The Phase Structure and Rheological Properties of Surfactant Solutions via Dissipative Particle Dynamics

This thesis presents a study of anionic surfactant solutions via experimental and simulation methods. The surfactants studied in this work are some of the most common ionic surfactants used in consumer products. Therefore understanding and predicting their behaviour, as a function of concentration, is crucial to the manufacturing of these products. Surfactant solutions can take the form of different `mesophases' depending on the concentration, which all show widely different properties as a result of molecular self-assembly in solution. The simulation methods used in this work contribute to understanding the behaviour of fluids on the macro-scale, by simulating the individual molecules in solution. Experimental analysis in this work makes use of techniques such as polarised optical microscopy (POM), rheological measurements, Raman spectroscopy and dynamic light scattering (DLS). These experiments were performed not only to establish the phase behaviour of the surfactant solutions, but also to uncover aspects of the structure at different concentrations. These results are then compared with simulations which are performed for similar systems. For example, Raman spectroscopy is used to show that the conformation of molecules is influenced by structural changes within the fluid, which is later found to be reproducible using simulation. The simulation technique of dissipative particle dynamics (DPD) is used in this work for studying the equilibrium phase behaviour of solutions at room temperature. Following an establishment of the equilibrium behaviour, a study is performed investigating the effect of the application of shear to these solutions. From these simulations we can also calculate a viscosity vs. shear rate profile for comparison with experimental results. A small selection of equilibrium molecular dynamics (MD) simulations are performed, in order to demonstrate that the simplifications made in performing DPD simulations vs. MD simulations have minimal impact on the final results.