摘要:

It is shown that rotation near the magic angle, owing to the coherent reduction of all anisotropic interactions, provides a way to obtain first order spectra in liquid crystal solvents which can be analysed through all the classical two dimensional experiments.

ISSN:0267-8292    

DOI:10.1080/02678298808086530

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The orientation of the director of a lyotropic liquid crystal with negative anisotropy of the diamagnetic susceptibility is studied when the sample is spun at various speeds and angles from the magnetic field. Different behaviours is observed at low and high spinning speeds where the orientation is governed by the magnetic and the inertial torques, respectively. At intermediate frequencies it is possible to reach an equilibrium between these two torques. The evolution of this equilibrium with the average magnetic torque allows one to form a conclusion on the inertial nature of this effect.

ISSN:0267-8292    

DOI:10.1080/02678298808086531

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Octasubstituted derivatives of tetrabenzocyclododecatetraene are found to exhibit thermotropic mesophases. The molecules of these mesogens are highly flexible with an average four-fold symmetry. Optical and differential scanning calorimetry measurements are presented on five ester and ether derivatives. All mesophases appear to be columnar with a negative optical anisotropy. From the five compounds studied the four which are derived from aliphatic ester and ether are optically uniaxial, while the mesophase of the decylbenzoyloxy derivative could be biaxial. Miscibility measurements performed with some discotic and pyramidic mesogens of similar molecular dimensions suggest that the mesophases of the tetrabenzocyclododecatetraene core form new families of columnar mesophases.

ISSN:0267-8292    

DOI:10.1080/02678298808086532

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

A series of lyotropic nematic liquid crystals based on aniso-dimensional micelles can be subdivided according to micelle shape and diamagnetic susceptibility anisotropy. The proton magnetic resonance spectra of benzene dissolved in dilute solution in several of these phases have been analysed. The signs of the partially averaged dipole–dipole coupling give information about the relative alignment of the six-fold symmetry axis of benzene, the director and the magnetic field. The system caesium perfluorooctanoate/water is unusual in two respects. The system forms a nematic phase with only a binary mixture and the disk micelles align in a perpendicular plane to the magnetic field. As a result the binary system has been studied previously in some detail (see N. Boden, S. A. Come and K. W. Jolley, 1987,J. phys. Chem.91, 4092). New measurements on the interesting phase diagram have been accomplished using caesium-133 and deuterium magnetic resonance measurements at various temperatures. Some dilatometric measurements have been made to complement the N.M.R. studies.

ISSN:0267-8292    

DOI:10.1080/02678298808086533

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The phase equilibria in phosphatidylcholine (PC)-n-alkane-2H2O systems have been studied to elucidate the driving forces for the transition between a lamellar liquid-crystalline (Lα) phase and a reversed hexagonal (HII) phase. A tentative phase diagram for the system dioleoyl-PC (DOPC)-n-dodecane-2H2O was determined. DOPC forms anLαphase up to at least 90°C in excess water. However, anHIIphase was formed at room temperature at both low and high water concentrations in DOPC-n-dodecane-2H2O mixtures. The phase equilibria were also studied in PC-n-dodecane-2H2O systems containing PC with different degrees of acyl chain unsaturation. The water and dodecane concentrations required to induce the formation of anHII(or isotropic) phase increase in the order dilinoleoyl-PC ∼ DOPC < 1-palmitoyl-2-oleoyl-PC < dipalmitoyl-PC. The effect ofn-alkanes with different chain lengths (C8–C20) on the phase equilibria in DOPC-n-alkane-2H2O mixtures was studied. Although the number of alkane carbon atoms added per DOPC molecule was kept constant, the ability of the alkanes to promote the formation of anHIIphase was strongly chain length dependent; the ability decreased when going from octane to eicosane. Finally, some PC-peptide-2H2O systems were investigated. Gramicidin (hydrophobic) had a similar influence on the phase equilibria as the alkanes. Melittin (amphiphilic) induced the formation of an isotropic phase, while insulin and duramycin (water soluble) had no, or a very limited, ability to induce a non-lamellar phase, respectively. Our results are discussed in the light of simple physical models dealing with the self-assembly of amphiphiles.

ISSN:0267-8292    

DOI:10.1080/02678298808086534

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The alkyl chain order in the lamellar liquid-crystalline phase formed in the octylamine/octylammonium chloride/water system has been investigated by2H N.M.R. at constant mole ratio water/amphiphile and varying charge of the lamellae. The results are correlated to the dimensions of the aggregate obtained from X-ray low angle diffraction measurements.

