Orthogonal hexatic smectic phase-rare or common?

The hexatic smectic B (Hex B) phase is commonly encountered among hydrogen-bonded liquid crystal materials. Among enaminoketone and Schiff's base compounds, twenty homologous series (c. 10² compounds) exhibiting orthogonal mono-or bi-layer hexatic phases were identified by combined microscopy and DSC. Phase transitions from crystal B (CrB) of S_f to Hex B as well as from Hex B to S_f, S_a or S_c phases were observed within the range 50 to 190°C. Temperature ranges of the Hex B phase detectable by DSC varied from 0.2 to 20°C. Within two groups of three-ring enaminoketones, the formation of the Hex B phase was found to be controlled by both inductive and mesomeric components of the Hammett constants of the terminal substituents of the molecules.     

Complex formation in mixtures of 4,4'-disubstituted diphenyldiacetylenes with cyanobiphenyls

We investigated 1:1 mixtures of a variety of 4,4'-disubstituted diphenyldiacetylenes (PTTP series) and cyano-containing mesogens for possible complex formation. Complexes were formed when alkyl/alkoxy cyanobiphenyls (CB series) were mixed with alkyl/alkoxy PTTP homologues. We found complexes having a melting temperature higher than that for either component, and complexes with lower melting temperatures. A fluoro substituent on the PTTP gave an enhanced nematic phase and a cyclohexane ring in place of a benzene ring yielded an enhanced smectic A phase, but a cyano on the PTTP gave poor liquid crystalline properties. Phase diagrams were constructed for PTTP-24/5-CB and PTTP-24/80-CB mixtures. Both complexes formed had melting temperatures higher than those for either component. These complexes were recrystallized and shown by X-ray studies to be 1:1 complexes. The PTTP-24/5-CB complex showed only a monotropic nematic phase, but the PTTP-24/80-CB complex showed induced smectic A and hexatic B phases in addition to an enantiotropic nematic phase.     

Phase transitions in two-dimensional colloidal particles at oil/water interfaces

Enhanced digital video microscopy is applied to study the equilibrium structure of a two-dimensional charged sulfate-polystyrene particle (2 mum in diameter) monolayer at decane/water interfaces. When the surface density is decreased, a sequential phase transition, pure solid phase-->pure hexatic phase-->liquid-hexatic-coexisting phase-->pure liquid phase, is observed. In addition, the transition between liquid and hexatic phases is first order, while the solid-hexatic phase transition is second order. The temperature effect on this two-dimensional melting transition is discussed by performing the experiments at three different temperatures. The Voronoi [J. Reine Angew. Math. 134, 198 (1908)] construction is applied to analyze the defect structure in the two-dimensional particle monolayer. The pair interaction potential of the two-dimensional colloidal particles is found to be a very long range repulsion and to decay with distance to the power of -3.     

The role of molecular shape in bilayer elasticity and phase behavior

A previously developed molecular level model for lipid bilayers [G. Brannigan and F. L. H. Brown, J. Chem. Phys. 120, 1059 (2004)] is extended to allow for variations in lipid length and simulations under constant surface tension conditions. The dependence of membrane elasticity on bilayer thickness is obtained by adjusting lipid length at constant temperature and surface tension. Additionally, bilayer fluidity at various lipid lengths is quantified by analysis of a length versus temperature phase diagram at vanishing tension. Regions of solid, gel-like (hexatic) and fluid bilayer behavior are established by identification of phase boundaries. The main melting transition is found to be density driven; the melting temperature scales inversely with lipid length since thermal expansion increases with lipid aspect ratio.     

Direct transition from the SmA phase to the tilted hexatic phase in liquid crystals with several lactate units

Compounds with differing numbers of lactate units in the chiral part were synthesized. For all materials, at least two smectic phases were found. In addition to the SmA, the SmC* and/or the tilted hexatic SmI*(F*) phase appear according to the length of the non-chiral alkyl chain. For the shortest non-chiral chain, a direct transition from the SmA phase to the SmI*(F*) phase has been discovered and studied. For compounds with the 2-(S)-methylbutyl alkyl chain and two lactate units in the chiral part the antiferroelectric SmC*_A phase occurs. The ferroelectric character of the hexatic phase has been confirmed even just below the SmC*_A phase.     

