Hexagonal Liquid Crystalline Research Articles

Dependence of Alkyl Chain Asymmetry on the Phase Equilibria of Three Catanionic Surfactant Mixtures Containing Dodecyltrimethylammonium Chloride−Sodium Alkylcarboxylate−Water

The pseudoternary phase behavior of three aqueous systems with catanionic surfactant mixtures was studied at 20 °C, with emphasis on the effect of the level of asymmetry between the parent surfactants. The three mixtures all contain the cationic surfactant dodecyltrimethylammonium chloride (DoTAC), combined with one of the surfactants sodium hexanoate (SH), sodium octanoate (SO), or sodium decanoate (SD). As could be expected, all three systems form a variety of phases deriving from the binary DoTAC−water axis, and the extension of those phases vary with the anionic surfactant chain length. The binary sodium alkylcarboxylate/water systems do not show as rich phase behavior as DoTAC−water at this temperature, and the small hexagonal liquid crystalline phase present in the binary SO−water are destabilized by DoTAC in the pseudoternary system. This can be explained in terms of asymmetry between the surfactant chain lengths, which causes packing problems. Neither SH nor SD forms a hexagonal phase at room temperature, the former due to its short alkyl chain and the latter due to its high Krafft temperature. The DoTAC−SD−water phase diagram is dominated by a lamellar liquid crystalline phase, whereas the range of existence of the lamellar phases in the SO and SH systems are much more limited. The hexagonal phases deriving from the binary DoTAC system can incorporate varying amounts of alkyl carboxylate. The largest extension is found in the SO system, indicating a favorable level of asymmetry between the two surfactants. All three systems form a large isotropic solution phase with water. In the SD system, the solution phase has a discontinuity around the equimolar line at high water content. Up to 13 wt % of total surfactant, there is coacervation resulting in two translucent solutions, one dense and one dilute. In the most dilute region, the solutions within and close to the small two-phase area have a bluish appearance. All solution phases were studied by 1H self-diffusion NMR. The results show that all systems are most likely to form mixed aggregates at anionic/catanionic molar ratios <1. In the DoTAC−SH and −SO systems, there are indications of micelles and anionic surfactant monomers coexisting in the monophasic isotropic solutions at anionic/catanionic molar ratios ≥1. In the DoTAC−SD system, on the other hand, there seems to be coexistence of two different types of aggregates at the molar ratios larger than unity. The phase boundaries of the pseudoternary phase diagrams in this work were constructed on the basis of 2H NMR experiments and visual observations, and the microstructures of the one-phase regions were clarified by SAXS.

Electro-Optic Switching by Helicene Liquid Crystals

Binary mixtures of the aggregating helicene 1 with dodecane have, at dodecane concentrations >30 vol %, a nematic liquid crystalline phase and, at dodecane concentrations ≲5 vol %, a hexagonal columnar liquid crystalline phase. The helical structure with donor and acceptor groups at opposite ends gives rise to a molecular dipole moment that is parallel to the helix axis and a positive dielectric anisotropy. Accordingly, the helix axes orient parallel to an applied electric field, providing the basis for electro-optic switching.

Cuprite Nanowires by Electrodeposition from Lyotropic Reverse Hexagonal Liquid Crystalline Phase

Cuprite (Cu2O) nanowires with diameter of 25−100 nm were electrodeposited from anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate reverse hexagonal liquid crystalline phase. The nanowires can be grown to up to tens of micrometers in length by simply changing the electrodeposition time. Resistivity measurements suggest improved alignment of the reverse hexagonal liquid crystalline phase under electric field during electrodeposition. The enhanced alignment of the liquid crystalline phase appears essential for the growth of nanowires with high aspect ratio. This new synthesis approach is fast and simple, and a great variety of organized assemblies are potentially available by simply adjusting the liquid crystal type and composition.

Polyaniline nanowires by electropolymerization from liquid crystalline phases

Conductive polyaniline nanowires in emeraldine form can be deposited by potentiodynamic electropolymerization from a reverse hexagonal liquid crystalline phase where one-dimensional (1D) aqueous channels can serve as space-confined reactors. Polyaniline nanowire bundles with single-wire diameter of 50–70 nm and length of several microns were obtained from surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse hexagonal liquid crystalline phase. Impedance measurement suggests improved ordering of the reverse hexagonal liquid crystal under external electric field during the electropolymerization process. This enhanced ordering appears essential for the nanowire growth. The nanowires obtained can be easily collected by a simple washing process.

