Studies On Molecular Dynamics In Aqueous Solution Of Superparamagnetic Iron Oxide Nanoparticales And Surfactants By Proton NMR Transverse Relaxation Dispersion

This thesis first introduced briefly the principles and methods of transverserelaxation dispersion, a term used in nuclear magnetic resonance (NMR)spectroscopyreferring to the fact that the presence of self-diffusion and/or chemical exchange willreduce the spin-spin relaxation time T₂ of related spins measured by the CPMGsequence. The method was then applied to the aqueous solution of superparamagneticiron oxide (SPIO), surfactant CTAB and surfactant 12-4-12[dimer of thealkanediyl-α-ω-bis(dodecyldimethylam-monium bromide)] to study the dynamics ofwater molecules and surfactant molecules/micelles in these systems.In the first experiment, the transverse relaxation dispersion curves of waterprotons in the aqueous solution of SPIO were measured under different concentrations.A diffusion model by Jensen and Chandra was used to fit the experimental data, andto calculate the size of SPIO particles and field inhomogeneity induced by SPIOparticles.In the second experiment, the transverse relaxation dispersion curves of waterprotons were measured in the aqueous solution of surfactants CTAB and 12-4-12 ofdifferent concentrations, respectively. Luz-Meiboom's two-site chemical exchangemodel was used to fit the experimental data and to calculate the chemical exchangerate (Ï„_ex) of the water protons. The result shows thatÏ„_ex of water protons decreased asthe concentrations of surfactants increased. The amplitudes ofÏ„_ex decreases werefound to be largest at the critical micelle concentrations (CMC).In the third experiment, the transverse relaxation dispersion curves of methylprotons in the hydrophilic group (N-CH₃) of the surfactants were measured in aqueoussolution of CTAB or 12-4-12 of different concentrations, respectively.Luz-Meiboom's two-site chemical exchange model was used to fit the experimentaldata and to calculate the chemical exchange rate of the hydrophilic methyl protons.The result shows that the calculated chemical exchange rate for the hydrophilicmethyl group of CTAB, at concentrations higher than CMC, probably reflected the slow exchange process in solution associated with the formation and dissociation ofmicelles. On the other hand, the calculated chemical exchange rate for the hydrophilicmethyl group of 12-4-12, at concentrations higher than CMC, probably describes thefast exchange process between free 12-4-12 molecules in the solution and those in themicelles.