The Behavior of Surfactants in Water / Oil System by Dissipative Particle Dynamics
Doctor of Philosophy
Dissipative Particle Dynamics is a mesoscale simulation model that is widely used in simulation of complex fluids. This method simulates the fluid in the scales between microscopic and macroscopic scales. The scope of this research is to study the influence of different type of surfactants and the phase behavior into the water/oil system by using dissipative particle dynamics simulation. The DPD method shows to be a reliable tool to get a better understanding about the prediction of the phase behavior of surfactants/oil/water system and to estimate the properties of surfactant. The DPD simulation is applicable to support the complicated experiments or to obtain data unavailable from experiment. To avoid expensive experiments, this simulation can be used to predict the properties of surfactants by suggesting promising information about different type of surfactants. In this thesis, the DPD interaction parameters between the beads as an input for DPD simulations are estimated using the COSMO-RS model (Conductor-like Screening Model for Real Solvents) through the Flory-Huggins interaction parameter matching the infinite dilution activity coefficient. The outcomes of this thesis are: 1. Interfacial tension (IFT) study: The DPD simulation was used to study the interfacial phenomena of water/alkanes and surfactant/water/octane systems. The computed interfacial tension of water/alkanes systems agrees very well with the experimental value for all temperatures. For surfactant/water/octane systems, dissipative particle dynamic (DPD) simulations were performed to study the interfacial properties such as interfacial tension, area compressibility, stress profile, and conformation of surfactant at water/octane interface. The IFT results of DPD agree qualitatively with experimental measurement. We perform a series of experimental study on the interfacial tension of surfactants with different structures at the octane/water interface to study the effect of different head group and tail group of surfactants, and the adsorption of Triton X-100 surfactant on water/octane interface. Moreover, the effect of sodium chloride (NaCl), calcium chloride (CaCl2), and temperature for one surfactant concentration and varying concentrations of salt or temperature were investigated on water/octane interface. 2. Critical micelles concentration (CMC) of SDS surfactant study: dissipative particle dynamics to determine the CMC has been used for sodium dodecyl sulfate (SDS) surfactant. The effect of NaCl salt and temperature upon the critical concentration for micelle formation have been investigated. The effects of salt on the CMC values were found to be consistent with experimental data. From DPD results, the CMC increases with increasing the temperature but the experimental values propose a minimum in the CMC around 25 °C. The DPD simulation cannot capture the minimum value as cores grained model. 3. The adsorption of SDS surfactant on carbonate surface study: Various concentration of SDS surfactant in water/ calcium carbonate system are evaluated to determine the amount of adsorption of surfactant to the surface by DPD. Simulation results show that the Freundlich isotherm models were well fitted better than Langmuir model of adsorption on calcium carbonate surface with a good agreement with the experimental fitting parameters. 4. The phase behavior of C8E4 surfactant study: Several DPD calculations were carried out at different simulation conditions with the C8E4 surfactant for water/octane model. From the simulation results, we are able to find the phase transition with changing the fraction volume of oil and temperature. We found five different phase structure depending on the oil/water volume fractions and temperature: spherical (inverse spherical) micelles, cylindrical (inverse cylindrical), hexagonal (inverse hexagonal), lamellar, and bicontinuous phase.