From the microscopic to the mesoscopic properties of lyotropic reverse hexagonal liquid crystals

In the present study we aimed to explore a correlation between the microstructural properties of the lyotropic reverse hexagonal phase (HII) of the GMO/tricaprylin/phosphatidylcholine/water system and its mesoscopic structure. The mesoscopic organization of discontinuous and anisotropic domains was examined, in the native state, using environmental scanning electron microscopy. The topography of the HII mesophases was imaged directly in their hydrated state, as a function of aqueous-phase concentration and composition, when a proline amino acid was solubilized into the systems as a kosmotropic (water-structure maker) guest molecule. The domain structures of several dozen micrometers in size, visualized in the environmental scanning electron microscopy, were found to possess fractal characteristics, indicating a discontinuous and disordered alignment of the corresponding internal water rods on the mesoscale. On the microstructural level, SAXS measurements revealed that as water content (Cw) increases the characteristic lattice parameter of the mesophases increases as well. Using the water concentration as the mass measure of the mixtures, a scaling relationship between the lattice parameter and the concentration was found to obey a power law whereby the derived fractal dimension was the relevant exponent, confirming the causal link between the microscopic and mesoscopic organizations. The topography of the HII mesophase was found to be affected by the microstructural parameters and the composition of the samples. Thermal analysis experiments involving these systems further confirmed that the behavior of water underpins both microscopical and mesoscopic features of the systems. It was shown that both the swelling of the lattice parameter and the mesoscopic domains is correlated to the bulk water concentration in the water rods.     

Solubilization of nutraceuticals into reverse hexagonal mesophases

The solubilization of four bioactive molecules with different polarities, in three reverse hexagonal (HII) systems has been investigated. The three HII systems were a typical reverse hexagonal composed of glycerol monooleate (GMO)/tricaprylin/water and two fluid hexagonal systems containing either 2.75 wt % Transcutol or ethanol as a fourth component. The phase behavior of the liquid crystalline phases in the presence of ascorbic acid, ascorbyl palmitate, D-alpha-tocopherol and D-alpha-tocopherol acetate were determined by small-angle X-ray scattering (SAXS) and optical microscopy. Differential scanning calorimetry (DSC) and Fourier-transform infrared (FT-IR) techniques were utilized to follow modifications in the thermal behavior and in the vibrations of different functional groups upon solubilizing the bioactive molecules. The nature of each guest molecule (in both geometry and polarity) together with the different HII structures (typical and fluids) determined the corresponding phase behavior, swelling or structural transformations and its location in the HII structures. Ascorbic acid was found to act as a chaotropic guest molecule, localized in the water-rich core and at the interface. The AP was also a chaotropic guest molecule with its head located in the vicinity of the GMO headgroup while its tail embedded close to the surfactant tail. D-alpha-tocopherol and D-alpha-tocopherol acetate were incorporated between the GMO tails; however, the D-alpha-tocopherol was located closer to the interface. Once Transcutol or ethanol was present and upon guest molecule incorporation, partial migration was detected.     

SYNERGETIC EFFECT OF SUCROSE AND ETHANOL ON FORMATION OF TRIGLYCERIDE MICROEMULSIONS

The phase behavior of soybean oil, a nonionic surfactant (ethoxylated monodiglycerides) and an aqueous phase of water containing ethanol, and sucrose was investigated at 35 and 40°C. A minimum concentration of 20 wt% ethanol was required for the formation of isotropic solutions. Addition of sucrose to the aqueous phase decreased the amount of ethanol required to form these solutions. The solubilization mechanism of the oil was investigated by small angle x-ray diffraction and polarized light microscopy. A stable lamellar liquid crystalline phase was formed for a mixture of 75/25 surfactant/sucrose solution (2.5 wt% sucrose). This phase was destabilized with increased concentrations of sucrose and liquid crystalline phases having hexagonal structures were favored at 8.75 wt% sucrose. At a ratio of 55/45 wt% of surfactant/sucrose solution (9 wt% sucrose) hexagonal structures were formed and could be destabilized or destroyed by addition of ethanol. The concept of stabilization and destabilization of liquid crystalline mesophases was applied to the solubilization of triglycerides in aqueous solutions. Two microemulsion regions were identified; oil-in-water (L₁) and water-in-oil (L₂) in systems containing soybean oil, ethoxylated monodiglycerides, and 20 wt% ethanol solution. At 55/45 wt% surfactant/20 wt% ethanol solution,7.5 wt% of soybean oil was solubilized. Addition of 10, 20, and 30 wt% sucrose, at the same ratio of surfactant to ethanol solution, increased the solubility of the oil to 9, 13.5, and 18 wt% respectively. In addition, the size of the L₁ phase increased and moved to the aqueous corner of the phase diagram and the size of the L₂ phase decreased.     

