Calorimetric Investigation of Conversion to The Most Stable Subgel Phase of Phosphatidylethanolamine-water System

The conversion of either the gel or the liquid crystal phase to the most stable subgel phase in dimyristoylphosphatidylethanolamine (DMPE)-water system at a water content of 25 mass% was studied by differential scanning calorimetry and isothermal calorimetry. The calorimetric experiments were performed for two samples depending on whether the thermal treatment of cooling to -60°C was adopted or not. In DSC of varying heating rate, exothermic peaks due to the partial conversion were observed at either temperatures just below the gel-to-liquid crystal phase transition at 50°C or temperatures where the liquid crystal phase is present as a metastable state. The enthalpies of conversion for both the gel and the liquid crystal phase were measured directly by the isothermal calorimetries at 47 and 53°C, respectively, where the exothermic peaks were observed by DSC and were compared with the enthalpy difference between the gel and subgel phases and that between the liquid crystal and subgel phases. This revised version was published online in July 2006 with corrections to the Cover Date.     

Role of liquid crystal in the emulsification of a gel emulsion with high internal phase fraction

A gel emulsion with high internal oil phase volume fraction was formed via an inversion process induced by a water–oil ratio change. The process involved the formation of intermediate multiple emulsions prior to inversion. The multiple emulsions contain a liquid crystal formed by the surfactant with water; this was both predicted by the equilibrium phase diagram as well as observed using polarization microscopy. These multiple emulsions were more stable compared to alternative multiple emulsions prepared in the same way with a surfactant that does not form liquid crystals. While the formation of a stable intermediate multiple emulsion may not be a necessary condition for the inversion to occur, the transitional presence of a liquid crystal proved to be a significant factor in the stabilization of the intermediate multiple emulsions. The resulting gel emulsion contained a small fraction of the liquid crystal according to the phase diagram, and it exhibited excellent stability.     

A novel viscoelastic system from a cationic surfactant and a hydrophobic counterion

The phase behavior of 2-hydroxy-1-naphthoic acid (2,1-HNC) mixed with cetyltrimethylammonium hydroxide (CTAOH) is reported. This novel system is compared with the published one of 3-hydroxy-2-naphthoic acid (3,2-HNC) mixed with CTAOH. We investigated the phase behavior and properties of the phases in aqueous solutions of 100 mM CTAOH with 2,1-HNC. In both systems a multilamellar vesicle phase is formed when the naphthoate/surfactant ratio (r) reaches unity. When an increasing amount of 2,1-HNC is mixed with a micellar solution of 100 mM CTAOH, an isotropic low-viscous micellar solution, a viscoelastic gel (consisting of rodlike micelles), a turbid region (two-phase region), and a viscoelastic liquid crystalline gel (consisting of multilamellar vesicles, MLV) were formed. The vesicular phase is highly viscoelastic and has a yield stress value. The transition from the micellar to the vesicle phase occurs for CTAOH/2,1-HNC over a two-phase region, where micelles and vesicles coexist. Also it was noticed that 2,1-HNC is dissolved in 100 mM CTAOH until the naphthoate/surfactant ratio reaches approximately 1.5, and the liquid crystalline phases were found to change their color systematically when they were viewed between two crossed polarizers. The vesicles have been characterized by differential interference contrast microscopy, freeze-fracture electron microscopy, and cryo-electron microscopy (cryo-TEM). The vesicles were polydisperse and their diameter ranged from 100 to 1000 nm. The interlamellar spacing between the bilayers was determined with small angle neutron scattering and agrees with the results from different microscopical methods. The complex viscosity rises by six orders of magnitude when rodlike micelles are formed. The complex viscosity decreases again in the turbid region, and then rises approximately six orders of magnitude above the water viscosity. This second rising is due to the formation of the liquid crystalline MLV phase.     

