title: Nanoscale Responsive Mechanisms of a Carbon Dioxide-Responsive Surfactant in an Oil-Water-Surfactant System: Molecular Dynamics Simulation Studies
Creators: name: Lei, Xuantong
Publisher: name: Schulich School of Engineering
Year: 2024
Subjects: subject: Carbon Dioxide-Responsive Surfactants
subject: Molecular Dynamics Simulation
subject: Oil-Water Interface
subject: Chemical Enhanced Oil Recovery
subject: Engineering--Chemical

description: In petroleum and chemical engineering, mixtures of oil, water, and other chemicals are commonly encountered in various applications, such as oil extraction, transportation, and refining. Surfactants are widely used in these applications for their ability to stabilize dispersion systems, including foams, emulsions, and suspensions. However, the impact of surfactants as stabilizers is a double-edged sword, as system destabilization is often required in subsequent processes and typically demands much more energy. Furthermore, the use of surfactants poses significant challenges in product purity control and wastewater disposal, resulting in both economic and environmental issues. Consequently, the search for a novel surfactant that is efficient and eco-friendly with minimal side effects has been a long-standing pursuit in petroleum and chemical engineering. The presence of a CO2-responsive surfactant is considered a promising solution for the petroleum industry, as this material can switch between its interface-active and interface-inactive forms when exposed to external CO2/N2 stimuli. Compared to other stimuli-responsive surfactants, using CO2/N2 as the stimuli has a significant advantage: their by-products do not accumulate in a system, and the external stimuli are easy to be introduced or removed. Because of these superior advantages, CO2-responsive surfactants are currently a focus in many fields, including enhanced oil recovery and oilfield chemistry. However, the CO2-responsiveness of this material raises new questions for the research community. What is the origin of its CO2-responsiveness? How can we regulate its CO2-responsiveness in different environments? Is there a possibility that its CO2-responsiveness could be disabled under specific conditions? To answer these questions, it is essential to thoroughly investigate the physicochemical interactions between a CO2-responsive surfactant and other chemicals. For this purpose, molecular dynamics simulations are considered the most appropriate tool. In this thesis, we employed molecular dynamics simulations to explore the CO2-responsive mechanisms at an atomistic level. Additionally, we utilized various advanced analytical tools of MD simulations to address different topics and investigate related issues comprehensively. The results of this thesis have shown that CO2-responsiveness significantly depends on the molecular interactions between a surfactant and its surrounding molecules. The switching between active and inactive forms is controlled by the deprotonation/protonation of the CO2-responsive surfactant, which alters the distribution of atomic charges and the polarity of molecules. It was discovered that the deprotonation/protonation process could occur at different locations within a system, with deprotonation being more likely at an oil-water interface due to van der Waals interactions with oil molecules, as indicated by free energy perturbation calculations. Additionally, it was found that the molecular interactions between different types of co-surfactants formulated with the CO2-responsive surfactant can modify the CO2-responsive mechanisms of a system. Typically, the protonation process activates a CO2-responsive cationic surfactant and stabilizes the system, but formulating with another anionic surfactant can reverse this effect, causing the protonation process to eventually destabilize the system. Furthermore, other types of molecular interactions, such as amide-π interactions, can impair the CO2-responsiveness of a system due to undesirable conformations of surfactants. Lastly, the curvature of an interface and modeling of the system can either screen or amplify the effects of specific molecular interactions, potentially changing the dominant factors of the CO2-responsive mechanisms. To conclude, this dissertation deepens our understanding of how intermolecular interactions influence the responsive mechanisms of a CO2-responsive surfactant at the nanoscale. The achievements will provide helpful guidance on how to optimize and control these interactions under suitable conditions to maximize the performance of CO2-responsive surfactants, facilitating a mature application of this novel surfactant in various fields of chemical and petroleum engineering.