Interactions of surfactants in nonionic/anionic reverse hexagonal mesophases and solubilization of α-chymotrypsinogen A

In an attempt to form H_II mesophases at room temperature we prepared lyotropic liquid crystals with two surfactants of the same lipophilic tails (glycerol monooleate, GMO, and oleyl lactate, OL) but differing in the size and charge of the headgroups.Increasing OL concentration significantly affected the hydration of the headgroups and subsequently the lipids packing. At low OL content the cubic mesophase was formed, while at higher OL contents the formation of hexagonal mesophase was favored. It was assumed that OL competed on the water binding, tuning the headgroups’ curvature and the packing parameter inducing the formation of reverse hexagonal mesophase. It was detected that cubic mesophase transformed upon heating to hexagonal structures. The hexagonal mesophases, which were formed both immediately after preparation and after aging, remained stable at elevated temperatures.α-Chymotrypsinogen was solubilized into the obtained LLCs at relatively high concentration (up to 1 wt%). The lattice parameter of the host LLCs exhibited a decrease as a function of the protein content. This process was assigned to partial dehydration of the GMO polar moieties in favor to CTA hydration.Generally speaking, the present study indicated that adding anionic to nonionic lipid is highly beneficial to gain additional compositional and structural characteristics of LLCs.     

Alpha-tocopherol-induced hexagonal HII phase formation in egg yolk phosphatidylcholine membranes

The effect of alpha-tocopherol and its acetate on the membrane structure of egg yolk phosphatidylcholine (egg PC) dispersions was investigated using phosphate-31 nuclear magnetic resonance (31P-NMR) and small-angle X-ray diffraction. The incorporation of alpha-tocopherol into egg PC dispersions induced a change in the 31P-NMR spectrum from a multilamellar bilayer line shape to a hexagonal HII one. The phase transition by alpha-tocopherol was also confirmed by small-angle X-ray diffraction analysis. The amount of hexagonal HII phase increased with increase in concentration of alpha-tocopherol. Egg PC dispersions containing a molar ratio of 0.8 of alpha-tocopherol gave a 31P-NMR spectrum of an approximately hexagonal HII type at 37 degrees C. The amount of hexagonal HII phase increased with increasing temperature, indicating that the alpha-tocopherol-induced phase transition is thermotropic and that the transition temperature of egg PC membranes from the lamellar to the hexagonal HII phase is lowered by alpha-tocopherol. The incorporation of alpha-tocopherol acetate did not induce any phase transition. This fact indicates that the hydroxyl group of alpha-tocopherol may play an important role in the hexagonal HII phase formation of egg PC dispersions.     

Low viscosity reversed hexagonal mesophases induced by hydrophilic additives

This study reports on the formation of a low viscosity H(II) mesophase at room temperature upon addition of Transcutol (diethylene glycol mono ethyl ether) or ethanol to the ternary mixture of GMO (glycerol monooleate)/TAG (tricaprylin)/water. The microstructure and bulk properties were characterized in comparison with those of the low viscosity HII mesophase formed in the ternary GMO/TAG/water mixture at elevated temperatures (35-40 degrees C). We characterized the role of Transcutol or ethanol as inducers of disorder and surfactant mobility. The techniques used were rheology, differential scanning calorimetry (DSC), wide- and small-angle X-ray scattering (WAXS and SAXS, respectively), NMR (self-diffusion and (2)H NMR), and Fourier transform infrared (FTIR) spectroscopies. The incorporation of either Transcutol or ethanol induced the formation of less ordered HII mesophases with smaller domain sizes and lattice parameters at room temperature (up to 30 degrees C), similar to those found for the GMO/TAG/water mixture at more elevated temperatures (35-40 degrees C). On the basis of our measurements, we suggest that Transcutol or ethanol causes dehydration of the GMO headgroups and enhances the mobility of the GMO chains. As a result, these two small molecules, which compete for water with the GMO polar headgroups, may increase the curvature of the cylindrical micelles and also perhaps reduce their length. This results in the formation of fluid H(II) structures at room temperature (up to 30 degrees C). It is possible that these phases are a prelude to the H(II)-L(2) transformation, which takes place above 35 degrees C.     

