Nuclear Magnetic Relaxation in Micellar Solution and Lyotropic Liquid Crystals. Tools and Applications

University dissertation from Stockholm : Kemi

Abstract: Structurally, this Thesis is divided into two parts thatdescribe (i) the hardware development for NMR relaxationstudies and (ii) its applications for studies of the surfactantsystems.The developed hardware solutions allow to expand thefrequency range covered by conventional NMR spectrometers tomake NMR experiments feasible at resonance frequencies down to100 kHz. The newly designed parts optimize the system recoveryand signal-to-noise ratio at low frequencies. The stability ofthe NMR system required for NMR relaxation experiment at lowmagnetic fields was improved after modification of the "lock"system used with the electromagnet.The applications of field-dependent NMR relaxation concernfour systems which presents questions about different aspectsof the aggregation phenomena. The micellar growth observed forthe decylammonium chloride/water system was studied andinterpreted using free Brownian diffusion model up to theconcentration (28wt%) where it became inconsistent with theexperimental data. To improve the fitting results the dynamicmodel was suitably modified to account for new featuresappeared in concentrated micellar solutions. Thecetyltrimethylammonium bromide/water system exhibits complexgrowth behavior upon benzene solubilization. To characterizethe microstructural transformations the field-dependentrelaxation data of both the surfactant and benzene moleculeswere used. The obtained consistency of the model parametersfrom the fitting to those two relaxation data sets allowed usto trace a transformation path of a micelle into amicroemulsion droplet. Perfluoroactonoate micelles revealmarkedly different internal structure from that of knownprotonated analogues, which were studied by 13C relaxationdispersion. Information about local order and dynamics offluorinated aliphatic chains were obtained. The projection ofcubic symmetry on the restricted rotational diffusion of themicelles was explored studying the structure of a micellarcubic I1liquid crystalline phase. The obtained dynamicparameters, however, were not sufficient to clarify explicitlythe structure of this phase.Generally, the studied systems show featureless isotropicNMR lineshapes and therefore neither symmetry selection of thespectral densities nor diversity of NMR relaxation experimentsthat probe different combinations of spectral densities can beapplied. For these systems the only way to extract thedynamical and structural information is to use NMR relaxationdispersion (besides other physical methods). The usage of thistool was emphasized and may serve as a common denominator for.The strengths and the limitations of the method were furtherinvestigated and clarified.Keywords: NMR, NMR relaxation dispersion, NMR spectrometer,low NMR frequencies, NMR duplexer, NMR preamplifier, NMR lock,surfactant systems, micellar solutions, lyotropic liquidcrystals, cubic mesophase.

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