Formation and properties of reverse micellar cubic liquid crystals and derived emulsions

The structure of the reverse micellar cubic (I2) liquid crystal and the adjacent micellar phase in amphiphilic block copolymer/water/oil systems has been studied by small-angle X-ray scattering (SAXS), rheometry, and differential scanning calorimetry (DSC). Upon addition of water to the copolymer/oil mixture, spherical micelles are formed and grow in size until a disorder-order transition takes place, which is related to a sudden increase in the viscosity and shear modulus. The transition is driven by the packing of the spherical micelles into a Fd3m cubic lattice. The single-phase I2 liquid crystals show gel-like behavior and elastic moduli higher than 104 Pa, as determined by oscillatory measurements. Further addition of water induces phase separation, and it is found that reverse water-in-oil emulsions with high internal phase ratio and stabilized by I2 liquid crystals can be prepared in the two-phase region. Contrary to liquid-liquid emulsions, both the elastic modulus and the viscosity decrease with the fraction of dispersed water, due to a decrease in the crystalline fraction in the sample, although the reverse emulsions remain gel-like even at high volume fractions of the dispersed phase. A temperature induced order-disorder transition can be detected by calorimetry and rheometry. Upon heating the I2 liquid crystals, two thermal events associated with small enthalpy values were detected: one endothermic, related to the "melting" of the liquid crystal, and the other exothermic, attributed to phase separation. The melting of the liquid crystal is associated with a sudden drop in viscosity and shear moduli. Results are relevant for understanding the formation of cubic-phase-based reverse emulsions and for their application as templates for the synthesis of structured materials.     

Thermotropic phase behaviour of long-chain alkylmaltosides

The thermotropic phase behaviour and phase structure of crystalline and non-crystalline n-tetradecyl-beta-D-maltoside (C14G2) and n-hexadecyl-beta-D-maltoside (C16G2) have been investigated by means of differential scanning calorimetry and X-ray techniques. Upon lyophilisation, both compounds form a solid, lamellar phase comprising disordered head groups and hexagonally packed alkyl chains that are suggested to be tilted and interdigitated. This ordered lamellar phase melts into a metastable lamellar liquid crystal, which re-crystallises to a high-temperature crystalline polymorph comprising interdigitated, non-tilted alkyl chains. Remarkably, the high-temperature polymorph of C14G2 has the same melting point as that of C16G2, namely 105 degrees C for both surfactants. A low-temperature polymorph of anhydrous C14G2 crystallises from water at room temperature, whereas the hemihydrate of C14G2 crystallises at 6 degrees C from water, or from chloroform containing trace water. X-ray data suggest both these crystalline modifications to comprise interdigitated and tilted alkyl chains.     

Preparation and characteristics of multiple emulsions containing liquid crystals

In this paper, multiple emulsions containing liquid crystals were prepared successfully and the influence of formulation parameters on the formation mechanism was studied. Moreover, differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS) spectra analysis and stability analysis were used to characterise the property of them. The results showed that the chemical structure of water-in-oil (W/O) emulsifiers directly impacted on the formation of multiple structure, but the effect on the formation of liquid crystal structure was negligible. With the gap of the polarity between inner and outer liquid oils decreased, both multiple structure and liquid crystal structure were harder to form. The content of sodium chloride in internal aqueous phase, which should be neither too high nor too low, has great impact on the formulation of multiple structure. It was easier to form two structures simultaneously when the carbon chain length of fatty alcohols was closer to that of emulsifier C₂₂ alkyl polyglucoside (202). DSC elucidated the phase transitions of water in the liquid crystal layer and the W/O emulsions. SAXS indicated that the liquid crystal orientation was lamellar. The stability analysis showed that the presence of liquid crystal structure had a significant contribution to the stability of the multiple emulsions.     