Structural properties of lecithin based reverse hexagonal (HII) liquid crystals and in vitro release of dihydromyricetin

Dihydromyricetin (DMY) was encapsulated to lecithin based reverse hexagonal (H_II) liquid crystals to improve its solubility limitation. PEG 400 was used as the representative oil phase. The H_II mesophases were identified by means of polarized light microscopy (POM) and small angle X-ray scattering (SAXS). The DMY was solubilized in interface layer inferred from the increase of the interfacial area of per surfactant a_s and the infrared spectra. The hexagonal samples showed highly elastic Maxwell properties and shear thinning properties indicated by their rheological spectra. Moreover with the decrease of PEG 400 content, the internal structure of samples apparently becomes more stable, as indicated by the increase in the storage and loss moduli and the decrease in a_s. Oleic acid enhances the viscoelasticity of sample and increases the release stability for DMY under acidic conditions. The in vitro release of DMY in H_II matrices showed that carriers have an ideal sustained release effect. The release of DMY was controlled by concentration diffusion.     

Columnar and interdigitated structures from apolar discotic mesogens with radial dipoles: a Monte Carlo study

We have simulated, using a Monte Carlo method, a system of apolar Gay-Berne discotic particles without dipoles and with two or three dipoles symmetrically embedded in the disc. The dipoleless system can form nematic and hexagonal columnar mesophases. Adding two in-plane antiparallel radial dipoles has the effect of destabilizing the columnar ordering, in favour of a fully interdigitated arrangement of the molecular stacks, with strong correlations in the plane perpendicular to the director and large biaxiality. No columnar phase is observed for the systems with three planar radial dipoles with threefold symmetry up to very low temperatures. Finally, on decreasing the strength of the three dipoles, a columnar non-interdigitated phase is observed.     

Water Solubilization in Nonionic Microemulsions Stabilized by Grafted Siliconic Emulsifiers

Microemulsions containing octanol, decanol, or dodecanol as the oil phase and oligomeric, grafted nonionic amphiphiles based on ethoxylated polymethylsiloxanes (Silwets) have been studied. It was demonstrated that significant amounts of water can be solubilized only when the hydrophobic siliconic backbone is very short (trimers). The water solubilization was evaluated using SAXS, DSC, and conductivity measurements. It was found that up to 40 wt% of water can be solubilized in dodecanol and Silwet L-7607 (MW 1000 and 75 wt% ethylene oxide (EO)). Surprisingly, no free water was detected in the aggregate core. All the solubilized water was confined in the vicinity of the interphasal region and froze at -10 degrees C and below. Up to three molecules of water can be associated with each EO headgroup. Based on SAXS measurements, the structural units of the microemulsions were interpreted to be lamellar-like, a form previously found for the related monomeric microemulsions. Copyright 2001 Academic Press.     

The role of glycerol and phosphatidylcholine in solubilizing and enhancing insulin stability in reverse hexagonal mesophases

The potential of reverse hexagonal mesophases based on monoolein (GMO) and glycerol (as cosolvent) to facilitate the solubilization of proteins, such as insulin was explored. H(II) mesophases composed of GMO/decane/water were compared to GMO/decane/glycerol/water and GMO/phosphatidylcholine (PC)/decane/glycerol/water systems. The stability of insulin was tested, applying external physical modifications such as low pH and heat treatment (up to 70°C), in which insulin is known to form ordered amyloid-like aggregates (that are associated with several neurodegenerative diseases) with a characteristic cross β-pleated sheet structure. The impact of insulin confinement within these carriers on its stability, unfolding, and aggregation pathways was studied by combining SAXS, FTIR, and AFM techniques. These techniques provided a better insight into the molecular level of the "component interplay" in solubilizing and stabilizing insulin and its conformational modifications that dictate its final aggregate morphology. PC enlarged the water channels while glycerol shrank them, yet both facilitated insulin solubilization within the channels. The presence of glycerol within the mesophase water channels led to the formation of stronger hydrogen bonds with the hosting medium that enhanced the thermal stability of the protein and remarkably affected the unfolding process even after heat treatment (at 70°C for 60 min).     

Use of the ternary phase diagram of a mixed cationic/glucopyranoside surfactant system to predict mesostructured silica synthesis

Mixed surfactant systems have the potential to impart controlled combinations of functionality and pore structure to mesoporous metal oxides. Here, we combine a functional glucopyranoside surfactant with a cationic surfactant that readily forms liquid crystalline mesophases. The phase diagram for the ternary system CTAB/H(2)O/n-octyl-beta-D-glucopyranoside (C(8)G(1)) at 50 degrees C is measured using polarized optical microscopy. At this temperature, the binary C(8)G(1)/H(2)O system forms disordered micellar solutions up to 72 wt% C(8)G(1), and there is no hexagonal phase. With the addition of CTAB, we identify a large area of hexagonal phase, as well as cubic, lamellar and solid surfactant phases. The ternary phase diagram is used to predict the synthesis of thick mesoporous silica films via a direct liquid crystal templating technique. By changing the relative concentration of mixed surfactants as well as inorganic precursor species, surfactant/silica mesostructured thick films can be synthesized with variable glucopyranoside content, and with 2D hexagonal, cubic and lamellar structures. The domains over which different mesophases are prepared correspond well with those of the ternary phase diagram if the hydrophilic inorganic species is assumed to act as an equivalent volume of water.     

