Theoretical Studies On The Aggregation Behavior Of Hydrophobic Layer On Rock Surface

Wettability alteration on the rock surface which adsorbed some pollutants, altered the hydrophilic rock surface to a hydrophobic one. This phenomenon is widespread in the natural world. The study of hydrophobic layer on rock surface mainly contains crude oil and surfactants’ layer et al. It is widely used in many industrial fields, such as oil exploration, mineral flotation, decontamination, that studies to the hydrophobic layer on the rock surface. Many researchers studied the detachment and aggregation of hydrophobic layer on rock surface by using a variety of experimental methods, but cannot research the microstructure and mechanism of the micro level, even molecular scale. Therefore, the development and use of the knowledge of theoretical chemistry, which is used to study the detachment and aggregation of hydrophobic layer, have become one of the important methods.In this paper, we study the change and mechanism of aggregation of several typical hydrophobic layers on the rock surface by molecular dynamics simulations. The main results of this paper are as follows:1. Molecular dynamic simulations have been performed to explore the mechanism of oil detachment from hybrid hydrophobic and hydrophilic surface in aqueous solution. The12carbon alkane molecule was used as the model oil molecule. The hybrid surface contains hydrophobic area, which formed by some alkyl chains linking with silica surface through C-O bonds, and hydrophilic area, which is a circle hydrophilic silica area on the middle of the hybrid surface. The simulated results showed that the key to the detachment of alkane molecules is the formation of water channel in oil layer between water phase and solid surface. In the detachment process, water molecules can penetrate oil layer to the silica surface through the strong H-bonding interaction among water molecules in water channel, and soon these molecules can form a gel layer along the silica surface by fast diffusion under the H-bonding and electrostatic interaction between water molecules and silica surface. At last, the half-hydrophilic area on hybrid surface becomes hydrophilic again after the oil layer’s detachment, and alkane molecules aggregate on the modified surface linked the alkyl chains. For the hybrid surface, some of alkane molecules insert into the interstice among the alkyl chains, and thus the oil drop cannot be dispatched thoroughly from the surface linked alkyl chains in aqueous solution. Our results showed that the detachment mechanism of oil from hybrid surface is different, compared with the whole pure hydrophilic surface.2. Molecular dynamic simulations were performed to study the effects of asphaltenes in the detachment process of crude oil. In this thesis, there are three systems. In the systems, the mixture of toluene, benzene, hexane, heptane, octane, nonane, cyclohexane, cycloheptane was used as the model crude oil. To contrast, system I has no asphaltenes and surfactants, and the results show that crude oil can be detachment from the silica surface. In system II, there are some surfactant molecules and nonionic asphaltenes, and the simulation results show that the crude oil cannot be detached from the silica surface, and the crude oil aggregated around the aggregates of asphaltenes and formed a big cylindrical aggregate. In system Ⅲ, there are crude oil and ionic asphaltenes, and the simulation results show that crude oil and asphaltenes aggregate to form a big spherical oil drop. Compared the three systems, there are two reasons that make crude oil detach difficult from the silica surface:one is the formation of H-bonds between asphaltene and silica surface, the other one is the formation of π-π-conjugated among asphaltenes aggregates.3. Molecular dynamic simulations were used to study the mechanism of restructuring of surfactant monolayer on mica surface in aqueous solution. In this thesis, the research of the process of restructuring of surfactant monolayer by observing the diffusion of bromide ion and water molecules, the reversion of surfactant at different stages and the restructuring of surfactant monolayer. The results show that the electrostatic interaction between the negative charge on mica surface and the head group of surfactant makes the bromide ion diffusion into the aqueous solution. At the same time, water molecules can penetrate the monolayer under electrostatic interaction and form water channel which bridge the aqueous solution and silica surface. The diffusion of bromide ion and the formation of water channel disturb the order of the monolayer, and some surfactants become reversed under bromide ion and water molecules. With the increasing of the simulation time, the number the reversion surfactants increases, the monolayer become restructure into bilayer structure, and then form a cylindrical aggregate. The results show that CTAB monolayer can restructure on the mica surface and form bilayer structure and then form a cylindrical aggregate at the end of simulation.