Theoretical Studies On The Behavior Of Surfactants At Complex Interface And Bulk Phase

Surfactants are amphiphilic substances due to its hydrophilic headgroups and hydrophobic tails. Thus, surfactants exhibit many unique characters, such as aggregate in the bulk water and array orderly at the interface spontaneously, which is defined as self-assembly. At the interface, the surfactants can form ordered adsorption structures, which can reduce the interfacial tension, improve the capillary number, and change the wetting properties, etc. In the bulk liquid, the self-assembly can induce to form special systems. It’s vital to learn more about the properties of the surfactants in the industrial fields.In the enhanced oil recovery (EOR), the surfactants are often added to the drive liquid to enhance the oil recovery efficiency. Besides to reduce the oil/water interface tension, the surfactants can decrease the adsorption loss of crude oil, increase the solubility of oil, reduce the viscosity of crude oil, and enhance the oil recovery. Because of the special nature of the amphiphilic surfactants, in the process of oil recovery, the surfactants exist in the gas/liquid interface, liquid/liquid interface, solid/liquid interface and the bulk phase. It is desired to understand the surfactants deeply due they are vital in the industrial application. Molecular dynamics simulation is the useful tool to learn the microscopic characters of the surfactants. Furthermore, molecular dynamics simulation can greatly reduce the experimental costs and provide useful suggestions on the theoretical level.Molecular simulations are used to study the behavior of the surfactants in the reservoir in this paper. Our work can be divided into two parts:First, we use the steered molecule dynamics to mimic the surfactants at the gas/liquid interface. The foam films are on the behalf of the gas/liquid system. We discuss in detail about the mechanism of the foam rupture and the vital factors to impact the foam stability. Through the simulations, we can learn better about how to control the foam volume, enhance the foam stability, and reduce the industry cost. Second, we use the conventional molecular dynamics to mimic the characters of surfactants at the oil/water interface. The behaviors of surface-active substances of the crude oil in the bulk and at the oil/water interface are discussed in details. The phenomena about the self-assembly and the influences of the structures of the surfactants to the self-assembly are studied. The researches provide suggestions on how to construct the reasonable flooding systems and create the suitable flooding environment. In addition, we also use the fluorescent probe to study the internal structure of the surfactant micelles. We focus on the location of the probe in the micelle and discuss in details about the effects of the probe concentration to the micelle, which provide the supplements to the experiments.The main contents and innovations of this paper are summarized as follows:1. Steered molecular dynamics study the process of foam rupture, which reveals the behavior of the surfactants in this process and the relative elements about the foam stability, and provide the theoretical guidance to control the foam volume reasonable.(1) Bridge-stretching mechanism is reasonable to explain the process of the foam rupture. In that mechanism, the antifoams will enter into the foam films to form an oil-bridge in the middle of the foam films. With the influence of the external factors, the oil-bridge is stretched in the radial direction. Finally, the oil-bridge ruptures. In this process, we focus on the details about the pseudoemulsion film, which is formed when the antifoams close to the foam films. The surfactants in the foam films will form H-bonds with water molecules. These water molecules are named as bounded water, which plays important role to the pseudoemulsion film. When the external force is applied to the foam films, the bounded water move to the bulk water gradually, which results the disappearance of the pseudoemulsion film. After the disappearance of the pseudoemulsion film, a hydrophobic oil-bridge is formed in the middle of the foam films. The stability of the oil-bridge is also vital to the foam films. When the oil-bridge is stretched, its center will become thin, and rupture eventually. This process is depended on the stretching force and the characters of the oil molecules.(2) On the basis of bridge-stretching mechanism, we investigate the reason of foam boosters to increase the foam stability. In this part, we take the betaine as the foam booster, which is commonly used in the industry application. In the presence of betaine, the structures of the foam films are significant affected. Betaine is the zwitterionic surfactant, therefore, betaine can reduce the repulsion of the foam surfactants, and they can interact with each other to form the stability structure. In addition, we study the influence of the concentration of betaine to the foam stability. With the concentration increasing, the foam stability is enhanced, and it’s difficult for antifoams to enter the foam films.2. Molecular dynamics are used to investigate the effect of the headgroups to the surface-active substance in the crude oil. Neutral and anionic surfactants present different properties in the crude oil and at the oil-water interface. Through the simulation, we can learn more about the "structures can influence the natures".Asphaltene molecules are the vital parts of the crude oil, which are also surface-active substances. The neutral asphaltene is easily deprotonated to be anionic asphaltene. Due to the asphaltene molecules have polyaromatic rings, they can form aggregations in the bulk phase. However, the anionic asphaltene molecules will repulse each other, thus, they gather loosely. Because of the anionic headgroups can form H-bonds with the water molecules, the anionic asphaltene molecules can accumulate at the oil/water interface easily. The neutral asphaltene molecules aggregate compactly and they cann’t form H-bonds with the water molecules, therefore, the neutral asphaltene molecules stay in the bulk phase stability during the simulation.3. Molecular dynamics mimic the solubilization of pyrene in the cetyltrimethylammonium bromide micelle. Through the simulation, the solubilization process is obtained intuitively at the molecular level, which provides the supplement to the experiments. When one pyrene molecule is in the micellar solution, it mainly locates in the interior cavity or the palisade layer of the micelle. Furthermore, we study the influence of the concentration of probe to the solubilization. When two probe molecules are in the micelle, they can form the n-n conjugated structure. However, during the most of the simulation time, they are separated with each other and they mainly exist in the palisade layer. The simulation provides the vital information to the experiments, which is difficult obtained through the experimental technology. And the results are helpful to understand the characters of the amphiphilic surfactants.