Lamellar Liquid Crystalline Phase Research Articles

Spontaneous Emulsification of Oil in Aerosol-OT/Water/Hydrocarbon Systems

Drops containing a hydrocarbon, the anionic surfactant Aerosol-OT (AOT), and water were injected into water at 30 °C, and the resulting behavior was observed by videomicroscopy. Spontaneous emulsification of most of the injected oil yielding droplets a few microns in diameter was seen when n-octane was the hydrocarbon and when the initial drop contained at least 25 wt % AOT and no more than about 10 wt % water. Emulsification was also observed when the initial drop contained small amounts of NaCl solution instead of water and for suitable conditions when n-hexane was the hydrocarbon. However, much less emulsification occurred for n-decane and n-dodecane, probably because surfactant-rich phases in equilibrium with excess oil in these systems solubilized little hydrocarbon. The mechanism of emulsification is closely related to that of an earlier study by Rang and Miller (Prog. Colloid Polym. Sci. 1998, 109, 101) using drops containing n-hexadecane, the pure nonionic surfactant C12E6, and n-octanol. In both systems diffusion produced inversion from an oil-continuous to a water-continuous phase, leading to emulsification at locations where supersaturation in oil occurred. One difference in the present case was that inversion was not continuous but involved formation of the lamellar liquid crystalline phase. Another was that the shift to more hydrophilic conditions leading to inversion was caused by decreases in ionic strength of water in the injected drop which, in turn, was produced by diffusion of water into the drop and of AOT and, in some cases, NaCl out of the drop. Owing to this ionic strength effect, nearly complete emulsification to small oil droplets was observed in some ternary AOT/hydrocarbon/water systems in the present study but was not possible in ternary nonionic surfactant/hydrocarbon/water systems investigated previously.

α-Methylene ordering of acyl chains differs in glucolipids and phosphatidylglycerol from Acholeplasma laidlawii membranes: 2H-NMR quadrupole splittings from individual lipids in mixed bilayers

A Acholeplasma laidlawii strain A-EF22 was grown in a medium supplemented with α-deuterated oleic acid. Phosphatidylglycerol (PG), the glucolipids monoglucosyldiacylglycerol (MGlcDAG), diglucosyldiacylglycerol (DGlcDAG) and monoacyldiglucosyldiacylglycerol, and the phosphoglucolipid glycerophosphoryldiglucosyldiacylglycerol (GPDGlcDAG) were purified, and the phase behaviour and molecular ordering for the individual lipids, as well as for mixtures of the lipids, were studied by 2H-, 31P-NMR and X-ray scattering methods. The chemical structure of all the A. laidlawii lipids, except PG, has been determined and verified previously; here also the chemical structure of PG was verified, utilising mass spectrometry and 1H and 13C high resolution NMR spectroscopy. For the first time, lipid dimers were found in the mass spectrometry measurements. The major findings in this work are: (1) addition of 50 mol% of PG to the non-lamellar-forming lipid MGlcDAG does not significantly alter the transition temperature between lamellar and non-lamellar phases; (2) the 2H-NMR quadrupole splitting patterns obtained from the lamellar liquid crystalline phase are markedly different for PG on one hand, and DGlcDAG and GPDGlcDAG on the other hand; and (3) mixtures of PG and DGlcDAG or MGlcDAG give rise to 2H-NMR spectra consisting of a superposition of splitting patterns of the individual lipids. These remarkable features show that the local ordering of the α-carbon of the acyl chains is different for PG than for MGlcDAG and DGlcDAG, and that this difference is preserved when PG is mixed with the glucolipids. The results obtained are interpreted in terms of differences in molecular shape and hydrophilicity of the different polar headgroups.

Surface Charge Markedly Attenuates the Nonlamellar Phase-Forming Propensities of Lipid Bilayer Membranes: Calorimetric and 31P-Nuclear Magnetic Resonance Studies of Mixtures of Cationic, Anionic, and Zwitterionic Lipids

