asphaltenes and resins than the other two systems.Finally,by calculating the free energy of adsorption of model molecule on the surface of silica,it is further proved that the fully hydrophilic system has stronger adsorption capacity.