Amphitropic liquid crystals. Two lamellar phases in a surfactant containing thermotropic and lyotropic mesogenic groups

Abstract

We have studied the lyotropic liquid-crystalline behaviour of cationic surfactants containing a potentially thermotropic moiety, a terminal cyanobiphenyloxy group. Both mono-alkyl and mid-chain substituted dialkyl surfactants have been examined using optical microscopy and NMR spectroscopy. Incorporation of the cyanobiphenyloxy group destabilizes the hexagonal and bicontinuous cubic phases, with only an extensive lamellar region being observed. For the dialkyl surfactant there is a range of compositions where two lamellar phases co-exist, one water-rich and the second surfactant-rich.     

Phase behaviour of lithium and sodium salts of phenylstearic acid

Phenylstearic acid, prepared from oleic acid and benzene, using the Friedel-Crafts reaction, has been confirmed to be a reproducible mixture of twelve positional isomers. Lithium and sodium salts of this acid are semi-crystalline solids which behave in many ways like pure single substances. The thermotropic polymorphism of these soaps has been studied using DSC and polarizing microscopy (as well as X-ray diffraction and ⁷Li NMR spectroscopy for the former soap). Both soaps exhibit characteristic stepwise melting behaviour and form stable reversed hexagonal mesophases at elevated temperatures, in contrast to the lamellar phases exhibited by the unsubstituted soaps.     

Synthesis and phase characterization of a double-tailed pyrrole-containing surfactant: a novel tecton for the production of functional nanostructured materials

A double-tailed polymerizable (pyrrolylalkyl) ammonium amphiphile has been synthesized, and its interfacial properties and aqueous phase behavior have been studied by polarized optical microscopy and X-ray diffraction. The Krafft temperature is about 27 degrees C, and the critical micelle concentration at 40 degrees C is about 1 mM, as obtained from surface tension measurements, potentiometry, and isothermal titration calorimetry. The lyotropic behavior of the surfactant is found to be of a complex nature. At concentrations higher than the micellar (L1) region, two mesophases have been identified: a second isotropic (L2) phase, which is probably micellar but not fully miscible with water, and a lamellar (L(alpha)) phase, showing interesting alignment properties. Small-angle X-ray scattering analysis of the mesophases has been evaluated in terms of a model of spherical micelles, which describes a mutual arrangement by a structure factor derived from a hard-sphere potential (Percus-Yevick, "PY", approach). Interest in the comprehensive phase behavior of the polymerizable surfactant is based on the desire to integrate the system into a composite material to obtain potentially conducting self-assembled hybrid mesostructures.     

Energy spectra for decaying 2D turbulence in a bounded domain

We use results derived in the framework of the replica approach to study the liquid-glass thermodynamic transition. The main results are derived without using replicas and applied to the study of the Lennard-Jones binary mixture introduced by Kob and Andersen. We find that there is a phase transition due to the entropy crisis. We compute both analytically and numerically the value of the phase transition point T(K) and the specific heat in the low temperature phase.     

Applications of proton magnetic resonance spectroscopy to structural problems–XXXI Hindered internal rotation in 2-dimethyl-aminopyridine derivatives

ΔH? and ΔS? values for the restricted rotation of the dimethylamino groups in 5-nitro-and 3,5-dinitro-2-dimethylaminopyridine and 4-dimethylaminopyrimidine are reported and discussed.     

Lyotropic liquid crystal phase behaviour of the zwitterionic surfactants 4-n-tridecyl- and 4-n-heptadecyl-pyridinium N-oxide in water

The phase behaviour, liquid crystal structures and head group hydration of two 4-n-alkylpyridine-N-oxide surfactants have been studied using optical microscopy, DSC and ²H NMR spectroscopy. Only a limited swelling of the surfactant phase occurs in water, so that no micellar solution phase (L₁) occurs. The lamellar phase is the only mesophase observed. Water (²H) quadrupole splittings indicate that the head group binds c. 6 water molecules.     

Quantitative analysis of the packing of alkyl glycosides: a comparison of linear and branched alkyl chains

In an attempt to relate the geometry of glycolipid assemblies with molecular packing constraints, the surface areas per molecule for straight and branched-chain alkyl glycosides with varying chain length are calculated. Effects of temperature, water content, sugar size and paraffin chain length are analysed based on closest packing assumption. The results show a continuous increase of the interface between the hydrophilic and the hydrophobic domain per molecule with growing dominance in bulkiness of either domain, until it reaches a maximum in hexagonal phases. The surface area per molecule, on the other hand, exhibits a sudden jump upon the phase transition from a lamellar to a hexagonal phase, reflecting different values of the packing parameter in both assemblies. This increase is primarily based on the assembly, rather than on molecule-based domain sizes. Therefore, estimations of molecular region sizes can serve only to determine the principal ability of compounds to form certain phases, but not predict the actual phase exhibited under given conditions. Within straight-chain glycosides the surface area per molecule is practically constant, whereas it increases with growing chain length for branched-chain analogues. This can be explained with differences in the volume–length ratio of the hydrocarbon domain.