Direct transition from the SmA phase to the tilted hexatic phase in liquid crystals with several lactate units

Compounds with differing numbers of lactate units in the chiral part were synthesized. For all materials, at least two smectic phases were found. In addition to the SmA, the SmC* and/or the tilted hexatic SmI*(F*) phase appear according to the length of the non-chiral alkyl chain. For the shortest non-chiral chain, a direct transition from the SmA phase to the SmI*(F*) phase has been discovered and studied. For compounds with the 2-(S)-methylbutyl alkyl chain and two lactate units in the chiral part the antiferroelectric SmC*_A phase occurs. The ferroelectric character of the hexatic phase has been confirmed even just below the SmC*_A phase.     

Condensation of free volume in structures of nematic and hexatic liquid crystals

Eight novel liquid crystalline materials were prepared containing highly branched terminal chains, either 2,4,4-trimethylpentyl or 3,5,5-trimethylhexyl. All materials exhibit nematic mesophases, with additional smectic (Sm) C, hexatic B and SmI phases for certain homologues. Analysis by small- and wide-angle X-ray scattering reveals continual build-up of the correlation length within the nematic phases, where we also observe splitting of the small angle peak into four lobes, indicating pretransitional Sm fluctuations. Connoscopy confirms the nematic phase to be uniaxial and optically positive. We observe that in the solid state, the molecules exist as staggered antiparallel pairs as a consequence of the sterically demanding bulky terminal group, and this would also appear to manifest in the hexatic B phase, where the layer spacing was found to be greater than the molecular length. If true, this is an example of pair formation driven by sterics rather than dipole–dipole interactions and suggests that reentrant systems driven purely by steric frustration may be found.     

Experimental characterization of hexatic smectic phases through electro-optic studies and dielectric relaxation spectroscopy

The electro-optic and complex dielectric behaviour of an antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl)phenyl 4'-(n-butanoyloxyprop-1-oxy)biphenyl-4-carboxylate, having chiral SmC_A* and hexatic smectic phases, have been investigated. Complex dielectric permittivities were measured as a function of frequency, d.c. bias field and temperature. Spontaneous polarization was measured by the current reversal technique; tilt angle was measured under a polarizing microscope using a low frequency electric field. The electro-optic properties and dielectric behaviour of the material are compared with results obtained by DSC and polarizing optical microscopy. Dielectric relaxation processes in SmC_A* and hexatic smectic phases were determined. The dielectric strength at the SmC_A* to hexatic smectic phase transition is discussed in terms of coupling between the long range bond orientational order and smectic C director. It seems from the results of spontaneous polarization and dielectric relaxation spectroscopy that the material might possess an additional phase between the SmC_A* and hexatic smectic I* phases.     

Low-dimensionality effects in the melting of a Langmuir-Blodgett multilayer

Low-dimensionality effects in the melting behavior of a cadmium arachidate Langmuir-Blodgett multilayer have been studied. Depth resolved information about structural changes occurring with temperature is obtained using in-plane X-ray diffraction under standing wave conditions. The surface region exhibits a distinctly different melting behavior as compared to the bulk of the film. While in the bulk of a 13-monolayer cadmium arachidate multilayer, the crystalline phase directly transforms to a tilted hexaticlike phase at 360 K, in the near surface region transformation occurs via an intermediate smectic phase. This behavior of the surface region is similar to that observed in two-dimensional crystals. Thus even in a thick Langmuir-Blodgett multilayer, the surface region exhibits low-dimensionality effects.     

Implicit-solvent mesoscale model based on soft-core potentials for self-assembled lipid membranes

An efficient implicit-solvent model for self-assembled lipid bilayers is presented and analyzed using Langevin molecular dynamics simulations. The model is based on soft interactions between particles and short-range attractive interaction between lipid tails, leading for the self-assembly of a lipid bilayer without an explicit solvent. This allows for efficient simulations of large membranes over long times. The model exhibits a fluid phase at high temperatures and a gel phase at low temperatures, identified with the Lbeta-phase. The melting transition is investigated via analysis of the diffusivity of the lipid molecules, the chain-orientational order parameter, the sixfold bond-orientational order parameter, and the positional and bond-orientational correlation functions. The analysis suggests the existence of a hexatic phase over a narrow range of temperatures around the melting transition. The elastic properties of the membrane in the fluid phase are also investigated.     

