Subdiffusive dynamics of a liquid crystal in the isotropic phase

The isotropic phase dynamics of a system of 4-n-hexyl-4'-cyano-biphenyl (6CB) molecules has been studied by molecular dynamics computer simulations. We have explored the range of 275-330 K keeping the system isotropic, although supercooled under its nematic transition temperature. The weak rototranslational coupling allowed us to separately evaluate translational (TDOF) and orientational degrees of freedom (ODOF). Evidences of subdiffusive dynamics, more apparent at the lowest temperatures, are found in translational and orientational dynamics. Mean square displacement as well as self-intermediate center of mass and rotational scattering functions show a plateau, also visible in the orientational correlation function. According to the mode coupling theory (MCT), this plateau is the signature of the beta-relaxation regime. Three-time intermediate scattering functions reveal that the plateau is related to a homogeneous dynamics, more extended in time for the orientational degrees of freedom (up to 1 ns). The time-temperature superposition principle and the factorization property predicted by the idealized version of MCT hold, again for both kinds of dynamics. The temperature dependence of diffusion coefficient and orientational relaxation time is well described by a power law. Critical temperatures Tc are 244+/-6 and 258+/-6 K, respectively, the latter is some 10 K below the corresponding experimental values. The different values of Tc we obtained indicate that ODOF freezes earlier than TDOF. This appears due to the strongly anisotropic environment that surrounds a 6CB molecule, even in the isotropic phase. The lifetime of these "cages," estimated by time dependent conditional probability functions, is strongly temperature dependent, ranging from some hundreds of picoseconds at 320 K to a few nanoseconds at 275 K.     

Serial coupled columns reversed-phase separations in high-performance liquid chromatography. Tool for analysis of complex real samples

High-performance liquid chromatography is considered as the selected analytical tool for a huge number of applications, including the carotenoid analysis. However, due to the great complexity of some natural samples containing this kind of compounds, conventional LC could not have enough separation power. In this work, serial connection of several columns is proposed as an alternative to LC and also to comprehensive two-dimensional LC for the analysis of carotenoids from complex real matrices. Different parameters regarding these separation procedures are studied and discussed, such as the stationary phase used or the application of high separation temperatures. The applicability of connecting two C30 columns to significantly increase the separation power, resolution and peak capacity for the analysis of carotenoids has been demonstrated for the first time. In fact, a peak capacity of 79 was obtained when using two C30 serial coupled columns, compared to 61 achieved using a single column. Similar improvements were also observed for the other serial couplings studied. The present methodology could be applied to the analysis of carotenoids in a great variety of samples. To the best of our knowledge, this is the first development carried out to study natural products such as carotenoids using columns coupled in series.     

Ion-pairing of octyl viologen diiodide in low-polar solvents: an experimental and computational study

We have investigated the ion-pairing and solvent effect on the NMR and UV/vis spectra of 1,1'-di- n-octyl-4,4'-bipyridinium diiodide in various solvents. A strikingly different behavior is observed in the low polar solvent dichloromethane. A large deshielding of the meta bipyridinium core resonance occurs and charge transfer (CT) transitions are observed in the visible region due to the formation of ion-pairs. The CT bands show a marked blue-shift as the polarity of the solvent is increased. Experimental data have been compared with the results of DFT calculations of proton's chemical shifts and TD-DFT calculations of the vertical electronic transitions of model ion-pairs (using the smaller methyl viologen dication) in the gas phase and after the inclusion of the solvent reaction field by means of the PCM scheme. Different geometrical arrangements of the ion-pairs have been investigated, and the direct and indirect solvent effect has been elucidated. A good agreement is obtained which allows one to get insights concerning the CT transitions of this system and the geometry of the ion-pairs in solution of low-polar solvents.     

Elastic incoherent neutron scattering on systems of biophysical interest: mean square displacement evaluation from self-distribution function

In the present work an operational recipe for the mean square displacement (MSD) determination, highlighting the connection between the self-distribution function and average statistical values, is presented. The determination of the MSD and of its contributions associated with different mechanisms, together with their thermal behavior, is performed by evaluating the self-distribution function derived by elastic incoherent neutron scattering (EINS). The approach is tested on EINS data collected by the backscattering spectrometer IN13 (ILL, Grenoble, France) on two model systems such as dry myoglobin in trehalose and poly(ethylene glycol) with mean molecular weight M(w) = 400 (PEG 400).     