Phase Behavior and Aggregate Formation for the Aqueous Monoolein System Mixed with Sodium Oleate and Oleic Acid

The phase behavior and microstructure of the two ternary systems monoolein (MO)−sodium oleate (NaO)−water (2H2O) and MO−oleic acid (OA)−2H2O are studied by a combination of optical microscopy, cryo-transmission electron microscopy, small-angle X-ray diffraction, and NMR methods. The results demonstrate significant differences in phase behavior between the two systems. The isothermal phase diagram of the MO−NaO−2H2O system is dominated by a large lamellar liquid crystalline phase that shows an ideal swelling up to high water contents. Stable vesicles are the dominant aggregates at water concentration >90 wt %. The existence of a lamellar phase is, however, absent from the MO−OA−2H2O system, where the largest single-phase region is a reversed hexagonal liquid crystalline phase, HII, at low water content. A similar water-poor HII phase is also identified for the MO−NaO−2H2O system. The two types of bicontinuous cubic structures, gyroid (CG) and diamond (CD), formed by the binary MO−2H2O system are also prese...

Electrodeposition of Nanostructured Mesoporous Selenium Films (HI-eSe)

Adherent and uniform nanostructured mesoporous films of selenium have been produced by electrodeposition from the hexagonal liquid-crystalline phase of a nonionic surfactant. The regular nanostructure consists of 25-A diameter cylindrical pores distributed on a long-ranged hexagonal lattice. Electron diffraction reveals that selenium is single crystalline with an hcp structure. Current−voltage curves recorded by STM indicate that the electrodeposits are intrinsic semiconductors with an apparent band gap of ≈1.0 eV.

Silver Nitrate/Oligo(ethylene Oxide) Surfactant/Mesoporous Silica Nanocomposite Films and Monoliths

A lyotropic, liquid crystalline (LC) phase of a silver nitrate/oligo(ethylene oxide), water, and acid mixture was used for one-pot synthesis of mesoporous silica materials in which Ag+ ions are uniformly distributed. We established that the AgNO3-to-surfactant mole ratio is very important in a 50 wt% surfactant/water system to preserve the hexagonal LC phase before and after the addition of the silica source. Below a 0.6 AgNO3-to-surfactant mole ratio, the mixture is liquid crystalline and serves as a template for silica polymerization. However, between 0.6 and 0.8 AgNO3-to-surfactant mole ratios, one must control the composition of the mixture during the polymerization processes. Above a 0.8 mole ratio, Ag+ ions undergo phase separation from the reaction mixture by complexing with the surfactant molecules. The resulting silica materials obtained from AgNO3/surfactant ratios above 0.8 have anisotropy but without a hexagonal mesophase. Here, we establish a AgNO3 concentration range in which the LC phase is preserved to template the synthesis of mesoporous silica, and we discuss the structural behavior of the mixtures at AgNO3/surfactant mole ratios of 0.00–2.00, using POM, PXRD, FTIR, and UV–Vis absorption spectroscopy.

Electrochemical characterization of a templated insulating polymer-modified electrode.

The controllable permselective behavior oftemplated poly(1,2-diaminobenzene)-modified electrodes is illustrated. The electrochemical behavior of polymer films prepared from several media, conventional aqueous solution, liquid crystalline micellar solution, and liquid crystalline hexagonal phase solution, were compared using several cationic and anionic redox probes. Films deposited from the hexagonal liquid crystalline phase were shown to be ion selective, allowing positively charged ions through to react at the electrode surface while excluding negatively charged ions. This permselective behavior was unique to the hexagonally templated films and was not exhibited by films deposited from either the conventional aqueous or the micellar media (which were completely blocking to both types of ion). In addition, it was shown that by altering the ionic strength of the background electrolyte control over the charge selective behavior of the hexagonally templated films was possible.

Mesoporous Pt/Ru Alloy from the Hexagonal Lyotropic Liquid Crystalline Phase of a Nonionic Surfactant

Mesoporous and well-mixed Pt/Ru alloys (1:1 atomic ratio) were prepared by co-reduction of metal salts in the hexagonal liquid crystalline phase of a nonionic surfactant. Preliminary data indicate that oxidation of carbon monoxide by the mesoporous alloy is more facile than by a commercial platinum catalyst or by mesoporous platinum.

