Micellar Liquid Crystals Research Articles

Self-assembly of the imidazolium surfactant in aprotic ionic liquids. The anion effect of aprotic ionic liquids.

The structure of ionic liquids (ILs) has an influence on their physiochemical properties, determining their performance as self-assembly media. In this study, we focus on the anion effect of aprotic ionic liquids (AILs). The aggregation behaviours of the cationic surfactant 1-hexadecyl-3-methylimidazolium bromide (C16mimBr) have been investigated in the imidazolium AILs with the 1-ethyl-3-methyl imidazolium cation and different anions, including nitrate, ethylsulfate, bis(trifluoromethylsulfonyl) imide and tetrafluoroborate. Surface adsorption parameters of C16mimBr were determined using surface tension measurements, and the critical micellization concentration values in AILs vary for their different cohesive energy. The micellar and lamellar lyotropic liquid crystal phases emerge with the increase of C16mimBr concentrations. The structure and properties of aggregates were determined using small angle X-ray scattering, polarized optical microscopy, rheology and differential scanning calorimetry. The anion effects of AILs on the phase behaviours and structure and properties of aggregates were analysed and discussed. The lamellar lyotropic liquid crystals have shown good conductivity, as confirmed by electrochemical impedance spectroscopy characterization. Our results enhance the understanding of the structure effect of ILs as self-assembly media and contribute to the design of tailorable solvents.

Studying the Structure of Sodium Lauryl Ether Sulfate Solutions Using Dissipative Particle Dynamics.

Sodium lauryl ether sulfate (SLES) is a common anionic surfactant used in a large number of personal care products. Commercial products typically contain a distribution in the number of ethoxy groups; despite this, there is limited existing work studying the effect of the ethoxy groups on the phase formation and structure. This is particularly important for the effect the structure has on the viscosity, an important consideration for commercial products. Dissipative particle dynamics is used to simulate the full phase diagram of SLES in water, including both micellar and lyotropic liquid crystal phases. Phase transitions occur at locations which are in good agreement with experimental data, and we find that these boundaries can shift as a result of varying the number of ethoxy groups. Varying the ethoxy groups has a significant effect on the micellar shape and crystalline spacing, with a reduction leading to more nonspherical micelles and decreased periodic spacing of the hexagonal and lamellar phases. Finally, while typical commercial products contain a distribution of ethoxy groups, computational work tends to focus on simulations containing a single chain length. We show that it is valid to use monodisperse simulations to infer behavior about solutions with a polydisperse chain length, based on its mean molecular length.

Self-assembly of an imidazolium surfactant in aprotic ionic liquids. 2. More than solvents.

As tailorable solvents, the physiochemical properties of ionic liquids can be tuned by the structure of ions. Herein, we investigate the structural effects of ILs on the self-assembly of surfactants. It has been confirmed that the cationic surfactant 1-hexadecyl-3-methylimidazolium bromide (C16mimBr) can self-assemble into micellar and lamellar lyotropic liquid crystal phases in the aprotic ionic liquid (AIL) 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF4). In this work, we explore the aggregation behaviours in AILs with different alkyl chains on the imidazolium group, i.e., 1-propyl-3-methylimidazolium tetrafluoroborate ([Pmim]BF4), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4), 1-hexyl-3-methylimidazolium tetrafluoroborate ([Hmim]BF4) and 1-octyl-3-methylimidazolium tetrafluoroborate ([Omim]BF4). With the increase of the cation chain length, AILs have better solubility of the solvophobic part of the surfactants and hence a weaker driving force for self-assembly. Therefore, the critical micellization concentration of C16mimBr in AILs increases as confirmed by the surface tension and small angle X-ray scattering characterizations. More interesting things happen to the phase behaviours. Besides the micellar and lamellar lyotropic liquid crystal phases, a hexagonal lyotropic liquid crystal phase is formed in [Pmim]BF4 while hexagonal and bicontinuous cubic lyotropic liquid crystal phases are formed in [Bmim]BF4, [Hmim]BF4 and [Omim]BF4. It is surprising to observe richer phase behaviours in solvents of lower cohesive energy. The detailed structural information of various aggregates has been obtained by small-angle X-ray scattering. It is demonstrated that AILs work as not only solvents but also co-surfactants.