The lamellar/nonlamellar phase preferences of lipid model membranes composed of mixtures of several cationic lipids with various zwitterionic and anionic phospholipids were examined by a combination of differential scanning calorimetry and 31P NMR spectroscopy. All of the cationic lipids utilized in this study form only lamellar phases in isolation. Mixtures of these cationic lipids with zwitterionic strongly lamellar phase-preferring lipids such as phosphatidylcholine form only the lamellar liquid-crystalline phase even at high temperatures, as expected. Moreover, mixtures of these cationic lipids with strongly nonlamellar phase-preferring zwitterionic lipids such as phosphatidylethanolamine exhibit a markedly reduced propensity to form inverted nonlamellar phases, again as expected. However, when mixed with anionic lipids such as phosphatidylserine, phosphatidylglycerol, cardiolipin, or phosphatidic acid, a marked enhancement of nonlamellar phase-forming propensity occurs, despite the fact both components of the mixture are nominally lamellar phase-preferring. An examination of the lamellar/nonlamellar phase transition temperatures and the nature of the nonlamellar phases formed, as a function of temperature and of the composition of the mixture, indicates that the propensity to form inverted nonlamellar phases is maximal in mixtures where the mean surface charge of the membrane surface approaches neutrality and decreases markedly with increases in the density of positive or negative charge at the membrane surface. Moreover, the onset temperatures of the reversed hexagonal phase rise more steeply than do those of the inverted cubic phase as the ratio of cationic and anionic lipids is varied, suggesting that the formation of inverted hexagonal phases is more sensitive to this surface charge effect. These results indicate that surface charge per se is a significant and effective modulator of the lamellar/nonlamellar phase preferences of membrane lipids and that charged group interactions at membrane surfaces may have a major role in regulating this particular membrane property.

Viscoelasticidad lineal de la fase líquido-cristalina laminar de un sistema tensioactivo no iónico/hidrocarburo/agua

A linear viscoelastic characterization by small amplitude oscillatory shear tests was carried out for heptane/polyoxyethylene glycol nonylphenyl ether (EOa=10)/water system exhibiting a lamellar liquid crystalline structure. The influence of composition on the dynamic functions was considered. The lamellar liquid crystalline phase shows a small linear viscoelastic range. The relaxation spectrum of the lamellar liquid crystalline phase presents two regions: the transition region and the plateau region, characteristic of highly structured systems. The maximum values of the plateau modulus correspond to the systems of intermediate compositions within the realm of existence of the lamellar mesophase.

Phase Behavior and Polymerization Kinetics of a Semifluorinated Lyotropic Liquid Crystal

Recently, amphiphilic monomers that are capable of forming lyotropic liquid crystalline phases have been utilized in a variety of applications including emulsion polymerization, development of nanocomposites, and the formation of polymeric surfactants. A new class of fluorinated amphiphilic monomers that exhibit lyotropic mesophases has shown great promise in ophthalmic applications. Initial studies indicate that the monomer possesses a lamellar morphology at certain concentrations that when polymerized would yield a material with anisotropic properties ideal for repairing retinal tears. Characterization of the polymerization kinetics of these fluorinated monomers provides a better understanding of conditions such that structure retention can be obtained. The fluorinated amphiphiles exhibit varying phase morphology ranging from an isotropic micellar phase to discontinuous cubic and lamellar liquid crystalline phases with increasing concentration and variation in the percent neutralization of the acid moiety. The lyotropic liquid crystalline order changes significantly with respect to percent neutralization. The polymerization kinetics follow a trend of decreasing order with increasing neutralization in that the fastest rates are seen in samples with higher degrees of order specifically in the lamellar liquid crystalline phase. The polymerization rate decreases to a minimum in samples of cubic morphology with low degrees of overall order. The higher polymerization rates in the lamellar phase are due to a decrease in the termination rate. Additionally, the polymerization behavior and morphology have a tremendous impact on the resulting polymer.

Evaluation of the rheological properties of sulfonic acids and sodium sulfonates

AbstractSulfonic acids of linear alkylbenzene (LAB) are converted into the corresponding sodium salts to produce the most widely used anionic surfactant worldwide, linear alkylbenzene sulfonate (LAS). Used in many industrial applications and consumer products, the physical and mechanical properties of the sulfonates are strongly dependent on the LAB manufacturing process. Until recently, commercial alkylation of benzene has employed aluminum chloride or hydrogen fluoride catalysts, but a new fixed‐bed alkylation process (DETAL) has been developed with improved 2‐phenyl isomer selectivity and low tetralin concentration. In order to better understand the rheology of LAS in aqueous media, a comprehensive comparative evaluation of sulfonic acids and sodium sulfonates of the three LAB process derivatives has been done using dynamic mechanical rheometry, steady shear viscometry, and X‐ray diffraction for phase identification. LAB sulfonic acids are Newtonian fluids in the temperature range of 20–60°C. The neat AlCl3, HF, and DETAL sulfonic acids are Newtonian fluids within the temperature range of 20–60°C. At 30 wt%, all three sulfonates are Newtonian at 20–60°C, and the 40 wt% AlCl3 sodium sulfonate remains in the Newtonian regime within this temperature range. Lamellar liquid crystalline phases have been identified for the sulfonates in the concentration range of 40–60 wt% in water at 20–60°C, and a hexagonal lattice phase also has been identified for DETAL sodium alkylbenzene sulfonate at 40 wt%, 60°C. The presence of anisotropic phases results in non‐Newtonian pseudoplastic behavior with time‐dependent viscosity functions.