Modulation of the optical response of polyethylene films containing luminescent perylene chromophores

In this work, two perylene derivatives containing different peripheral alkyl chains (i.e., N,N'-bis-(hexyl)perylene-3,4,9,10-tetracarboxyldiimide (ES-PTCDI) and N,N'-bis-(2'-ethylhexyl)perylene-3,4,9,10-tetracarboxyldiimide (EE-PTCDI)) were synthesized and efficiently dispersed at low loadings (from 0.01 to 0.1 wt %) into linear low-density polyethylene (LLDPE) by processing in the melt. Spectroscopic investigations (UV-vis and fluorescence) combined with quantum-mechanical studies demonstrated the ability of both chromophores to generate aggregates among the planar structure of dyes when dissolved in solution or dispersed into LLDPE above a certain concentration. The data acquired for dyes' dispersions into the polymer matrix reveal that the optical properties and responsiveness to mechanical stimuli are strongly dependent on the compactness of perylene aggregates provided by the different molecular structure of dyes. In particular, the strong intermolecular aggregates of ES-PTCDI resulted in being more resistant toward mechanical stress and less orientable by uniaxial drawing along the drawing direction of the film, whereas the less compact and distorted supramolecular architecture of EE-PTCDI chromophores provided composite films with a remarkable optical response to mechanical solicitations.     

Non-Abelian duality from vortex moduli: A dual model of color-confinement

It is argued that the dual transformation of non-Abelian monopoles occurring in a system with gauge symmetry breaking G→H is to be defined by setting the low-energy H system in Higgs phase, so that the dual system is in confinement phase. The transformation law of the monopoles follows from that of monopole-vortex mixed configurations in the system (with a large hierarchy of energy scales, v₁v₂) , under an unbroken, exact color-flavor diagonal symmetry . The transformation property among the regular monopoles characterized by π₂(G/H), follows from that among the non-Abelian vortices with flux quantized according to π₁(H), via the isomorphism π₁(G)π₁(H)/π₂(G/H). Our idea is tested against the concrete models—softly-broken supersymmetric SU(N), SO(N) and USp(2N) theories, with appropriate number of flavors. The results obtained in the semiclassical regime (at v₁v₂Λ) of these models are consistent with those inferred from the fully quantum-mechanical low-energy effective action of the systems (at v₁,v₂Λ).     

Thermo-physical characterization of Pharmacoat? 603, Pharmacoat? 615 and Mowiol? 4-98

Hydroxypropyl methylcellulose (HPMC) and polyvinyl alcohol (PVA) are important polymers in pharmaceutical, food and other industries being largely used as encapsulation agents. The characterization of two reference grades of HPMC (Pharmacoat^? 603 and Pharmacoat^? 615) and one reference grade of PVA (Mowiol^? 4-98), through X-ray diffraction (XRD) and thermogravimetry (TG) is described. Specific analyses were performed by means of dynamic vapour sorption analysis of water adsorption/desorption from vapours at 10, 25, 40, 55 and 70?°C. Guggenheim?CAnderson?Cde Boer (GAB), Brunauer?CEmmett?CTeller (BET), Park and n-layer BET models were successfully used to fit the experimental data. The glass transition temperature as function of water content was measured by means of differential scanning calorimetry (DSC). The experimental data were analysed according to Linear, Gordon?CTaylor, Fox and Roos equations. XRD studies revealed amorphous structure for the Pharmacoat^??603 and Pharmacoat^??615 and crystalline for Mowiol^??4-98. Single and multi-step temperature degradation point was found for Pharmacoat^??603 and Pharmacoat^??615 and Mowiol^??4-98, respectively. The water uptake is higher for Pharmacoat^??603 and Pharmacoat^??615 than Mowiol^??4-98. The influence of temperature on water uptake is opposite for the two types of polymers. GAB and n-layer BET were found to better model Pharmacoat^??603 and Pharmacoat^??615 and Mowiol^??4-98 data, respectively. The water makes the glass transition to decrease quite drastically. Gordon?CTaylor is better fitting the experimental data both for Pharmacoat^??603 and Pharmacoat^??615 and Mowiol^??4-98.     

History of the attempts to measure orbital frame-dragging with artificial satellites

An annotated guide to navigate the intricate history of the attempts to measure the Lense-Thirring orbital precessions with artificial satellites is offered to the reader.