Extremely small twist elastic constants in lyotropic nematic liquid crystals

Recent measurements of the elastic constants in lyotropic chromonic liquid crystals (LCLCs) have revealed an anomalously small twist elastic constant compared to the splay and bend constants. Interestingly, measurements of the elastic constants in the micellar lyotropic liquid crystals (LLCs) that are formed by surfactants, by far the most ubiquitous and studied class of LLCs, are extremely rare and report only the ratios of elastic constants and do not include the twist elastic constant. By means of light scattering, this study presents absolute values of the elastic constants and their corresponding viscosities for the nematic phase of a standard LLC composed of disk-shaped micelles. Very different elastic moduli are found. While the splay elastic constant is in the typical range of 1.5 pN as is true in general for thermotropic nematics, the twist elastic constant is found to be one order of magnitude smaller (0.30 pN) and almost two orders of magnitude smaller than the bend elastic constant (21 pN). These results demonstrate that a small twist elastic constant is not restricted to the special case of LCLCs, but is true for LLCs in general. The reason for this extremely small twist elastic constant very likely originates with the flexibility of the assemblies that are the building blocks of both micellar and chromonic lyotropic liquid crystals.

Dynamic light scattering characterization of hard–soft gel phase transition in micellar crystal and comparison with rheological data

We report the dynamic light scattering study of polymer dynamics in the broad vicinity of the hard-soft gel phase transition in micellar crystal. Inside the region of micellar crystal phase with increasing temperature the ratio of the magnitudes of two dynamic modes (slow and fast) monotonously decreases below 54 °C in the hard gel. Above 54 °C in the soft gel it is almost independent of temperature, remaining at a minimum. The method provides phenomenon details inaccessible with conventional rheological method, where simple detection of sharp phase transition is available. We show that the diffusion dynamic mode, which is common for a liquid phase, is also present in a micellar crystal despite the obvious difference in the rheological properties of micellar liquid and micellar crystal. This study demonstrates the advantages of the dynamic light scattering method at micro-scale level.

One-step synthesis of ordered mesoporous silica from olivine and its pore size tailoring

Abstract This study investigates the reaction of sulfuric acid with olivine for the synthesis of long-range ordered mesoporous silica at low-temperatures. By synthesizing mesoporous silica directly from olivine, no high temperatures or reactants with a high CO2 footprint are required. The silicic acids released from olivine are directly guided to form high quality ordered mesoporous silica in a one-step synthesis with the use a micellar liquid crystal prepared from the non-ionic surfactant Triton X-100. The produced mesoporous structures feature specific surface areas of up to 1000 m2/g and narrow pore size distributions around 2–4 nm. In addition, the optimal conditions and the limits of the preparation method are investigated. It was shown that Triton X-100 concentrations between 20 and 30% and olivine contents between 10 and 20% are the optimal conditions. Moreover, the washing process of the product is investigated, resulting in high silica purities. Furthermore, the pore size can be tailored into larger sizes with the addition of heptane. Therefore, olivine can be a green alternative source for the synthesis of well-tailored ordered mesoporous silica.

From single gemini surfactants in water to catanionic mixtures with the bile salt sodium taurodeoxycholate: Extensive micellar solutions, coacervation and liquid crystal polymorphism as revealed by phase behavior studies