New evidence for gel-liquid crystalline phase coexistence in the ripple phase of phosphatidylcholines

Experimental evidence supporting the hypothesis of gel-liquid crystalline phase coexistence in the stable ripple phase of diacylphosphatidylcholines has been obtained from time-resolved X-ray small- (SAXS) and wide-angle diffraction (WAXS) in the millisecond to second time domain. The pretransition of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) exhibits a thin lamellar liquid crystalline intermediate phase (designated Lalpha) if driven far away from equilibrium by an infrared temperature jump (T-jump) technique. The findings can be described by a two-step model. (1) Instantaneously with the T-jump, an anomalously thin lamellar liquid crystalline intermediate phase (d = 5.6-5.8 nm) forms, coexisting with the original gel-phase Lbeta'. Within the first seconds, the lamellar repeat distance of the intermediate increases to a value of about 6.7 nm. A closer examination of these kinetics reveals two relaxation components: a fast process, proceeding within tenths of a second, and a slow process, on the time scale of a few seconds. (2) Finally, both the liquid crystalline and the gel-phase relax into the stable ripple phase Pbeta'. The total process time of the transition is nearly independent of the addition of NaCl, but varies strongly with the chain length of the lecithin species.

Frictional properties of lyotropic liquid crystalline mesophases at surfaces

The lubricating effect of assemblies of amphiphiles at surfaces has been studied. Liquid crystalline mesophases were investigated in terms of frictional and wear properties in a pin on disc rig. It is shown that systems forming lamellar liquid crystals indeed serve as lubricating liquids, indicating that the lamellar liquid crystalline phase adsorb on the steel surface forming a lubricating tribofilm. Poor performance is obtained when the lubricating system is in a single phase, i.e. in this case a lamellar liquid crystalline region. However, good lubrication is found when the lamellar liquid crystalline phase is dispersed in the water. This is attributed to a low viscosity of the system rendering a fast relaxation of the system in order to form a new film after the disturbing action of the two sliding surfaces. The lamellar packing is altered by the addition of non-charged amphiphiles, such as short chain alcohols. Using different alcohols it was shown that aromatic short chain alcohols are detrimental to the wear. This was attributed to the Rebinder effect, i.e. a strong adsorption of the alcohol, which in turn facilitates crack formation and thus the wear. In systems with mixtures of cat- and anionic amphiphiles it was, not surprisingly found that the best lubrication is obtained with a slight excess of either surfactant.

Buccal permeation of [ d-Ala 2, d-Leu 5]enkephalin from liquid crystalline phases of glyceryl monooleate

The ex vivo buccal permeability of a [ d-Ala 2, d-Leu 5]enkephalin (DADLE) and glyceryl monooleate (GMO) was examined from the cubic and lamellar liquid crystalline phases of GMO and aqueous phosphate-buffered saline (pH 7.4, PBS) solution across excised porcine buccal mucosa mounted in a Franz cell. GMO was released in vitro from the liquid crystalline phases indicating the erosion of the liquid crystal matrices. GMO released from the liquid crystalline matrices permeated the porcine buccal mucosa with fluxes of 0.10±0.03 and 0.07±0.00%/cm 2 per h for the cubic and lamellar phases, respectively. The flux of DADLE (1.21±0.32 and 1.15±0.11%/cm 2 per h for the cubic and lamellar phases, respectively) from the liquid crystalline phases was significantly enhanced by the GMO compared with PBS solution (0.43±0.08%/cm 2 per h) during the initial permeation phase ( t<3 h). Our results suggest that the cubic and lamellar liquid crystalline phases can be considered as promising buccal drug carriers for peptide drugs as well as acting as permeation enhancers.

In vitro peptide release from liquid crystalline buccal delivery systems

Swelling and [ d-Ala 2, d-Leu 5]enkephalin (DADLE) release from the lamellar and cubic liquid crystalline phases of glyceryl monooleate (GMO) were studied using two in vitro methods, a total immersion method and a Franz cell method. The swelling of the lamellar phase and glyceryl monooleate (0% w/w water content) and DADLE release from the liquid crystalline phases were temperature dependent. The swelling ratio was greater at 20°C than 37°C while DADLE release increased at 37°C compared to 20°C for both the lamellar and cubic phases. The water uptake increased dramatically with decreasing initial water content of the liquid crystalline phases. However, DADLE release increased with increasing initial water content, which corresponded to increased viscosity. The swelling and DADLE release profiles obtained using a Franz cell method with a moist nylon membrane to mimic buccal drug release conditions were slower than the total immersion method. These results show that the swelling and DADLE release strongly depended on temperature, the initial water content of the liquid crystalline matrix and the methodology employed for determining the swelling and DADLE release.