Thermally induced fluid reversed hexagonal (H(II)) mesophase

In the present study we characterized the microstructures of the Lc and HII phases in a glycerol monooleate (GMO)/tricaprylin (TAG)/water mixture as a function of temperature. We studied the factors that govern the formation of a low-viscosity HII phase at relatively elevated temperatures (>35 degrees C). This phase has very valuable physical characteristics and properties. The techniques used were differential scanning calorimetry (DSC), wide- and small-angle X-ray scattering (WAXS and SAXS, respectively), NMR (self-diffusion and (2)H NMR), and Fourier transform infrared (FTIR) spectroscopies. The reverse hexagonal phase exhibited relatively rapid flow of water in the inner channels within the densely packed cylindrical aggregates of GMO with TAG molecules located in the interstices. The existence of two water diffusion peaks reflects the existence of both mobile water and hydration water at the GMO-water interface (hydrogen exchange between the GMO hydroxyls and water molecules). Above 35 degrees C, the sample became fluid yet hexagonal symmetry was maintained. The fluidity of the HII phase is explained by a significant reduction in the domain size and also perhaps cylinder length. This phenomenon was characterized by higher mobility of the GMO, lower mobility of the water, and a significant dehydration process.     

Phase behavior of lipid-based lyotropic liquid crystals in presence of colloidal nanoparticles

We have investigated the microstructure and phase behavior of monoglyceride-based lyotropic liquid crystals in the presence of hydrophilic silica colloidal particles of size comparable to or slightly exceeding the repeat units of the different liquid crystalline phases. Using small angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC), we compare the structural properties of the neat mesophases with those of the systems containing silica colloidal particles. It is found that the colloidal particles always macrophase separate in inverse bicontinuous cubic phases of gyroid (Ia3d) and double diamond (Pn3m) symmetries. SAXS data for the inverse columnar hexagonal phase (H(II)) and lamellar phase (L(α)) suggest that a low volume fraction of the nanoparticles can be accommodated within the mesophases, but that at concentrations above a given threshold, the particles do macrophase separate also in these systems. The behavior is interpreted in terms of the enthalpic and entropic interactions of the nanoparticles with the lamellar and hexagonal phases, and we propose that, in the low concentration limit, the nanoparticles are acting as point defects within the mesophases and, upon further increase in concentration, initiate nucleation of nanoparticles clusters, leading to a macroscopic phase separation.     

Effect of quenching on the structural and dynamic properties of non-aqueous lyotropic mesophases

We focused to highlight the effect of quenching on the development and ordering of non-aqueous lyotropic liquid crystalline phases. Lyotropic mesophases are prepared from binary mixtures of sodium dodecyl sulphate and ethylene glycol at varying concentrations 30:70 and 50:50 wt%. The obtained self-assembled phases are characterised by X-ray diffraction, polarisation optical microscopy, differential scanning calorimetry and dielectric spectroscopy to evaluate the structural, optical, thermal and dielectric behaviours. Structural and textural measurements confirmed mesomorphic and crystalline phases for both mixtures. Calorimetric study gives insight about the growth of new phases at ≈335 K and isotropic temperatures of these mixtures. Both the mixtures are quenched from 335 K to the 303 K to analyse the effect of quenching on the structure and ordering of mesophases. We noticed well-defined hexagonal liquid crystalline mesophases for both concentrations after quenching at 303 K. Dielectric and relaxation behaviours of quenched mesophases were also examined. Higher capacitance and dielectric strength are noticed for quenched mixtures. The application prospective of such phases is also discussed.     

Determination of water content in internally self-assembled monoglyceride-based dispersions from the bulk phase

We determined the water intake of internally structured oil-loaded monoglyceride-based dispersions. This was possible through small-angle X-ray scattering (SAXS) experiments on the corresponding bulk mesophases because the structural parameters in full hydration conditions are identical to those of the dispersed particles. From low water contents to full hydration, the bulk phases depend strongly on the amount of oil. At room temperature in excess water and with increasing oil concentration, successive bicontinuous cubic, reverse hexagonal, micellar cubic, and inverse micellar-type isotropic fluid phases are found. The solubilized water is determined as a function of the oil content for each phase, and it is found to range from 5-33 wt %.     