Biocompatible lipidic formulations: phase behavior and microstructure

Biocompatible systems formulated for use in the food, cosmetic, and pharmaceutical fields are characterized. Ternary phase diagrams of mixtures of natural lipids (glycerol trioleate, glycerol monooleate, diglycerol monooleate, and lecithin) and water were investigated by means of optical microscopy in polarized light and by multinuclear NMR spectroscopy. All systems showed a microemulsion region at high oil content and a large area of coexistence of two liquid crystalline (hexagonal and lamellar) phases. 1H and 13C NMR self-diffusion measurements were used to characterize microstructural features of the microemulsions. On water dilution, the two-phase liquid crystalline region transforms into a creamy emulsion area where the droplets of water are stabilized by both the lamellar and the hexagonal phases, as indicated by 2H NMR measurements. Due to the very effective dispersing action of the two liquid crystalline phases, these emulsions show a high stability toward phase separation.     

LYOTROPIC LIQUID CRYSTALS IN BINARY SYSTEMS N-ALKYL GLYCOSIDES/WATER*

The formation of lyotropic mesophases (liquid crystals) in four binary systems n-alkyl glycosides/water was examined in dependence on surfactant concentration, temperature and the chain lengths (alkyl = heptyl, octyl, nonyl, decyl). The binary phase diagrams were established and the enthalpies of phase transitions were measured. The following phase transitions were detected by texture observation and calorimetry: hexagonal phase H, lamellar phase L, cubic phase Q, gel phase G and crystalline phase C. The positions of the corresponding regions of these phases in the phase diagram were determined. Sequence of phases and the localization of the phase regions were depended on the chain length of the alkyl group. So in the binary system n-decyl-β-D-glucoside/water the H-phase was not observed.     

Effect of temperature on dynamic rheological behavior of discontinuous cubic liquid crystal

The dynamic rheological properties of discontinuous cubic liquid crystal, formed by nonionic surfactant C(12-14)E(12), were investigated in the discrete and continuous patterns of raising temperature. In the discrete pattern, the discontinuous cubic phase appears in two types of viscoelastic behaviors under the melting points of cubic phase: elastic gel and viscoelastic liquid. When the discontinuous cubic phase begins to melt, it has the weak polymer-like viscoelasticity. Temperature and shear frequency have completely different effects on the ratio of viscous and elastic components of samples in these three states. At low temperature, the samples dominate in elasticity and temperature and shear frequency has hardly any effect on viscoelasticity. At moderate temperature, its ratio of viscous and elastic components increases with increasing temperature and decreasing shear frequency. At the vicinity of the melting point of cubic liquid crystal, the cubic liquid crystal appears to have almost equaled viscous and elastic component; shear has obvious effect on the ratio of viscoelasticity at low frequency. The results from the continuous pattern of raising temperature are consistent with those from the discrete pattern.     

The electro-optical response of nematic emulsions

Switchable nematic emulsions are micron-sized droplets of nematic liquid crystal, floating in isotropic fluid matrices. Such droplets can be switched from an opaque (off) to a transparent (on) state by application of very low electric fields. It is known that the electro-optical properties of liquid crystal dispersions are affected by several parameters, including the liquid crystal loading. The electro-optical response of nematic emulsions has been investigated as a function of liquid crystal weight percentage. Almost transparent films with a reduced contrast ratio are obtained with lower liquid crystal contents. A macroscopic phase separation is observed when liquid crystal content exceeds 45 wt %. On the contrary, large contrast ratios and very low switching fields can be obtained if liquid crystal ranges from 25 to 35 wt %. Consequently, nematic emulsions prepared in this liquid crystal range can be used as promising systems for electro-optical applications. In addition to technological developments, these results can help computational and basic studies of phase separation in novel multiphase liquid crystalline materials.     