Experimental characterization of hexatic smectic phases through electro-optic studies and dielectric relaxation spectroscopy

The electro-optic and complex dielectric behaviour of an antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl)phenyl 4′-(n-butanoyloxyprop-1-oxy)biphenyl-4-carboxylate, having chiral SmC_A* and hexatic smectic phases, have been investigated. Complex dielectric permittivities were measured as a function of frequency, d.c. bias field and temperature. Spontaneous polarization was measured by the current reversal technique; tilt angle was measured under a polarizing microscope using a low frequency electric field. The electro-optic properties and dielectric behaviour of the material are compared with results obtained by DSC and polarizing optical microscopy. Dielectric relaxation processes in SmC_A* and hexatic smectic phases were determined. The dielectric strength at the SmC_A* to hexatic smectic phase transition is discussed in terms of coupling between the long range bond orientational order and smectic C director. It seems from the results of spontaneous polarization and dielectric relaxation spectroscopy that the material might possess an additional phase between the SmC_A* and hexatic smectic I* phases.     

Simultaneous calorimetric and optical reflectivity measurements of extremely thin liquid crystal films

Abstract

An extremely sensitive measuring system has been developed to obtain simultaneously heat capacity and optical reflectivity data from thin free-standing liquid crystal films. Results from 3(10)OBC near the smectic A to hexatic B transition will be reported and discussed in the context of recent theory regarding phase transitions in two dimensions.     

Zero spontaneous curvature and its effects on lamellar phase morphology and vesicle size distributions

Equimolar mixtures of dodecyltrimethylammonium chloride (DTAC) and sodium octyl sulfonate (SOSo) show a vesicle phase at >99 wt % water and a single, fluid lamellar phase for water fractions below 80 wt %. This combination is consistent with the bilayer bending elasticity kappa approximately k(B)T and zero bilayer spontaneous curvature. Caillé line shape analysis of the small-angle X-ray scattering from the lamellar phase shows that the effective kappa depends on the lamellar d spacing consistent with a logarithmic renormalization of kappa, with kappa(o) = (0.8 +/- 0.1)k(B)T. The vesicle size distribution determined by cryogenic transmission electron microscopy is well fit by models with zero spontaneous curvature to give (kappa + (kappa/2)) = (1.7 +/- 0.1)k(B)T, resulting in kappa = (1.8 +/- 0.2)k(B)T. The positive value of kappa and the lack of spontaneous curvature act to eliminate the spherulite defects found in the lamellar gel phases found in other catanionic mixtures. Current theories of spontaneous bilayer curvature require an excess of one or more components on opposite sides of the bilayer; the absence of such an excess at equimolar surfactant ratios explains the zero spontaneous curvature.     

A molecular model for the smectic B phase. The stabilized molecular rotor

Transition temperatures and smectic B mesophase ranges are examined for a number of homologous series, and some general rules are proposed that relate the smectic B phase behaviour to molecular structure. A mechanical model is developed for the crystal and hexatic smectic B phases, based on the dynamics of rigid or elastic rotors, which suggests two possible types of behaviour. It is found that such predicted behaviour is observed for particular series, but in general the smectic B phase characteristics of homologous series may be between the limits established for the model.     

Applying the Z method to estimate temperatures of melting in structure II clathrate hydrates

Equilibrium melting temperatures for structure II THF hydrate and argon/xenon (Ar/Xe) binary hydrate have been calculated using molecular dynamics using two melting techniques, namely the Z method [Belonoshko et al., Phys. Rev. B, 2006, 73, 012201] (applied for the first time to complex molecular solids) and direct phase coexistence simulations. The two methods give results in moderate agreement: calculations with the Z method give T(fus) to be 250.7 K (0.77 katm) for THF and 244.3 K (1.86 katm) for Ar/Xe hydrate respectively; the corresponding direct phase coexistence calculations give T(fus) in the range 235-240 K (0.77 katm) for THF and 240-252.5 K (1.86 katm) for Ar/Xe hydrate. The Z method was found to define the key thermodynamic states with high precision, although required long simulation times with these multicomponent molecular systems to ensure the complete melting required by the method. In contrast, the direct phase coexistence method did bracket the equilibrium temperature with little difficulty, but small thermodynamic driving forces close to phase equilibrium generated long-lived fluctuations, that obscured the precise value of phase coexistence conditions within the bracketed range.     

