NMR Diffusion Studies of Association in Surfactant Systems Inclusion Complexes, Micellar Solutions and Microemulsions

University dissertation from Lund University (Media-Tryck)

Abstract: The aim of this thesis has been to investigate self-diffusion behavior of host-guest complex, microemulsion and polymer solution. Pulsed Field Gradient NMR (PFG-NMR) was shown to provide detailed molecular information in all system studied. The general strategy was to measyre the diffusivities of the species and from there get molecular insight into the aggregation and dynamics of the system. Guest host interaction has been studied between β-cyclodextrin (β-CD) and different cationic surfactant. The equilibrium constant (K11) increases as a function of the number of CH2 groups in the surfactant chain. It could be concluded that the cavity of β-CD could incorporate approximately 10-12 CH2 groups and that it is the hydrophobic interaction that mainly is responsible for the inclusion complex formed. The interaction between the gemini surfactant and β-CD formed a 2:1 (CD:gemini) complex in a two step mechanism with the first association constant higher than the second one (K2:1), but both relatively small in comparison with the analogue singled tailed surfactant. The values of the K11 and K21 increased with gemini spacer length which indicate that the available space on the gemini molecule is important. The second association constant shows no cooperatively and its magnitude are discussed in terms of steric constrains. Bolaform surfactant complexation was investigated with both β-CD and α-CD with Isothermal Titration Calorimetry (ITC). The K11, enthalpy and entropy of formation were obtained. From ITC a molecular interpretation is made explaining the enthalpy and entropy difference between α- and β-CD. In a second study diblock copolymers of the type methoxy poly(ethylene oxide)- poly(alkylene oxide) were incorporated into the nonionic bicontinuous microemulsion at 298 K. An enhancement of the microemulsion in solubilising water and oil were observed with a boosting factor of 9. The self-diffusion coefficients of the components in the system were measured and provided molecular information on how the microstructure of the bicontinuous microemulsion changed upon addition of the polymers. The diffusion study concerning the aqueous solution of the synthetic polymers, PEO- PPO-PEO, is focused towards analyzing the PFG-NMR decay correctly. The polydispersity has consequences on the polymer self-assembly, which were highlighted, modeled and discussed based on PFG-NMR data. The main conclusion is that the curved NMR decay as obtained for self-assembly can be explained with a multi-component ideal solution.