嵌段共聚物反介观液晶相新方法合成多级有序排列的二氧化硅纳米棒

在非极性溶剂体系中,开辟了利用反相液晶法制备多级有序纳米材料的新方法.利用嵌段共聚物表面活性剂在非极性溶剂中形成反相胶束,无机硅物种可以进入胶束的内部,在溶剂挥发后,有机-无机物种进一步组装成为反相六方液晶相.除去模板剂后即制备出尺寸规则的二氧化硅纳米棒材料.由于嵌段高分子的作用,六方排列的二氧化硅纳米棒进一步排列成层状结构(层间距约150nm).通过选择表面活性剂及改变其浓度,纳米棒的尺寸可以在9～15nm范围内调变.该反应途径对于合成其它尺寸均一、多级有序排列的纳米棒材料是非常有意义的.     

The influence of sodium chloride and urea on chromonic liquid crystals formed by CI Acid Red 266

Chromonic liquid crystals are currently receiving renewed interest with particular attention on the Edicol Sunset Yellow (ESY)/water system, which forms columnar nematic and hexagonal phases. CI Acid red 266 is structurally fairly similar to ESY and also forms columnar nematic and hexagonal phases but at much lower concentrations (>1%). In this study, we have examined the influence of sodium chloride and urea on chromonic liquid crystals formed by CI acid red 266. The techniques employed were polarising microscopy, X-ray diffraction and ²H NMR. Sodium chloride moves the concentration at which mesophases form to higher values. Once formed, the mesophases are stable to slightly higher temperatures. Screening of the interstack electrostatic repulsions by added electrolyte appears to be responsible for the changes. Urea can be added in fairly large concentrations (up to 25 wt%) without significant changes in mesophase stability. X-ray diffraction measurements show that there is little change in the aggregate structure with added urea. NMR measurements on urea and water ordering show that urea has much larger order parameters than water. Both order parameters are much smaller than values reported for ESY, but this is simply because of the lower dye concentrations. The larger order parameters for urea appear to arise from some intercalation of urea into the acid red 266 stacks. There is no evidence for changes in ‘water structure’ by the addition of urea.     

Phosphatidylcholine embedded micellar systems: enhanced permeability through rat skin

Micellar and microemulsion systems are excellent potential vehicles for delivery of drugs because of their high solubilization capacity and improved transmembrane bioavailability. Mixtures of propylene glycol (PG) and nonionic surfactants with sodium diclofenac (DFC) were prepared in the presence of phosphatidylcholine (PC) as transmembrane transport enhancers. Fully dilutable systems with maximum DFC solubilization capacity (SC) at pH 7 are presented. It was demonstrated that the concentrates underwent phase transitions from reverse micelles to swollen reverse micelles and, via the bicontinuous transitional mesophase, into inverted O/W microstructures. The SC decreases as a function of dilution. DFC transdermal penetration using rat skin in vitro correlated with SC, water content, effect of phospholipid content, presence of an oil phase, and ethanol. Skin penetration from the inverted bicontinuous mesophase and the skin penetration from the O/W-like microstructure were higher than that measured from the W/O-like droplets, especially when the micellar system containing the nonionic surfactant, sugar ester L-1695, and hexaglycerol laurate. PC embedded within the micelle interface significantly increased the penetration flux across the skin compared to micellar systems without the embedded PC at their interface. Moreover, the combination of PC with HECO40 improved the permeation rate (P) and shortened the lag-time (T(L)).     

Cryo-field emission scanning electron microscopy imaging of a rigid surfactant mesophase

The aerosol OT/ L-alpha-phosphatidylcholine/isooctane/water system forms a rigid mesophase that transitions from reverse hexagonal to multilamellar in structure at specific water contents. This study shows that characteristics of ordered liquid-crystalline mesophases can be distinguished and imaged in high clarity using cryo-field emission scanning electron microscopy (cryo-FESEM). The reverse hexagonal phase consists of bundles of long cylinders, some with length scales of over 2 microm, that are randomly oriented as part of a larger domain. Cryo-imaging allows the visualization of the intercylinder spacings and the details of transitions from one domain to another. The multilamellar structured mesophase consists of spherical vesicles of 100 nm to 10 microm in diameter, with intervening noncrystalline isotropic regions. Coexistence regions containing both the reverse hexagonal and lamellar structures are also observed in the transition from the reverse hexagonal to the lamellar phase. These results complement and qualitatively verify our earlier studies with small-angle neutron scattering, high-field nuclear magnetic resonance spectroscopy, and freeze-fracture direct imaging transmission electron microscopy. The information is useful in understanding materials templating in these rigid systems.