Emulsifying potency of new amino acid-type surfactant (II): stable water-in-oil (W/O) emulsions containing 85 wt% inner water phase

The ternary phase diagram for N-[3-lauryloxy-2-hydroxypropyl]-L-arginine L-glutamate (C12HEA-Glu), a new amino acid-type surfactant, /oleic acid (OA)/water system was established. The liquid crystal and gel complex formations between C12HEA-Glu and OA were applied to a preparation of water-in-oil (W/O) emulsions. Stable W/O emulsions containing liquid paraffin (LP) as the oil and a mixture of C12HEA-Glu and OA as the emulsifier were formed. The preparation of stable W/O emulsions containing 85 wt% water phase was also possible, in which water droplets would be polygonally transformed and closely packed, since the maximum percentage of inner phase is 74% assuming uniformly spherical droplets. Water droplets would be taken into the liquid crystalline phase (or the gel complex) and the immovable water droplets would stabilize the W/O emulsion system. The viscosity of emulsions abruptly increased above the 75 wt% water phase (dispersed phase). The stability of W/O emulsions with a lower weight ratio of OA to C12HEA-Glu and a higher ratio of water phase was greater. This unusual phenomenon may be related to the formation of a liquid crystalline phase between C12HEA-Glu and OA, and the stability of the liquid crystal at a lower ratio of oil (continuous phase). W/O and oil-in-water (O/W) emulsions containing LP were selectively prepared using a mixture of C12HEA-Glu and OA since the desirable hydrophile-lipophile balance (HLB) number for the emulsification was obtainable by mixing the two emulsifiers.     

Highly concentrated (gel) emulsions,versatile reaction media

Highly concentrated (gel) emulsions are characterised by dispersed phase volume fractions exceeding 0.74, the critical value for the most compact packing of monodispersed undistorted spheres. Their structure consists of polyhedral droplets separated by thin films of continuous phase, a structure resembling gas–liquid foams. Their rheological properties vary from elastic to viscoelastic having a gel appearance. One of the most promising applications is their use as reaction media. The recent advances in the preparation of low-density polymeric materials (solid foams, aerogels) are reviewed and new applications are described. These include the preparation of dual meso/macroporous inorganic oxide materials and the use of gel emulsions as alternative to conventional solvent media in chemical and enzyme-catalysed reactions.     

Phase behavior of the lecithin/water/isooctane and lecithin/water/decane systems

The isothermal pseudo-ternary-phase diagram was determined at 25 degrees C for systems composed oflecithin, water, and, as oil, either isooctane or decane. This was accomplished by a combination of polarizing microscopy, small-angle X-ray scattering, and NMR techniques. The lecithin-rich region of the phase diagram is dominated by a lamellar liquid-crystalline phase (Lalpha). For lecithin contents less than 60% and low hydration (mole ratio water/lecithin = W0 < 5.5), the system forms a viscous gel of branched cylindrical reverse micelles. With increase in the water content, the system phase separates into two phases, which is either gel in equilibrium with essentially pure isooctane (for lecithin < 25%) or a gel in equilibrium with Lalpha (for lecithin > 25%). These two-phase regions are very thin with respect to water dilution. For 8 < W0 < 54 very stable water-in-oil emulsions form. It is only after ripening for more than 1 year that the large region occupied by the emulsion reveals a complex pattern of stable phases. Moving along water dilution lines, one finds (i) the coexistence of gel, isooctane and Lalpha, (ii) equilibrium between reverse micelles and spherulites, and, finally, (iii) disconnected reverse micelles that fail to solubilize water for W0 > 54. This results in a Winsor II phase equilibrium at low lecithin content, while for lecithin > 20% the neat water is in equilibrium with a reverse hexagonal phase and an isotropic liquid-crystalline phase. The use of the decane as oil does not change the main features of the phase behavior.     

Effect of carbon nanotubes on phase transitions of nematic liquid crystals

Phase diagrams of multi-wall carbon nanotube (MWNT)/nematic liquid crystal (E7) and buckminsterfullerene (C₆₀-I _h)/nematic liquid crystal (E7) binary systems have been investigated by means of polarizing optical microscopy and differential scanning calorimetry. It was found that the isotropic-nematic phase transition temperature (T _NI) of the liquid crystal component was enhanced by the incorporation of MWNT within a small composition gap. A chimney-type phase diagram can be identified in the MWNT/E7 mixture over a narrow range of ∼0.1-0.2% MWNT concentration. Upon substituting the nanotubes with isotropic fillers such as fullerene, the (C₆₀-I _h)/E7 blend showed no discernible change of T _NI in the same concentration range of the chimney of the MWNT/E7 mixture, suggesting a significant contribution of anisotropy (or the aspect ratio) of the nanotubes to the entropy of the system containing liquid crystal molecules. This enhanced T _NI phenomenon may be attributed to anisotropic alignment of liquid crystal molecules along the carbon nanotube bundles.     