ISSN:0267-8292    

DOI:10.1080/02678298808086535

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), specifically deuteriated at various positions of thesn-2-chain, have been studied by N.M.R. relaxation methods. Analysis of the experiments, employing a density matrix treatment based on the stochastic Liouville equation, provides new information about the dynamic organization of the different membrane phases (liquid-crystalline, intermediate and gel phases). The complex molecular dynamics are characterized by a super-position of inter- and intramolecular motions, comprising overall reorientation of phospholipid molecules and trans-gauche isomerization of individual chain segments. In addition, there is evidence for two-site rotational jumps of thesn-2-chains in the plane of the membrane. The results clearly demonstrate the particular advantage of N.M.R. relaxation studies in characterizing complex chemical and biological systems.

ISSN:0267-8292    

DOI:10.1080/02678298808086536

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Lamellar phases of double tailed sodium alkyl-phosphates/water systems have been investigated by31Pand13Crelaxations enhanced by paramagnetic divalent ions exchanging rapidly among the polar heads of the surfactant molecules. In order to extract both structural and dynamic information from these experiments, the paramagnetic probes have been chosen such that their electron spin-lattice relaxation times are significantly larger (Mn2+, VO2+) or smaller (Ni2+) than the time scales of molecular motions generally found in the 0.1-2.0 ns range. The paramagnetic relaxation of surfactant nuclei results from intra- and intermolecular contributions, the latter corresponding to dipolar interactions in a two dimensional lattice. The sum of these contributions has been computed as an average over all accessible conformers resulting from the trans-gauche isomerizations of alkyl chains and fitted to the experimental relaxation rates at several magnetic field strengths. This procedure allows the determination of the populations of the main conformers, of the reorientational correlation times and of the lateral diffusion coefficient of the surfactant, which are sometimes difficult to obtain by other N.M.R. methods.

ISSN:0267-8292    

DOI:10.1080/02678298808086537

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

A thermotropic ionic lamellar phase from non-stoichiometric pyridinium octyl-phosphates has been investigated by multinuclear N.M.R. and X-ray diffraction. At room temperature and above, this phase is formed for pyridine to octylphosphoric acid molar ratios from 0.2 to 0.8.2H and13C relaxation experiments show that the pyridinium ion undergoes a very anisotropic motion withDzz> Dxx≫ Dyy, zandxbeing the perpendicular direction to the ring and thec2symmetry axis, respectively. The order parameters given by the2H quadrupolar splittings and the13C chemical shift anisotropy (CSA) areSzz= 0.13,Syy= -0.08 andSxx= -0.05, showing that the pyridinium ring is preferentially oriented parallel to the lamellar plane. The31P CSA and the C1-P dipolar splitting yieldSzz= 0.33 andSxx⋍Syyfor the octylphosphate anion. The order parameters of alkyl C-H bonds have been obtained from theJresolved two-dimensional13C N.M.R. spectra of oriented samples. Two limiting conformational models have been considered to calculate theSCH. One of them is reasonably consistent with the structure derived from X-ray experiments and has been used to calculate the dipolar31P relaxation. Taking into account the CSA contribution, the relaxation measurements performed at 36, 121 and 202 MHz show that the octylphosphate anion undergoes a quasi-axial reorientation about the long molecular axisxwithD∥/D⊥= 4 andD⊥ ⋍ 107rad/s at 300 K.

ISSN:0267-8292    

DOI:10.1080/02678298808086538

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Dielectric relaxation of water molecules in the lamellar, Lα, cubic and hexagonal, Hα, lyotropic structures of nonylphenoxy-poly(ethylenoxy)ethanol (Ark. 9) has been studied by dielectric time domain spectroscopy in the frequency range between 10 MHz and 10 GHz. The values of the relaxation times, obtained at room temperature, are the following: 41 ps for the Lαphase, 29 ps for the cubic phase and 22 ps for the Hαphase. As is seen, the relaxation time of bound water is distinctly higher than that of pure water, and it depends strongly on the phase structure. The relaxation times measured for the liquid-crystalline phases as well as for pure Ark. 9 obey the Arrhenius law, and the energy barriers obtained have the following values: (20 ± 2) kJ/mol for all the liquid-crystalline phases, and (30 ± 3) kJ/mol for pure Ark. 9. The former is in good agreement with the value found for bound water in lipid systems whereas the latter is characteristic of the isotropic phase of thermotropic liquid crystals.

ISSN:0267-8292    

DOI:10.1080/02678298808086539

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor