Tese Doutoramento
Molecular Dynamics in Magnetic Fluids by 1H NMR - Paramagnetic Relaxation Enhancement and Local Order Fluctuations
Maria José Jardim Beira
Abstract:
NMR relaxometry is widely applied in the study of very diverse chemical systems, such as polymers, ionic liquids, liquid crystals and magnetic fluids. The molecular motions that characterize each system, occurring within timescales that range from picoseconds to milliseconds, are encoded in their relaxation dispersion profiles. Such motions generally include rotations/reorientations, as well as translational selfdiffusion, which should, in theory, be the necessary and sufficient contributions to explain the NMRD profiles of non-magnetic, isotropic systems. Regarding liquid crystalline phases, order fluctuations are usually the most effective relaxation mechanism, while for magnetic systems paramagnetic relaxation often dominates the NMR relaxometry profile.
Recent works evidenced local order fluctuation dynamics in isotropic liquids, very far from the glass transition, either due to nano-segregation of polar-apolar domains, revealed by the X-ray profiles, or as a result of marked molecular anisometry. The present work mainly combines 1H NMR relaxometry and diffusometry with X-ray scattering experiments in the study of the molecular dynamics of magnetic and non-magnetic ionic liquids based on the [P6,6,6,14] + and [C8mim]+ cations and on PEG-based magnetic systems.
The data obtained from all these three experimental techniques evidenced the need to consider either order parameter fluctuations or layer undulations in the NMRD model fitting analysis of these systems, despite their apparent isotropic nature. This observation indicates that NMR is sensitive both to the existence of local order, as well as to the characteristics of that order - orientational and positional. The electrical properties of the ionic liquids were additionally assessed via electrochemical impedance spectroscopy, being potentially interesting for dielectric applications, as relative permittivities of about 1000 were obtained.