Calorimetric study of the isotropic to nematic phase transition in an aligned liquid crystal nano‐colloidal gel

A high‐resolution calorimetric study of the specific heat (C_p ) has been carried out for the isotropic to nematic phase transition in an aligned liquid crystal (octylcyanobiphenyl ‐ 8CB) and aerosil nano‐colloid gel. A stable alignment was achieved by repeated thermal cycling of the samples in the presence of a strong uniform magnetic field, which introduces anisotropy to the quenched random disorder of the silica gel. In general, the specific heat features of the I?N transition in aligned (anisotropic) gel samples are consistent with those seen in random (isotropic) gel samples, namely the observance of two C_p peaks and non‐monotonic transition temperature shifts with increasing silica concentration. However, larger transition temperature shifts with silica density, modification of the phase conversion process in the two‐phase coexistence region, and a larger effective transition enthalpy are observed for the aligned samples. The lower‐temperature aligned C_p peak is larger and broader while exhibiting less dispersion than the equivalent peak for the random gel. This may be a consequence of the alignment altering the evolution from random‐dilution‐dominated to random‐field‐dominated effects. The exact origin of the larger transition temperature shifts is uncertain but the larger enthalpy suggests that the nematic state is different in the aligned system than in random gels. The general non‐monotonic behaviour of the transition temperature is interpreted using dimensional analysis as a combination of an effective elastic stiffening of the liquid crystal combined with a liquid crystal and aerosil surface interaction energy.     

Odd-even effect in miscibility of main-chain liquid crystal polymers with low molar mass nematogens

Qualitative phase diagrams were constructed using the contact method for binary mixtures of several chemically-distinct low molar mass nematogens (LMMN) with a main chain liquid crystal polymer (TPB-x) which has a mesogenic group, 1-(4-hydroxy-4'-biphenyl)-2-(4-hydroxy phenyl) butane, separated by flexible alkyl spacers of variable length, x. Several interesting effects were observed. TPB-x was found to exhibit an odd-even variation in miscibility in the nematic state (2n + 1 = miscible, 2n=immiscible) with 4'-pentyl-4-cyanobiphenyl (5CB), but not with 4'-pentyloxy-4-cyanobiphenyl (5OCB) in which most polymers were completely miscible. On prolonged isothermal annealing in the biphasic region in 5CB, TPB-2n exhibited spherulitic crystallization of the liquid crystal polymer. These observations are shown to be qualitatively consistent with a modification of the Flory-Huggins theory by Brochard et al.     

Vesicle Phases with Semipolar Additives

We examined the influence of semipolar additives on the phase behavior of mixed zwitterionic surfactant/consurfactant systems. It is shown that in these systems with increasing concentration esters like hexylacetate (HA) and ketones like hexylmethylketone (HMK) can behave both like consurfactants and like hydrocarbons. In solutions of 200 mM tetradecyldimethylamineoxide (TDMAO)/cosurfactant the additives cause first a phase transformation from the micellar L(1) phase to a lamellar L(alpha) phase. Upon further increasing concentration, the L(alpha) phase is transformed into a microemulsion. The L(alpha) phase consists of densely packed multilamellar vesicles. The vesicles are shown by electron microscopy. The multilamellar character of the vesicles is also reflected in the conductivity of the phase. It is up to 10 times lower than the conductivity of the L(1) phase. In some systems the vesicles are transformed on rest into a multidomain stacked L(alpha) phase. It is furthermore demonstrated that the two-phase L(1)/L(alpha) region in these systems is very narrow. In situations where enough HA is added to be close to the boundary of the L(1) phase, it is shown that very small amounts of cosurfactant can transform the L(1) phase into the L(alpha) phase. In extreme situations 1 mM cosurfactant is sufficient for transforming the L(1) phase with 200 mM TDMAO into the L(alpha) phase. In the investigated systems the L(alpha) phase is a highly viscoelastic fluid in which the storage modulus is 1 order of magnitude larger than the loss modulus. Besides the conventional way to prepare samples by adding all ingredients and stirring the solution intensively, all investigated systems were additionally prepared without applying any shear forces. In a surfactant/cosurfactant solution the additive was brought into the sample by diffusion. The phase behavior of both types of samples showed fundamental differences in some cases, which give insight into the influence of shear forces on these systems. Copyright 2001 Academic Press.     

