The Environmental-Responsive Behavior And Mechanism Of Surfactant/Polymer Systems

In this paper, molecular simulation, as well as experimental method, has been used to systematically investigate environmental-response behavior of surfactants/ polymers in enhanced oil recovery on different scale. By combining of experiment and computer simulation, the relationship between molecular structure, apparent properties and the environmental conditions has been established, provided a new path for designing smart Saa/Polymer flooding system by tturning disadvantage conditions to advantage factors for EOR.1. Study of environmental-response behavior of polymers in bulk phaseStimulus-response polymer is a class of water-soluble polymers, their physical conformation or chemical nature change corresponding with the variation of external environment conditions. The potential application of pH-sensitive polymers as a novel in-depth control system in EOR have been proposed recently. Numbers of experimental methods having been used to investigate self-assembly of polymer, such as light scattering, spectroscopy etc., but these methods are restricted by conditions, and can not information directly on molecular level.Dynamics simulation as well as macroscopic rheology experimental method, has been used to systematically investigate the environmental-response behavior and mechanism of PAA. It was found that the conformation and molecular aggregation behavior of PAA responded with pH, inorganic salts. The molecular conformation of polymer was non-swelling in acidic conditions, while swelled abruptly along with solution viscosity increasing in case sodium hydroxide was added into the solution. This feature provided facilitative means for people to control its properties, therefore it is necessary for us to study the response mechanism at the molecular level for expanding its application. The results showed that, hydrogen-bonding interactions of PAA (COOH) long-chain lead to the association of intra- or inter-molecular interaction. The former result in the coiling of the molecular easily, the latter led to the formation of spatial network structure; the electrostatic repulsion interaction between -COO- and -COO- induce the stretch of the molecular chains. When the ionic strength of the solution was increased, it is difficult to form the molecular network, because the interaction between PAA and water become weak. On the other hand, the electrostatic repulsion between PAA molecules is reduced, resulting in decrease of the viscosity of the solution. The simulation results about the phase structure agree very well with the phenomenon observed in experiments, it verified the relationship between the changing of molecular conformation and environmental conditions.2. Study of environment-response behavior of surfactantsDifferent from the previous surfactant research, this section focuses on investigating the response behavior and mechanism of surfactants in bulk or at g/l interface. It was found that sodium stearate molecule form a network structure at very low concentration, and the self-assembly process is affected by concentration, temperature, pH, salts and other conditions. Experimentally, TEM, AFM, DSC and rheology method were used to demonstrate the presence of hydrogels formed by sodium stearate molecule aggregation. The transformation of the self-organization from spherical micelle to rodlike micelle and then to a complex network gel was observed. Using molecular simulation methods the environment-response mechanism has been discussed. Different contribution of molecule interaction to the self-organization and transformation was discussed, which showed that the interaction between surfactants head groups and water plays an important role in the whole process. The environmental-response character of zwitterionic surfactant has also been determined. The electrical properties of surfactant head group change from cationic to anionic with pH values varied from acidic to basic, resulting in different molecular behavior at gas-liquid interface. The pH-response behavior of sodium dodecyl N-diethylamino has been studied, and it was found that the foam stabilized by anionic surfactants has better dynamic stability. The results show that it is the hydrophilic group that determines not only foam static stability but also dynamic stability. Foams stabilized by surfactants with ionic head group are much more stable than that stabilized by nonionic ones.Computer simulation methods provide detailed information about the conformation and aggregation behavior of surfactants and polymer molecule in solution or at interface, and revealed the environment-response nature and mechanism. These results help us understand better about the environmental-response properties and molecular behaviors of Saa and polymer, and provided us a new sight for choosing Saa/polymers for EOR.The innovations in the thesis are as follows.1. Simulations are employed to study the environmental-responsive behavior and mechanism of surfactants/polymers in aqueous solution, integrated with the rheological experimental measurement. environmental factors including pH, temperature, salt etc.. The results might provide impactful direction for the design of smart fluid in practice process.2. On the molecular level, the rheological properties of polymer aqueous solution under shear has been extended to the changes of molecular aggregation. By combining Mesodyn and DPD, the interaction parameters were adjusted carefully to study the change of environmental factors, such as molecular charge, inorganic salts, temperature etc.3. Using molecular simulation method to study and verify the formation and structure of thermo-reversed hydrogel formed by surfactant self-assembly. By combining experimental measurement and molecular simulation results, the mechanisms of effects of environmental conditions on surfactant interface behavior has been revealed.4.Molecular simulation method and experimental methods are combined to investigate the pH-response behavior of N-diethylamino on air/water interface.It is found that it's the hydrophilic group that determines not only foam static stability but also dynamicstability.