Gemini surfactants and bile acid salts are interesting amphiphiles owing to their unconventional and versatile molecular structure, and consequent unusual aggregation properties. In this work, we investigated the phase behavior of binary surfactant/water systems for the cationic bis-quat gemini surfactants 12-2-12 (short spacer) and 12-6-12 (intermediate spacer) and subsequently catanionic mixtures of each gemini surfactant and the bile salt sodium taurodeoxycholate, STDC. The phase diagrams and phase structure were unraveled by polarized light microscopy, differential scanning calorimetry, deuterium NMR and small angle X-ray scattering. The 12-2-12 and 12-6-12 gemini form a micellar-lamellar-cubic and micellar-hexagonal-cubic phase sequence, respectively, denoting the strong influence of the spacer length on the self-assembly properties. Both the catanionic 12-2-12/STDC/water and 12-6-12/STDC/water systems display a rich mesomorphic behavior. One of the most striking features is that the STDC micelles are able to incorporate enormous amounts of gemini surfactant, forming an extremely wide solution region of mixed micellar structures. By contrast, the gemini micelles can only solubilize limited amounts of STDC. The liquid crystalline phases from the binary systems incorporate sizeable amounts of the counterpart surfactant, forming mixed catanionic aggregates at high concentration. Interestingly, the dilute region of both systems is dominated by liquid-liquid phase separation implying the formation of aqueous two-phase micellar systems. The observed features are qualitatively rationalized in terms of packing and electrostatic effects between the surfactants at play.

Synthesis and thermophysical properties of two new protic long-chain ionic liquids with the oleate anion

This work reports the synthesis of 2-hydroxy ethylammonium oleate and bis(2-hydroxy ethyl)ammonium oleate ionic liquids, which have a long aliphatic chain as well as the study of some of their physical properties, in particular the effect of temperature on their density, speed of sound, viscosity, and refractive index. 1H and 13C NMR spectra were used to characterize the chemical structure of the species in concordance with FT-IR spectra. DOSY NMR spectra were used to determine the self-diffusion coefficients of 2-hydroxy ethylammonium oleate ionic liquid, which were consistent with the formation of a lamellar or micellar liquid crystal phase; due the similar structure, a similar aggregation in the bis(2-hydroxy ethyl)ammonium oleate ionic liquid it is expected.

Peptide separations by slab gel electrophoresis in pluronic F127 polymer liquid crystals.

Proteomics and peptidomics could benefit from simple methods for high-resolution separation of oligopeptides analogous to slab gel electrophoresis of proteins. Gels of Pluronic F127 copolymer surfactant were investigated as media for slab gel electrophoresis of oligopeptides using a trypsin digest of myoglobin. Concentrated solutions of Pluronic F127 are fluid at low temperatures (</= 5 degrees C), but become a gel-like micellar liquid crystal upon warming. Nucleic acids are well separated by electrophoresis in these gels as previously shown by Rill and Liu. Good separations of myoglobin tryptic peptides were accomplished by electrophoresis on slab gels of 24% Pluronic F127 or 15% polyacrylamide using the alkaline Laemmli buffer system (without sodium dodecyl sulfate). Labeling of peptides with the succinimidyl ester of Cascade Yellow (CY) prior to electrophoresis allowed sensitive detection with blacklight illumination at 365 nm. Labeled tryptic peptides were identified by matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF)-mass spectrometry. An inverse dependence of electrophoretic mobility on mass of peptides with charge Z = -1 was observed in both media. Two-dimensional (2-D) electrophoresis of myoglobin peptides on polyacrylamide, then on Pluronic media, at pH 8.3 indicated that the primary separation mechanism of most peptides was the same in both media. A few off-diagonal spots indicated that some peptides were preferentially retarded in Pluronic gels, perhaps due to hydrophobic effects. The ease of gel preparation and peptide recovery are advantages of Pluronic F127 gels for oligopeptide electrophoresis. The two media can be combined conveniently for 2-D electrophoresis, providing means to facilitate protein identification and peptidomics.

Effect of the thickness of the hydrophobic bilayer on the helical twisting power in micellar nematic liquid crystals

The effect of the thickness of the hydrophobic bilayer of host nematic phases on the helical twisting power (HTP) of some chiral guests has been investigated. The thickness of the hydrophobic bilayer of the host nematic phases was varied with varying chain length from C8 to C12 in the alkylammonium chloride series and from C12 to C16 in the alkyltrimethylammonium bromide series. The chiral guests were L-alanine esters with C4, C6, C8 and C10 hydrocarbon chains, mandelic acid (MA) and hexahydromandelic acid (HHMA). In the host chiral nematic phases of the alkylammonium chloride series, the HTPs of the chiral alanine ester guests increased as their chain length was increased and the thickness of the hydrophobic bilayer was extended. In both series, similar behaviour was observed for the chiral guests MA and HHMA, but in general, the HTPs of HHMA were larger than those of MA. Accordingly, the chirality transfer from the chiral guest to the achiral micelle was interpreted to be more effective if the chiral guest has a “certain fitting and orientation” in the micelle.