Interaction between colloidal silica and a nonionic surfactant hexagonal liquid crystalline phase

The hexagonal-micelle transition was investigated for Synperonic A7 at 40% w/w using rheology, microscopy, differential scanning calorimetry (DSC) and small angle X-ray scattering (SAXS). The pure phase showed signs of a two-step process indicated by inflections in SAXS peak height and interlayer spacing plots. There was also a small discrepancy between the transition temperature determined by rheology and that determined by microscopy. It was postulated that the two processes were the partial break up of the hexagonal phase leading to a loosening of the structure followed by the full conversion of the hexagonal phase to isotropic micelles. This idea was supported by the fact that the addition of silica considerably lowered the rheological transition temperature while having only a small effect on the microscopy value. The lowering in the rheology transition temperature was found to be much smaller at high ionic strength indicating an electrostatic mechanism involving the overlap of electrical double layers. A model was devised which explained the concentration of the silica on the formation of the hexagonal phase, a phenomenon observed experimentally by SAXS.

The Electrochemical Deposition of Nanostructured Cobalt Films from Lyotropic Liquid Crystalline Media

Lyotropic liquid crystalline phases formed at high concentrations of nonionic surfactants provide a versatile templating medium that can be used to produce nanostructured materials with regular arrays of pores of nanometer dimension and extended periodicity. In this work, we have used this technique to prepare nanostructured cobalt films on gold substrates by electrochemical deposition of cobalt from cobalt acetate dissolved in the aqueous domains of the hexagonal lyotropic liquid crystalline phase of Brij Low angle X-ray and transmission electron microscope studies show that the resulting cobalt films have a regular nanostructure consisting of a hexagonal array of cylindrical pores. The repeat distance for this nanostructure lies in the range of 7-11 nm and can be systematically varied by varying the amount of -heptane added to the template mixture. Preliminary magnetic measurements on the nanostructured samples suggest that the coercivity of these cobalt films is three to five times greater than that for polycrystalline cobalt and that it varies systematically with the size of the pores. © 2001 The Electrochemical Society. All rights reserved.

Micelles, Dispersions, and Liquid Crystals in the Catanionic Mixture Bile Salt−Double-Chained Surfactant. The Bile Salt-Rich Area

The phase behavior and phase structure for the catanionic pair sodium taurodeoxycholate−didodecyldimethylammonium bromide (DDAB) are investigated, at 25 °C. A combination of techniques is used including light and electron microscopy, small-angle X-ray scattering, and pulsed field gradient NMR self-diffusion. The bile salt micellar solution incorporates large amounts of the double-chained amphiphile, with the solution region extending to equimolarity. On the contrary, the hexagonal liquid-crystalline phase is destabilized by the addition of small amounts of DDAB. At equimolarity, coacervation instead of precipitation is observed, with formation of a viscous isotropic solution and a very dilute one. In the water-rich part of the phase diagram, a peculiar type of phase separation occurs, involving the formation of very fine bluish dispersions and a region of coexistence of two dispersions (double dispersion region). Microscopy and self-diffusion data for the solution region indicate limited growth of the mixed micelles. Large domains in which the micellar structure appears to be maintained are imaged in the bluish dispersions by electron microscopy. No other type of aggregate such as vesicles or precipitates is observed in the dilute bile salt-rich area of this mixture.

Self-Assembly-Induced Supramolecular Hexagonal Columnar Liquid Crystalline Phase Using Laterally Attached Nonmesogenic Templates

A novel poly[di(4-heptyl) vinylterephthalate] has been synthesized based upon a free radical polymerzation from a newly designed monomer having a nonmesogenic template with two double-swallow-tailed 4-heptyl (Y-shaped) groups. The resulting polymer consists of a polyethylene backbone and a 2,5-bis(4-heptyloxycarbonyl)phenyl lateral substitutent on every second carbon atom of the backbone. Although neither the backbone nor the pendent template exhibits liquid crystalline (LC) behavior, they couple together to form a supramolecular hexagonal columnar LC phase on a scale of greater than 1.5 nm in the polymer. Therefore, it is deduced that this supramolecular LC structure is constructed via a cooperative assembly of a helical arrangement of the backbones with the pendent templates.

