Lipid-detergent systems are interesting to study, as the two amphiphiles have very different spontaneous curvature, however readily form mixed micelles in solution. These micelles can be shorter cylindrical micelles or long worm-like micelles. For such a system the size of the micelles varies strongly with solute conditions, being dependent on the total amount of amphiphile in solution, as well as, the lipid to detergent ratio in the micelles. Although the broad phase behaviour of such systems has been studied and is relatively well understood, there are still many open questions remaining. Some of the questions that motivated the work presented are: how the length and composition of the micelles varies within the micellar region, and how the micelles grow? The biologically interesting system under study is lecithin and bile salt, where the equilibrium sizes were experimentally determined for different samples within the micellar region. A model, combining the length of the micelles, with the concentrations of lecithin and bile salt in the system is presented, and is used to calculate the composition of the micelles at equilibrium. The kinetics of the growth of the micelles after a pertubation causing a shift in the equilibrium size has not been studied in detail before. The kinetics of the system are studied using a stopped flow setup, which I specifically designed for neutron scattering experiments. The stopped flow setup allows for the measurement to start 200ms after the initial mixing of the two liquids, after dilution of a solution the micelles relax to a new longer length. The kinetics of this relaxation were studied as a function of the initial and final size of the micelles, as well as, the ionic strength of the solution. The micelles were found to grow through coalescence, where the rate of growth seems to be constant for different sizes of micelles and the time taken for the relaxation depends on the difference between the final and initial lengths of the micelles. The rate of growth is strongly influenced through changing the ionic strength of the solution, indicating the importance of an electrostatic barrier to the fusion of micelles.

Viscoelastic surfactant have found numerous application in the oil fields as fracturing and matrix acidizing fluid additives in the recent years. They have the ability to form long worm-like micelles with the increase in pH and calcium concentration, which results in increasing the viscosity and elasticity of partially spent acids. On one hand, concentration of surfactant in the fluids has profound effects on their performance downhole. Additionally, there is continuous debate in the industry on whether the gel generated by these surfactants causes formation damage, especially in dry gas wells. Therefore, being able to analyze the concentration of these surfactants in both live and spent acids is of great importance for production engineers who apply surfactant-based fluids in the oil fields. In the present work, a two-phase titration method was optimized for quantitative analysis of a carboxybetaine viscoelastic surfactant, and surfactant retention in calcite cores was quantitatively determined by two phase titration method and the benefits of using mutual solvents to break the surfactant gel formed inside the cores was assessed. On the other hand, high temperatures and low pH are usually involved in surfactant applications. Surfactants are subjected to hydrolysis under such conditions due to the existence of a peptide bond (-CO-NH-) in their molecules, leading to alteration in the rheological properties of the acid. The impact of hydrolysis at high temperatures on the apparent viscosity of carboxybetaine viscoelastic surfactant-based acids was evaluated in the present study, and the mechanism of viscosity changes was determine by molecular dynamics (MD) simulations. Our results indicate that, first, significant amount of surfactant has been retained in the carbonate matrix after acidizing treatment and there is a need to use internal breakers when surfactant-based acids are used in dry gas wells or water injectors. Second, hydrolysis at high temperatures has great impact on surfactant-acid rheological properties. Short time viscosity build-up and effective gel break-down can be achieved if surfactant-acid treatments are carefully designed; otherwise, unexpected viscosity reduction and phase separation may occur, which will affect the outcome of acid treatments.

No description available.