On the deformability of additivated phosphatidylcholine liposomes: Molecular dynamic regimes and membrane elasticity.

Liposomes with enhanced elasticity have been proven to increase the efficiency of drug transport across the skin. The understanding of the background physicochemical processes driving the liposome viscoelastic properties is an essential feature for the design of effective formulations involving different lipids and additive molecules. In this work we use field-cycled nuclear magnetic resonance relaxometry to analyze both the mechanical properties of liposome membranes, and their relationship with the involved molecular dynamics. Different liposomal formulations were considered. We show a correlation between the molecular dynamical regime and mesoscopic physical parameters that define the expected deformability of the vesicles. Results strongly suggest that the purity of the used lipids may influence the elastic properties of the membranes in an appreciable way. Common features in the behaviour of the involved dynamic variables were identified by comparing formulations with surfactants of similar molecular weight.

Air core notch-coil magnet with variable geometry for fast-field-cycling NMR.

In this manuscript we present details on the optimization, construction and performance of a wide-bore (71 mm) α-helical-cut notch-coil magnet with variable geometry for fast-field-cycling NMR. In addition to the usual requirements for this kind of magnets (high field-to-power ratio, good magnetic field homogeneity, low inductance and resistance values) a tunable homogeneity and a more uniform heat dissipation along the magnet body are considered. The presented magnet consists of only one machined metallic cylinder combined with two external movable pieces. The optimal configuration is calculated through an evaluation of the magnetic flux density within the entire volume of interest. The magnet has a field-to-current constant of 0.728 mT/A, allowing to switch from zero to 0.125 T in less than 3 ms without energy storage assistance. For a cylindrical sample volume of 35 cm(3) the effective magnet homogeneity is lower than 130 ppm.

On the role of collective and local molecular fluctuations in the relaxation of proton intrapair dipolar order in nematic 5CB.

We investigate the role that local motions and slow cooperative fluctuations have on the relaxation of the intrapair dipolar order in the nematic 5CB. With this purpose we present a theoretical and experimental systematic study which allow us to quantify the contribution from each type of molecular fluctuation to the intrapair dipolar order relaxation time, T(1D). The experimental work includes measurements of Zeeman and intrapair dipolar order relaxation times (T(1Z) and T(1D)) as a function of temperature at conventional NMR frequencies, in three complementary samples: normal and chain deuterated 4-n-pentyl-4(')-cyanobiphenyl (5CB and 5CB(d11)) and a mixture of normal 5CB and fully deuterated 4-n-pentyl-4'-cyanobiphenyl (5CB(d19)), 50% in weight. Additionally we perform T(1Z) field-cycling Larmor frequency-dependent measurements to obtain the spectral density of the cooperative fluctuations. The obtained results are as follows. (a) The cooperative molecular fluctuations have a strong relative weight in the relaxation of the intrapair dipolar order state, even at Larmor frequencies in the range of conventional NMR. (b) Alkyl chain rotations are an important relaxation mechanism of the intrapair dipolar order at megahertz frequencies. (c) Intermolecular fluctuations mediated by translational self-diffusion of the molecules is not an efficient mechanism of relaxation of the intrapair dipolar order.

Proton field-cycling nuclear magnetic resonance relaxometry in the smectic A mesophase of thermotropic cyanobiphenyls: Effects of sonication.

Proton field-cycling nuclear magnetic resonance relaxometry is used to study the spin-lattice relaxation dispersion of selected standard smectic A liquid crystals at different temperatures. Relaxation features at both, in the presence and absence of a monochromatic ultrasonic field are considered. We show that the laboratory-frame spin-lattice relaxation time is mainly governed by translational diffusion. Order director fluctuations (ODF) are less important while rotational diffusion seems to be only relevant near the clearing point. Our study suggests that sonication enhances the ODF contribution in the SmA mesophase. Within the framework of the approach we have outlined, different features associated with the ODF mechanism can be investigated.

Spin-lattice dispersion in nematic and smectic-A mesophases in the presence of ultrasonic waves: a theoretical approach.

We present a theoretical study of the Larmor frequency dependence of the nuclear spin-lattice relaxation caused by order director fluctuations for both nematic and smectic-A mesophases. The analysis is focused on the case where the molecular system is subjected to sonication during the relaxation process. The departure from the nonsonicated case is discussed for various values of the involved parameters. Two different approaches are discussed for the smectic case.

Apparent low-field spin-lattice dispersion in the smectic-A mesophase of thermotropic cyanobiphenyls.

Proton field-cycling spin-lattice relaxometry T1 of the smectic-A mesophase in cyanobiphenyls revealed the presence of steep dispersions in the low-frequency regime. We clearly show that the strong dispersion characteristic of smectic organizations cannot be attributed to the collective molecular dynamics (order director fluctuations), as it is usually interpreted. We present two independent experimental evidences: the dependence of the dispersion with the slew rate of the magnetic field cycle and the dependence of the dispersion with the presence and power of an ultrasonic field.

Low-frequency molecular dynamics studied by spin-lock field cycling imaging.

Spin-lock adiabatic field cycling imaging (SLOAFI) relaxometry was shown to be a useful technique for obtaining a fast study of spin-lattice relaxation dispersion in the rotating frame. The aim of the present article is to describe some technical aspects of the experiment in more detail, while showing simple examples that can be compared with laboratory frame relaxation. We also present here a general discussion of the equations for an off-resonance experiment used to analyze low-frequency molecular dynamics.