Construction of a DOPC/PSM/cholesterol phase diagram based on the fluorescence properties of trans-parinaric acid

Cell membranes have a nonhomogenous lateral organization. Most information about such nonhomogenous mixing has been obtained from model membrane studies where defined lipid mixtures have been characterized. Various experimental approaches have been used to determine binary and ternary phase diagrams for systems under equilibrium conditions. Such phase diagrams are the most useful tools for understanding the lateral organization in cellular membranes. Here we have used the fluorescence properties of trans-parinaric acid (tPA) for phase diagram determination. The fluorescence intensity, anisotropy, and fluorescence lifetimes of tPA were measured in bilayers composed of one to three lipid components. All of these parameters could be used to determine the presence of liquid-ordered and gel phases in the samples. However, the clearest information about the phase state of the lipid bilayers was obtained from the fluorescence lifetimes of tPA. This is due to the fact that an intermediate-length lifetime was found in samples that contain a liquid-ordered phase and a long lifetime was found in samples that contained a gel phase, whereas tPA in the liquid-disordered phase has a markedly shorter fluorescence lifetime. On the basis of the measured fluorescence parameters, a phase diagram for the 1,2-dioleoyl-sn-glycero-3-phosphocholine/N-palmitoyl sphingomyelin/cholesterol system at 23 °C was prepared with a 5 mol % resolution. We conclude that tPA is a good fluorophore for probing the phase behavior of complex lipid mixtures, especially because multilamellar vesicles can be used. The determined phase diagram shows a clear resemblance to the microscopically determined phase diagram for the same system. However, there are also significant differences that likely are due to tPA's sensitivity to the presence of submicroscopic liquid-ordered and gel phase domains.     

Phase behavior of polylactides in solvent–nonsolvent mixtures

Isothermal phase diagrams for the semicrystalline poly-L-lactide (PLLA) and the amorphous poly-DL-lactide (PDLLA) in combination with several solvent–nonsolvent combinations (dioxane/water, dioxane/methanol, chloroform/methanol, and NMP/water) have been determined. The locations of the liquid–liquid miscibility gap, the solid–liquid miscibility gap and the vitrification boundary in the isothermal phase diagrams at 25°C were identified. The liquid–liquid miscibility gap for the systems with PLLA was located in the same composition range as the corresponding systems with PDLLA. For the systems containing PLLA solid–liquid demixing was thermodynamically preferred over liquid–liquid demixing. Attempts were made to correlate the experimental findings with predictions on the basis of the Flory-Huggins theory for ternary solutions using interaction parameters derived from independent experiments. Qualitative agreement was found between the theoretical predictions and the experimentally obtained liquid–liquid miscibility gap. No good agreement was found for the solid–liquid miscibility gap. © 1996 John Wiley & Sons, Inc.     

Phase diagrams with critical and tricritical liquid phase solution points in a ternary system

Critical phenomena of liquid phase stratification in ternary systems are considered. Models of the T-x ₁-x ₂ phase diagrams with critical and tricritical liquid solution points and isothermal diagrams over the corresponding interinvariant intervals are given.