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Projeto de investigação
Study of NMR superoaramagnetic relaxation effects in magnetic ionic liquids
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Tuning the 1H NMR Paramagnetic Relaxation Enhancement and Local Order of [Aliquat]+-Based Systems Mixed with DMSO
Publication . Cordeiro, Rui; Beira, Maria J.; Cruz, Carlos; Figueirinhas, João L; Corvo, Marta C; Almeida, Pedro L; Rosatella, Andreia A.; Afonso, Carlos A. M.; Daniel, Carla I.; Sebastião, Pedro J.; CENIMAT-i3N - Centro de Investigação de Materiais (Lab. Associado I3N); LAQV@REQUIMTE; DQ - Departamento de Química; MDPI - Multidisciplinary Digital Publishing Institute
Understanding the behavior of a chemical compound at a molecular level is fundamental, not only to explain its macroscopic properties, but also to enable the control and optimization of these properties. The present work aims to characterize a set of systems based on the ionic liquids [Aliquat][Cl] and [Aliquat][FeCl4] and on mixtures of these with different concentrations of DMSO by means of 1H NMR relaxometry, diffusometry and X-ray diffractometry. Without DMSO, the compounds reveal locally ordered domains, which are large enough to induce order fluctuation as a significant relaxation pathway, and present paramagnetic relaxation enhancement for the [Aliquat][Cl] and [Aliquat][FeCl4] mixture. The addition of DMSO provides a way of tuning both the local order of these systems and the relaxation enhancement produced by the tetrachloroferrate anion. Very small DMSO volume concentrations (at least up to 1%) lead to enhanced paramagnetic relaxation without compromising the locally ordered domains. Larger DMSO concentrations gradually destroy these domains and reduce the effect of paramagnetic relaxation, while solvating the ions present in the mixtures. The paramagnetic relaxation was explained as a correlated combination of inner and outer-sphere mechanisms, in line with the size and structure differences between cation and anion. This study presents a robust method of characterizing paramagnetic ionic systems and obtaining a consistent analysis for a large set of samples having different co-solvent concentrations.
Water Molecular Dynamics in the Porous Structures of Ultrafiltration/Nanofiltration Asymmetric Cellulose Acetate-Silica Membranes
Publication . Cunha, João; da Silva, Miguel P.; Beira, Maria J.; Corvo, Marta C.; Almeida, Pedro L.; Sebastião, Pedro José; Figueirinhas, João L.; de Pinho, Maria Norberta; CENIMAT-i3N - Centro de Investigação de Materiais (Lab. Associado I3N); MDPI AG
This study presents the characterization of water dynamics in cellulose acetate-silica asymmetric membranes with very different pore structures that are associated with a wide range of selective transport properties of ultrafiltration (UF) and nanofiltration (NF). By combining 1H NMR spectroscopy, diffusometry and relaxometry and considering that the spin-lattice relaxation rate of the studied systems is mainly determined by translational diffusion, individual rotations and rotations mediated by translational displacements, it was possible to assess the influence of the porous matrix's confinement on the degree of water ordering and dynamics and to correlate this with UF/NF permeation characteristics. In fact, the less permeable membranes, CA/SiO2-22, characterized by smaller pores induce significant orientational order to the water molecules close to/interacting with the membrane matrix's interface. Conversely, the model fitting analysis of the relaxometry results obtained for the more permeable sets of membranes, CA/SiO2-30 and CA/SiO2-34, did not evidence surface-induced orientational order, which might be explained by the reduced surface-to-volume ratio of the pores and consequent loss of sensitivity to the signal of surface-bound water. Comparing the findings with those of previous studies, it is clear that the fraction of more confined water molecules in the CA/SiO2-22-G20, CA/SiO2-30-G20 and CA/SiO2-34-G20 membranes of 0.83, 0.24 and 0.35, respectively, is in agreement with the obtained diffusion coefficients as well as with the pore sizes and hydraulic permeabilities of 3.5, 38 and 81 kg h-1 m-2 bar-1, respectively, reported in the literature. It was also possible to conclude that the post-treatment of the membranes with Triton X-100 surfactants produced no significant structural changes but increased the hydrophobic character of the surface, leading to higher diffusion coefficients, especially for systems associated with average smaller pore dimensions. Altogether, these findings evidence the potential of combining complementary NMR techniques to indirectly study hydrated asymmetric porous media, assess the influence of drying post-treatments on hybrid CA/SiO2 membrane' surface characteristics and discriminate between ultra- and nano-filtration membrane systems.