NMR study of the slow dynamics and local structure in lyotropic micellar liquid crystals

The first experimental evidence of director fluctuations in a micellar lyotropic liquid crystal, studied by 1H spin-lattice relaxation rate, is reported. The system is constituted by the ternary mixture: potassium laurate/1-decanol/water. With the aim to test the slow dynamics, the experiment has been performed over a broad range of Larmor frequencies (2 × 103− 6.6 × 106 Hz), using fast field-cycling NMR relaxometry. The results evidence that in the nematic and isotropic mesophases and the poliphasyc region, director fluctuations are responsible for the spin-lattice relaxation dispersion in the low Larmor frequency range ( 105 Hz), two relaxation mechanisms are assigned: i) molecular reorientation by translational diffusion on the micellar surface, and ii) molecular exchange between the micelle and the bulk.

Z-scan measurement of the nonlinear refractive indices of micellar lyotropic liquid crystals with and without the ferrofluid doping

The laser-induced nonlinear optical response of micellar lyotropic liquid crystals in the nematic and isotropic phases is presented. The Z-scan technique is used to measure the amplitude and the sign of the nonlinear refractive indices of the lyotropic mixture. The amplitude of the nonlinear refractive indices $(\ensuremath{\sim}\ensuremath{-}{10}^{\ensuremath{-}6} \mathrm{esu})$ is two orders of magnitude smaller than the one observed in thermotropic liquid crystals. The effect of the ferrofluid doping is also discussed.

Self-screening of Langmuir-Blodgett films by a discotic micellar lyotropic liquid crystal

The influence of the surface treatment on the anisotropic part of the surface tension of a lyotropic discotic micellar nematic liquid crystal doped with ferrofluid is analysed. The treatment involves Langmuir-Blodgett (LB) films deposited on the glass substrates. Textural observations and phase-shift measurements of the transmitted light were carried out. A phenomenological model which allows calculation of the director average orientation in the bulk and at the surfaces is used for interpreting the experimental data. Our results show that the average director alignment depends on the concentration of the magnetic grains of ferrofluid at the glass surfaces. On the contrary, it is practically independent of the nature of the molecule deposited on the glass substrate. According to the model, the flat glass surface, independently of the treatment or the absence of any treatment, stabilizes onto itself a lamellar layer (or bilayer) formed by the amphiphilic molecules of the liquid crystal. This layer screens the effect of the LB film (or of the pure untreated substrate) on the alignment of the lyotropic nematic mesophase.

Determination of elastic constants and rotational viscosity of micellar liquid crystals by conductivity measurements

An experimental study of a lyotropic nematic liquid crystal (CsPFO in a water solution) confined to a cylindrical tube is presented. The structure of the sample is controlled by a magnetic field. A small ac voltage is applied along the tube for conductivity measurements. Elastic constants and conductivity constants of the material are determined from static measurements at different magnetic fields. The method is also used to determine the rotational viscosity γ1 from dynamic response under sudden changes of the magnetic field. The dynamic measurements are analyzed in the framework of the Leslie–Ericksen theory. It is shown that although the dynamic behavior can be qualitatively explained with a single effective rotational viscosity γ1, back flow effects have to be taken into account should the analysis yield consistent values for γ1. The results obtained by this method agree well with other experimental studies.