Spontaneous Organization of Supramolecular Rod-Bundles into a Body-Centered Tetragonal Assembly in Coil−Rod−Coil Molecules

The synthesis and characterization of coil−rod−coil molecules of 4,4‘-bis[4-methyloxypoly(propyleneoxy)propyloxy-4‘-biphenyloxymethyl]biphenyl with a poly(propylene oxide) coil of 3 (7), 6 (8), 9 (9), 13 (10), 17 (11), and 22 (12) propylene oxide units are described. These molecules self-assemble into ordered structures that differ significantly on variation of the length of poly(propylene oxide) coil. Coil−rod−coil molecule 7 self-organizes into lamellar crystalline and bicontinuous cubic liquid crystalline assemblies, while 8 shows a hexagonal columnar liquid crystalline assembly. Remarkably, increasing the length of coil induces discrete supramolecular aggregates that self-assemble into a birefringent 3-D superlattice. The molecules 9 and 10 assemble into discrete supramolecular aggregates that spontaneously organize into a novel 3-D tetragonal lattice with a body-centered symmetry in the crystalline and melt states. Further increasing the length of coil as in the case of 11 and 12 induces only a body-centered tetragonal crystalline phase, while the liquid crystalline phase in these molecules is suppressed. X-ray diffraction experiments and density measurements showed that the aggregation of these molecules into a discrete supramolecular structure gives rise to aromatic rod bundles with hockey puck-like cylindrical shape encapsulated by phase-separated coil segments which results in the formation of oblate aggregates. This nonspherical oblate shape is believed to be responsible for the formation of a body-centered tetragonal phase. These results demonstrate that supramolecular structures, from 1-D lamellar to 3-D tetragonal superlattices, formed by the self-assembling process of molecular rods can be controlled in a systematic and predictive way by simple variation of the length of grafted coils.

The Effect of Peptide/Lipid Hydrophobic Mismatch on the Phase Behavior of Model Membranes Mimicking the Lipid Composition in Escherichia coli Membranes

The effect of hydrophobic peptides on the lipid phase behavior of an aqueous dispersion of dioleoylphosphatidylethanolamine and dioleoylphosphatidylglycerol (7:3 molar ratio) was studied by 31P NMR spectroscopy. The peptides (WALP n peptides, where n is the total number of amino acid residues) are designed as models for transmembrane parts of integral membrane proteins and consist of a hydrophobic sequence of alternating leucines and alanines, of variable length, that is flanked on both ends by tryptophans. The pure lipid dispersion was shown to undergo a lamellar-to-isotropic phase transition at ∼60°C. Small-angle x-ray scattering showed that at a lower water content a cubic phase belonging to the space group Pn3m is formed, suggesting also that the isotropic phase in the lipid dispersion represents a cubic liquid crystalline phase. It was found that the WALP peptides very efficiently promote formation of nonlamellar phases in this lipid system. At a peptide-to-lipid (P/L) molar ratio of 1:1000, the shortest peptide used, WALP16, lowered the lamellar-to-isotropic phase transition by ∼15°C. This effect was less for longer peptides. For all of the WALP peptides used, an increase in peptide concentration led to a further lowering of the phase transition temperature. At the highest P/L ratio (1:25) studied, WALP16 induced a reversed hexagonal liquid crystalline (H II) phase, while the longer peptides still promoted the formation of an isotropic phase. Peptides with a hydrophobic length larger than the bilayer thickness were found to be unable to inhibit formation of the isotropic phase. The results are discussed in terms of mismatch between the hydrophobic length of the peptide and the hydrophobic thickness of the lipid bilayer and its consequences for lipid-protein interactions in membranes.

Ternary Phase Diagram of the Triton X-100/Poly(acrylic acid)/Water System

The phase behavior of the Triton X-100/poly(acrylic acid) aqueous system within the entire range of surfactant concentration and up to 30 wt % polymer was examined at various temperatures. At 25 °C two isotropic solution phases (L1 and L2) and one hexagonal liquid crystalline phase form in the ternary system. The hexagonal liquid crystalline and the isotropic solution phases were characterized by means of PGSE-NMR, SAXS, and rheological measurements. The presence of the polymer changes the melting temperature rather than the SAXS lattice spacing of the hexagonal liquid crystalline phase. Self-diffusion studies in the isotropic solution region show that surfactant diffusion is slower when polymer is added. The obtained data show that surfactant and polymer associate; the polymer molecules are probably adsorbed on the surface of the surfactant aggregates.