Enaminoketones as new hydrogen bonded liquid crystals

The relation between molecular structure of enaminoketone derivatives and their mesomorphic properties is discussed. It has been observed that the electron accepting terminal groups (F,Cl,Br,CN) promote formation of orthogonal phases while strongly electron donating groups (OC_nH_2n+1) favour tilted phases. Among the synthesised compounds, materials useful for structural studies of two dimensional (2D) melting in orthogonal, as well as the tilted systems, were found. It has been determined that the heat effects in the melting process from crystalline smectic into liquid-like smectic, through the hexatic state, are correlated to the density changes.     

Predicting the phase diagram of two-dimensional colloidal systems with long-range interactions

The phase diagram of a two-dimensional model system for colloidal particles at the air-water interface was determined using Monte Carlo computer simulations in the isothermic-isobaric ensemble. The micrometer-range binary colloidal interaction has been modeled by hard disklike particles interacting via a secondary minimum followed by a weaker longer-range repulsive maximum, both of the order of kBT. The repulsive part of the potential drives the clustering of particles at low densities and low temperatures. Pinned voids are formed at higher densities and intermediate values of the surface pressure. The analysis of isotherms, translational and orientational correlation functions as well as structure factor gives clear evidence of the presence of a melting first-order transition. However, the melting process can be also followed by a metastable route through a hexatic phase at low surface pressures and low temperatures, before crystalization occurs at higher surface pressure.     

Lamellar gels and spontaneous vesicles in catanionic surfactant mixtures

Caillé analysis of the small-angle X-ray line shape of the lamellar phase of 7:3 wt/wt cetyltrimethylammonium tosylate (CTAT)/sodium dodecylbenzene sulfonate (SDBS) bilayers shows that the bending elastic constant is kappa = (0.62 +/- 0.09)k(B)T. From this and previous results, the Gaussian curvature constant is kappa = (-0.9 +/- 0.2)k(B)T. For 13:7 wt/wt CTAT/SDBS bilayers, the measured bending elasticity decreases with increasing water dilution, in good agreement with predictions based on renormalization theory, giving kappa(o) = 0.28k(B)T. These results show that surfactant mixing is sufficient to make kappa approximately k(B)T, which promotes strong, Helfrich-type repulsion between bilayers that can dominate the van der Waals attraction. These are necessary conditions for spontaneous vesicles to be equilibrium structures. The measurements of the bending elasticity are confirmed by the transition of the lamellar phase of CTAT/SDBS from a turbid, viscoelastic gel to a translucent fluid as the water fraction is decreased below 40 wt %. Freeze-fracture electron microscopy shows that the gel is characterized by spherulite defects made possible by spontaneous bilayer curvature and low bending elasticity. This lamellar gel phase is common to a number of catanionic surfactant mixtures, suggesting that low bending elasticity and spontaneous curvature are typical of these mixtures that form spontaneous vesicles.     

Surface order-disorder phase transitions and percolation

In the present paper, the connection between surface order-disorder phase transitions and the percolating properties of the adsorbed phase has been studied. For this purpose, four lattice-gas models in the presence of repulsive interactions have been considered. Namely, monomers on honeycomb, square, and triangular lattices, and dimers (particles occupying two adjacent adsorption sites) on square substrates. By using Monte Carlo simulation and finite-size scaling analysis, we obtain the percolation threshold theta(c) of the adlayer, which presents an interesting dependence with w/k(B)T (w, k(B), and T being the lateral interaction energy, the Boltzmann constant, and the temperature, respectively). For each geometry and adsorbate size, a phase diagram separating a percolating and a nonpercolating region is determined.     

Variety of mesophases in compounds with an increasing number of lactate units in the chiral chain

Liquid-crystalline compounds with different numbers of lactate units, n, in the chiral part were synthesised and mesomorphic properties studied. Physical properties were compared with respect to n. In the compound with one lactate unit in the chiral part the TGBA–TGBC–SmC* phase sequence was detected. For two lactate units the antiferroelectric SmC*_A phase occurs. Finally, three-lactate material exhibits the tilted hexatic SmI*(F*) phase below the ferroelectric SmC* phase. Dielectric spectroscopy and spontaneous tilt and polarisation were measured. For the three-lactate compound the temperature dependences were analysed in the vicinity of the SmC*–hexatic phase transition, and these properties compared with the theoretically predicted behaviour.