Structural Polymorphism of Amphiphilic Copolymers: Six Lyotropic Liquid Crystalline and Two Solution Phases in a Poly(oxybutylene)-b-poly(oxyethylene)−Water−Xylene System

Six lyotropic liquid crystalline and two solution (equilibrium) phases can be formed (at 25 °C) by an amphiphilic diblock copolymer composed of poly(ethylene oxide) and poly(butylene oxide) in the presence of water (selective solvent for PEO) and “oil” (p-xylene, a selective solvent for PBO). Oil-in-water (“normal”) micellar solution (L1), micellar cubic liquid crystals (I1), hexagonal liquid crystals (H1), and a lamellar structure (Lα) are formed with an increase of the copolymer concentration along the oil-lean side of the ternary phase diagram. A micellar solution (L2), micellar cubic liquid crystals (I2), hexagonal liquid crystals (H2), and bicontinuous cubic liquid crystals (V2), all of the water-in-oil (“reverse”) morphology, are formed in the water-lean part of the phase diagram with increasing copolymer concentration and in the presence of some water. The oil-in-water [water-in-oil] structures can accommodate up to 0.33 [0.40] g of oil [water]/g of copolymer. The formation of a reverse micellar cu...

Experimental techniques for studying order on different length scales in micellar liquid crystals

Fluorosurfactants are described which, on solution in water, form discrete discotic micelles to relatively high concentrations. With increasing concentration, these micelles undergo a sequence of disorder-order transitions to form, first, a nematic phase and, secondly, a smectic one. The ordering of the micelles in these phases is described and the experimental techniques used to characterise it are outlined.

Slow relaxation effects at the second-order nematic to lamellar smectic phase transition in micellar liquid crystals.

The second-order nematic to lamellar-smectic phase transition is studied in the micellar systems decylammonium chloride--${\mathrm{NH}}_{4}$Cl--water and cesium perfluoro-octanoate--${\mathrm{D}}_{2}$O, by measuring the electric conductivity of aligned samples and the density as functions of temperature. In the nematic range, for temperatures higher than about 1 \ifmmode^\circ\else\textdegree\fi{}C above the transition, the data on cooling and heating coincide within experimental error. Closer to the lamellar-smectic phase significant differences occur between the values observed on heating and cooling. For the density, the difference between the two values in the smectic range is several ${10}^{\mathrm{\ensuremath{-}}4}$ g/${\mathrm{cm}}^{3}$. The processes responsible for the observed behavior have long relaxation times, estimated in the order of several hours to several days. A simple rearrangement of the micelles into layers should have relaxation times comparable to the relaxation times of the nematic to smectic-A phase transition in thermotropic liquid crystals; however, no such relaxation effect has been reported in thermotropic liquid crystals, even at very fast heating and cooling rates. Accordingly, the slow relaxation suggests significant change in the micellar structure at the nematic to lamellar-smectic phase transition, probably a divergence in the aggregation number, i.e., a transition to continuous lamellae. The observed long relaxation times reflect the slow approach to an equilibrium distribution of defects after the formation of the lamellae.

Effects of counterion identity on a micellar liquid crystal

Magnetic birefringence measurements are reported for mixtures of cesium- and rubidium-perfluorooctanoate in water above the nematic-isotropic phase transition temperature. Despite significant differences in the phase diagrams between the two materials, it was found that both the supercooling temperature T ∗ of the isotropic phase, as well as the micelle aggregation number, are quite insensitive to relative surfactant concentration.

New induced cholesteric micellar liquid crystals. Characterization and pitch determination

New induced cholesteric micellar liquid crystals have been prepared based on potassium laurate and sodium decyl sulfate. The chiral molecule D-glucose was used as dopant. The inverse pitch length is found to be proportional to the added guest. The twist induced by the dopant strongly depends on the nature of the detergent, the principal component of the mesophases. The mesophases are characterized on the basis of the microscopic textures. Deuterium NMR spectra in D2O shows positive diamagnetic anisotropy for these cholesteric mesophases.

Inequivalent quadrupolar couplings of the deuterium‐exchanged nitrogen in micellar liquid crystals

AbstractIn deuterium NMR, two inequivalent quadrupolar couplings of the deuterium‐exchanged nitrogen in a hydrocarbon chain near a chiral carbon atom have been observed in the nematic and cholesteric mesophases. The two quadrupolar doublets arise from the ND segments of the amphiphile and are interpreted as being due to the intramolecular motion in the amphiphile chain about the C—N bond. Different peak intensity of the two doublets suggests that one conformation is more stable than the other.