Student Seminar February 29, 2016 | 10:30 am | Donadeo ICE 7-395
Presenters: Cuiying Jian and Ling Zhang
Supervisors: Dr. Hongbo Zeng and Dr. Qingxia Liu
Title: Probing Molecular Interactions and Interfacial Properties of Asphaltenes and Violanthrone-79 Model Compound
Mohammad Reza Poopari1, Cuiying Jian1，2, Ling Zhang1, Qingxia Liu1, Nestor Zerpa3,
Tian Tang2, Hongbo Zeng1
1Department of Chemical and Materials Engineering,2Department of Mechanical Engineering, University of Alberta, Edmonton, Alberta, and 3Nexen Energy ULC, A CNOOC Limited Company, Calgary, Alberta
The molecular interaction and interfacial properties (e.g. interfacial tension, IFT) of an n-pentane (C5) extracted asphaltene and a model asphaltene compound, i.e. Violanthrone-79 (VO-79), were investigated via both molecular dynamics simulations and experimental measurements. The effects of solution concentration, solvent, temperature, and solution salinity were explored. The adsorption kinetics of asphaltenes and VO-79 were measured using a Quartz Crystal Microbalance with Dissipation monitoring (QCM-D). IFT of different organic solvents and aqueous solutions in the presence of asphaltenes and VO-79 was measured using a tensiometer. Overall, the IFT results for C5 asphaltenes and VO-79 show similar trends. The IFT decreases with increasing the concentration of asphaltenes or VO-79, and also decreases at an elevated temperature. The addition of an inorganic salt (i.e. NaCl) in water increases the IFT. QCM-D experiments showed that the adsorption of VO-79 on silica sensors was independent of the concentration of VO-79 below 1000 ppm, while the adsorption was more significantly at higher concentration. The adsorption of C5 asphaltenes on silica surfaces was also found to be independent of concentration in toluene, while the adsorption was enhanced and increased with increasing the asphaltene concentration in heptol. AFM imaging revealed more aggregates on the silica surfaces after adsorption in heptol as compared to that in toluene.
Molecular dynamics simulations were performed to investigate the underlying mechanisms for the reduction of IFT by model asphaltene compounds at the organic solvent/water interface. Two types of systems were employed: (1) one series of simulations starting from model asphaltene compounds initially dispersed in organic solvents and (2) the other series of simulations starting from pre-assembled layers formed by model asphaltene compounds at the interface. It was found that correlations of IFTs between simulations and experiments should be made based on “surface concentration” of asphaltene compounds, rather than the generally used “bulk concentration”. Furthermore, the simulations revealed that hydrogen bonding as well as other types of electrostatic interactions present between the model compounds and water plays important roles in reducing the IFT.