Dynamical processes and order behind the pronounced 1H NMR paramagnetic relaxation enhancement induced in [P6,6,6,14]3[GdCl6]-based ionic liquid mixtures
Publication . Beira, Maria J.; Silva, Gonçalo M.C.; Eusébio, Tiago M.; Figueirinhas, João L.; Cordeiro, Rui; Cruz, Carlos; Corvo, Marta C.; Almeida, Pedro L.; Rosatella, Andreia A.; Afonso, Carlos A.M.; Filipe, Eduardo J.M.; Parella, Teodor; Cabrita, Eurico J.; Sebastião, Pedro J.; CENIMAT-i3N - Centro de Investigação de Materiais (Lab. Associado I3N); DCM - Departamento de Ciência dos Materiais; Faculdade de Ciências e Tecnologia (FCT); UCIBIO - Applied Molecular Biosciences Unit; DQ - Departamento de Química; Elsevier
NMR relaxometry is a technique that allows the molecular dynamics study of chemical compounds across a broad time scale, ranging from slow translational diffusion or collective motions to fast rotations. The present work is a 1H NMR relaxometry and diffusometry study of systems based on [P6,6,6,14][Cl] and [P6,6,6,14]3[GdCl6] ionic liquids, complemented by X-ray diffractometry measurements. The use of the X-ray profiles enabled the determination of an almost temperature independent interdigitated disposition of the cations that is at the origin of local order fluctuations. This structure also affects the paramagnetic relaxation enhancement, which is very significant for these systems and is achieved at very low metal concentrations (around 1mM). The present study provides a comprehensive analysis that is consistent for all the analyzed systems and across the different experimental techniques, despite the experienced technical challenges related to extremely short relaxation times. Additionally, the Electrochemical Impedance Spectroscopy profiles of the neat [P6,6,6,14][Cl] sample were explained by an analogous equivalent circuit model that allowed for a global analysis consistent with the diffusometry and X-ray diffractometry results. The representation of the real and imaginary parts of the impedance allowed for a visual deconvolution of the contributions of the different circuit blocks considered in the model.
Observing short-range orientational order in small-molecule liquids
Publication . Gradišek, Anton; Apih, Tomaž; Beira, Maria J.; Cruz, Carlos; Fernandes, Susete N.; Godinho, Helena M.; Sebastião, Pedro J.; DCM - Departamento de Ciência dos Materiais; CENIMAT-i3N - Centro de Investigação de Materiais (Lab. Associado I3N); Nature Publishing Group
Local molecular ordering in liquids has attracted a lot of interest from researchers investigating crystallization, but is still poorly understood on the molecular scale. Classical nucleation theory (CNT), a macroscopic thermodynamic description of condensation, has shortcomings when dealing with clusters consisting of tens of molecules. Cluster formation and local order fluctuations in liquid media are difficult to study due to the limited spatial resolution of electron- and photon-imaging methods. We used NMR relaxometry to demonstrate the existence of dynamic clusters with short-range orientational order in nominally isotropic liquids consisting of elongated molecules. We observed clusters in liquids where the local ordering is driven by polar, steric, and hydrogen-bond interactions between the molecules. In the case of a liquid crystal, measuring the local orientational order fluctuations allowed us to observe the size of these clusters diverging when approaching the phase transition from the isotropic to the nematic phase. These fluctuations are described in terms of rotational elasticity as a consequence of the correlated reorientations of the neighbouring molecules. Our quantitative observations of the dynamic clusters in liquids, numbering about ten or fewer molecules, indicate that this is a general phenomenon in various types of liquids.
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Fundação para a Ciência e a Tecnologia
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OE
Número da atribuição
PD/BD/142858/2018