Effect of Mixing Oils on the Hexagonal Liquid Crystalline Structures

Structural change in hexagonal liquid crystal (H1) in the water/heptaoxyethylene dodecyl ether (C12EO7) system upon addition of mixed oil (m-xylene + decane) is investigated by phase study and small-angle X-ray scattering (SAXS). The surfactant layer curvature tends to be less positive or negative upon addition of the mixed oil at a high m-xylene/decane ratio, and the H1−Lα (lamellar liquid crystal) transition takes place. At a low m-xylene/decane ratio, however, the H1−I1 transition occurs with increasing mixed oil content, where I1 is a discontinuous micellar cubic phase. An excess oil phase directly separates from the H1 phase at a moderate m-xylene/decane ratio. The change in the effective cross sectional area per surfactant, aS, as a function of oil content was also measured at each mixing fraction of oils by SAXS. m-Xylene molecules tend to penetrate the surfactant palisade layer and widen aS, whereas decane molecules tend to be solubilized in the deep core of surfactant hydrophobic moieties and the...

Nanostructured Platinum (HI-ePt) Films: Effects of Electrodeposition Conditions on Film Properties

A systematic investigation into the effects of deposition conditions on the electrochemical properties and nanostructure of high surface area (up to ∼460 m2 cm-3) mesoporous platinum films (HI-ePt) obtained by electroreduction of hexachloroplatinic acid (HCPA) from an hexagonal lyotropic liquid crystalline phase is reported. The surface area, repeat distance, and regularity of the nanostructure and the surface morphology of the films were shown to depend on the deposition conditions. In particular, the deposition potential was found to have a notable effect on the film surface area and on the uniformity of nanostructure. The results obtained are explained by variations in the efficiency of the deposition process and the relative contributions of competing reactions. Increasing the temperature of electrodeposition was found to increase significantly the surface areas of the films and to increase the repeat distance of the nanostructure. In addition, a roughening of the films on both a micro- and a nanomete...

Platinum Microelectrodes with Unique High Surface Areas

Nanostructured platinum films were electrodeposited onto microelectrodes from an hexagonal lyotropic liquid crystalline plating mixture. Cyclic voltammetry in dilute sulfuric acid and copper underpotential deposition confirmed that the resulting platinum films had high surface areas (with roughness factors of ca. 210) as a direct consequence of their electrochemically accessible nanostructure. The microelectrodes were shown to combine a high real surface area with efficient mass transport characteristics. This combination of properties is unique and may be of significance to the fields of electrochemistry and electroanalysis.

Effect of Types of Perfume Compounds on the Hydrophile–Lipophile Balance Temperature

The effect of added perfume compounds (d-limonene (LN), α-hexylcinnamic aldehyde (HCA), β-ionone (IN), benzyl acetate (BA), linalool (LL), geraniol (GL), eugenol (EL), and cis-3-hexenol (HL)) on the three-phase behavior was investigated in water/octaethylene glycol dodecyl ether (C12EO8)/perfume compound systems. The HLB (hydrophile–lipophile balance) temperatures are considerably lower than those of saturated hydrocarbon systems and are in the order LN > HCA > IN > BA > LL > GL > EL > HL. It is considered that the perfume molecules tend to be solubilized in the vicinity of the interface of the water–hydrocarbon moieties of the surfactant or the surfactant palisade layer. To confirm this, the location of the solubilized perfume compound in aggregates of liquid crystals was investigated by small-angle X-ray scattering (SAXS). In the LN, HCA, IN, BA, LL, and GL systems, the hexagonal liquid crystalline phase changes to lamellar liquid crystals on addition of each perfume compound, whereas the hexagonal liquid crystalline phase is directly changed to a reverse micellar solution phase in the EL and HL systems. The effective cross-sectional area per surfactant molecule, as, was calculated by using interlayer spacing measured by SAXS. Perfume compounds, which reduce the HLB temperature greatly, tend to penetrate into the surfactant palisade layer and increase as. Thus, the perfume molecules change the surfactant curvature from positive to negative. The more the perfume compound penetrates into the palisade layer, the lower the HLB temperature becomes.