Behavior Of Surfactants On Oil/Water Interface And Foam Film For Enhanced Oil Recovery

Full-atom Molecular Dynamics (MD), mesoscopic Dissipative Particle Dynamics (DPD), as well as macroscopic experimental method, has been used to systematically investigate interfacial behavior of surfactants in enhanced oil recovery based on different scale. By combining of experiment and computer simulation, the relationship between molecular structure, film properties and the apparent phenomena has been established. These innovative results help us understand and predict properties and behaviors of surfactants adsorpted at interface and provide guiding significance for the various applications of surfactants and novel molecule design.1. Molecular modeling for surfactant adsorpted at oil/water interfaceWhen choosing between the many surfactants available, both natural and artificially prepared, or designing new ones, one would often like surfactants which reduce the interfacial tension both effectively and efficiently by adding as little surfactant as possible. Molecular simulations are an attractive alternative to provide additional information on distributions of the amphiphiles at interface and bulk, enhancing our understanding of surfactant interfacial efficiency and effectiveness. Therefore, as well as some experiment method, they have been used here to achieve the above goal. Different from efficiency used experimentally, the parameters interfacial efficiency and the maximal interfacial density have been presented in this part to characterize the properties of surfactants at interface.With a simple coarse-grained model, surfactants adsorption layer at different oil/water interface have been investigated firstly. The results show that surfactant has significant inducing effect on composition of interface, and it is beneficial to decrease interfacial tension if the hydrophobic chains of the surfactant and the oil have similar structure. With dodecane as oil phase, how variations in surfactants structure, concentration, and salinity influence their ability to reduce the interfacial tension has been investigated. An opposite trend has been found between interfacial efficiency and the maximal interfacial density for general surfactants used in EOR nowadays and the approach to solve the problem has been limited to adding salts, co-surfactants or designing novel ones. The results show that appropriate salts make ionic surfactant molecules adsorbed more at interface. Ionic and nonionic mixed surfactants system has also been studied in this part and the synergism mechanism has been discussed. In the last part of the chapter, the general target molecule structure has been confirmed as "Gemini" type and how variations in surfactants head and linker structure influence their interfacial behavior has been investigated.In the second part of this section, the adsorption behavior for surfactants with different structure has been investigated by experiment methods, such as pressure measuring instrument for oil droplets drilling through the capillary, interfacial tension, interfacial rheology and so on. The results show that pressure measurement can be a more scientific method than interfacial tension to characterize the capability of surfactant used in oil recovery. The simulation conclusions are well consistent with the experimental results.2. Structure and properties of foam film stabilized by surfactantFoam is an especial colloidal disperse system engendered by the dispersing of indiscerptible or tinily dissolved gas into liquid solution, which is widely used in petrochemistry, cosmetics, detergent chemistry, mineral floatation, food and so on. Because of its thermodynamic instability, foam stability is crucial for its application. Most current reports on foam are focused on its apparent properties, and the foam stability mechanism is still unclear, especially under dynamic disturbing condition, which is more important for foam applications.Foam stability both in static and dynamic state has been investigated by foam decay measurements and rotation disturbing method respectively. Texture analyzer and AR rheometer are used to study the foam film properties. Furthermore, surfactants arrangement at foam film has been "seen" by MD simulation on a molecule level. How the surfactant structure and the monolayer configuration in foam film be related to its apparent foam properties is analyzed. The results show that it is the hydrophilic group that determines foam static stability. Surfactants with ionic head group are much more stable than nonionic ones. While under disturbing condition, the hydrophobic chain has become curial. The nearly flat orientating of benzene group in SDBS foam films leads to its high elastic modulus, low viscosity modulus when shearing, and poor dynamic stability.Foam drainage is also very important in many actual processes. Electric conductivity method was used to measure the process, which was simple and accurate, and was very sensitive to the interfacial changes, but the drainage process is not clear because of the restrictions of this measurement scale. MD has been used here to investigate the state of water in foam film on a molecule level. Through the radial distribution function (RDF) among the headgroups and water molecules, water molecule in three states has been determined, named as bounded water, trapped water and free water, respectively. New model for drainage process has been proposed as K=K₀+ ae^-x/t. As a supplement to experiment, molecular simulationprovides new insights in understanding of foam systems and guidance for its applications in actual industry.The innovations in the thesis are as follows:1. The parameter such as interfacial density, interfacial efficiency has been used to characterize surfactant adsorption behavior at oil/water interface. Variations factors that influence surfactants tendency to adsorb at interface and ability to reduce the interfacial tension has been investigated.2. The synergistic mechanism between ionic surfactant SDBS and nonionic TX-100 at oil/water interface has been discussed. Compared to single SDBS molecular interfacial layers, which consists of large cavities, the mixed interface are arranged more compactly with TX-100 clusters filled some cavities, thus decrease the interfacial tension to ultra low.3. The study on the stability was extended from apparent static foam stability to dynamics stability under disturbing condition. By combining simulation and experiment method, such as Texture Analyzer which is used to investigate the character of foam films for the first time, foam stable mechanism under different condition has been analyzed.4. New model for drainage process has been put forward base on the description of three water states in foam film by combining full-atom MD simulation and foam conductivity method.