Pedro J. Sebastião

Proton NMR relaxation study on the nematic-nematic phase transition in A131 liquid crystal.

A study of the proton NMR spin-lattice relaxation time, T(1), of the A131 liquid crystal compound as a function of temperature and Larmor frequency, using a combination of fast field-cycling and standard NMR techniques, is presented. The frequency dispersion in a wide range (from 10 kHz to 300 MHz) at different temperatures and the temperature variation of T(1), in several frequency conditions, were analyzed considering the contributions of the molecular movements generally detected in liquid crystals. In the case of nematic phases of calamitic liquid crystals, the nuclear spin relaxation is dominated by collective movements and local molecular reorientations. The experimental results clearly show a transition within the nematic range of this compound, previously identified as one from the uniaxial to the biaxial phase. This transition can be associated with a slowing down of the molecular rotations around the long molecular axis, where the preferred orientation defines the principal director as detected in the T(1) dispersion analysis.

NMR molecular dynamics study of chromonic liquid crystals Edicol Sunset Yellow doped with salts

We investigate the effect of monoatomic salts on the molecular dynamics in the nematic and isotropic phases formed by the chromonic liquid crystal Edicol Sunset Yellow. The study was carried out using proton nuclear magnetic resonance relaxometry. To analyse the effect of incorporation of additional sodium chloride or lithium chloride on the solutions’ molecular dynamics, the spin‐lattice relaxation time was measured for Larmor frequencies between 10kHz and 100MHz. In the nematic phase, the presence of additional sodium or lithium ions seems to contribute to an increase of the rotations/reorientations correlation times in comparison with the mixture without extra ions. The collective motions detected by proton NMR relaxometry are associated with collective fluctuations of molecules within the stacks in the nematic phase and with order parameter fluctuations in the isotropic phase. Copyright

Molecular order and dynamics of water in hybrid cellulose acetate–silica asymmetric membranes

ABSTRACT In this work 2H NMR spectroscopy and 1H NMR relaxometry and diffusometry were used to characterise water order and dynamics in cellulose acetate/silica asymmetric membranes. Two hydrated membranes were characterised allowing the identification of extra ordering of the water molecules and the presence in each membrane of up to two spectral components with different degrees of order and different values. The mechanism behind this order increase was ascribed to the rapid exchange of the water molecules between the pore walls and its interior. relaxometry dispersions allowed for the identification of the relevant mechanisms of pore-confined water motion, with rotations mediated by translational displacements (RMTD) as the dominant mechanism in the low frequency region. Using the RMTD low cut off frequency along with the in situ directly measured diffusion constant it was possible do determine characteristic lengths of correlated water motion in both membranes studied, which fall in ranges compatible with typical pore dimensions in similar membranes. GRAPHICAL ABSTRACT

Dynamics of binary mixtures of an ionic liquid and ethanol by NMR

A study of molecular dynamics of the ionic liquid 1‐ethyl‐3‐methylimidazolium bis(trifluoro‐methylsulphonyl)imide ([Emim][Tf2N]) in solution with deuterated ethanol at different molar concentration and temperatures is presented. The study was performed using 1H and 2H nuclear magnetic relaxation and 2H 1D spectroscopy. The temperature dependence of the spin–lattice relaxation time T1 of the cations allows the evaluation of the activation energies of the rotational degree of freedom of these molecules. The viscosity in the binary system increases with the concentration of ionic liquid. However, the activation energy in the cation molecules decreases when the concentration of the ionic liquid increases, indicating that the rotational dynamics is facilitated. This behavior is explained from the fact that the presence of the ionic liquid in the system disrupts the degree of intermediate range order expected in the alcohol system. Copyright

Magnetic Flux Density Distribution in the Air Gap of a Ferromagnetic Core With Superconducting Blocks: Three-Dimensional Analysis and Experimental NMR Results

The design of magnetic cores can be carried out by taking into account the optimization of different parameters in accordance with the application requirements. Considering the specifications of the fast field cycling nuclear magnetic resonance (FFC-NMR) technique, the magnetic flux density distribution, at the sample insertion volume, is one of the core parameters that needs to be evaluated. Recently, it has been shown that the FFC-NMR magnets can be built on the basis of solenoid coils with ferromagnetic cores. Since this type of apparatus requires magnets with high magnetic flux density uniformity, a new type of magnet using a ferromagnetic core, copper coils, and superconducting blocks was designed with improved magnetic flux density distribution. In this paper, the designing aspects of the magnet are described and discussed with emphasis on the improvement of the magnetic flux density homogeneity (ΔB/B0) in the air gap. The magnetic flux density distribution is analyzed based on 3-D simulations and NMR experimental results.

FFC NMR relaxometers on education: Topologies, control techniques and electromagnetic devices

The Fast Field Cycling (FFC) Nuclear Magnetic Resonance (NMR) equipment has been mainly developed by engineers with a strong background in power electronics, control and physics. This technique has been widely used by physicists, chemists, biologists, pharmacists and food analysts. During the last decades, the development of this type of apparatus has been taking advantage of the power semiconductors, topologies of the power electronic converters, control techniques, computational tools and materials, among other aspects. In this paper, teaching aspects of using this type of equipment and technique in courses of physics and electrical engineering is described.