Complexation in the heptanoic acid-heptylamine system

The complexation between heptylamine and heptanoic acid has been elucidated at 298.15 K using spectroscopic methods and also by measuring macroscopic quantities such as viscosity, conductivity and surface tension. Fourier transform IR and 13C NMR measurements point towards the existence of a compound consisting of one amine molecule and one acid molecule in an equimolecular mixing ratio. These suggestions are supported by viscosity and surface tension measurements. This compound is further able to interact with excess acid, but similar behaviour is not observed with excess amine. The equimolecular compound behaves like a catanionic surfactant; this is seen in the phase diagram for the heptylamine–heptanoic acid–water system at 298.15 K, where the dominating phase is the lamellar liquid-crystalline phase. This phase is in equilibrium with almost pure water. At low water content a solution phase extending from the binary heptylamine–heptanoic acid axis and covering all mixing ratios between the amine and the acid is also present.

Structure-related aspects on water diffusivity in fatty acid–soap and skin lipid model systems

Simplified skin barrier models are necessary to get a first hand understanding of the very complex morphology and physical properties of the human skin barrier. In addition, it is of great importance to construct relevant models that will allow for rational testing of barrier perturbing/occlusive effects of a large variety of substances. The primary objective of this work was to study the effect of lipid morphology on water permeation through various lipid mixtures (i.e., partly neutralised free fatty acids, as well as a skin lipid model mixture). In addition, the effects of incorporating Azone® (1-dodecyl-azacycloheptan-2-one) into the skin lipid model mixture was studied. Small- and wide-angle X-ray diffraction was used for structure determinations. It is concluded that: (a) the water flux through a crystalline fatty acid–sodium soap–water mixture (s) is statistically significantly higher than the water flux through the corresponding lamellar (Lα) and reversed hexagonal (HII) liquid crystalline phases, which do not differ between themselves; (b) the water flux through mixtures of Lα/s decreases statistically significantly with increasing relative amounts of lamellar (Lα) liquid crystalline phase; (c) the addition of Azone® to a skin lipid model system induces a reduction in water flux. However, further studies are needed to more closely characterise the structural basis for the occlusive effects of Azone® on water flux.

POLYMERIC SURFACTANTS BASED ON OLEIC ACID IV. Lamellar Liquid Crystal Polymerization of Sodium Oleate/Oleic Acid/Aliphatic Diene/Water System

The lamellar liquid crystalline phase in the system consisting of sodium oleate (NaOL), oleic acid (OLA), and water was determined. The interlayer spacing (d) of the lamellar liquid crystal was measured through small angle X-ray diffraction, which indicated that oleic acid molecules were solubilized between the end methyl groups at low concentrations, and then were located within the hydrocarbon chain layer with further increase of its concentration. Cross-linking agents were added to the system, which were found being located partly in between the end methyl groups and partly within the hydrocarbon chain layers. The liquid crystal phase of NaOL/OLA/H2O system with the cross-linking agent was polymerized at 60 °C, which turned out to be a mixture of liquid crystals and solids. Interlayer spacing decreased by about 10 Å, indicating a disruption of the ordered structure by the polymerization. The polymerization took place not only within the hydrocarbon layer, but also in between the layers separated by the end methyl groups. The resulting polymer lowered the surface tension of water to below 30 mN/m, with a critical micellization concentration of about O.25g/L.

On the Location of Vitamin E in the Hexagonal II Structure of Phosphatidylethanolamine.

X-ray diffraction was used to obtain information on the orientation and location of vitamin E in phospholipid model membranes. It was found that addition of α-tocopherol and to α lesser extent α-tocopheryl acetate to dielaidoylphosphatidylethanolamine (DEPE) stabilizes the hexagonal II (HII) phase at the expense of the lamellar liquid-crystalline phase and reduces the lattice constant of the HII phase. This result can be interpreted as suggesting that the vitamin E molecules are located with their long axes parallel to the hydrocarbon chains of DEPE in the HII phase.

Rheo-optical investigations of lyotropic mesophases of polymeric surfactants

The shear orientation of hexagonal and lamellar liquid crystalline phases of polymeric surfactants was investigated by rheo-optical techniques (flow birefringence (Δn), small-angle light scattering) as well as by nuclear magnetic resonance and optical microscopy. The evolution of birefringence in the hexagonal phase is discussed for simple and oscillatory shear, and an alignment of rodlike micelles along the flow direction was found. A shear induced formation of vesicles (“onions”) is observed with the lamellar phase. They displayed a characteristic four-lobe pattern in depolarized light scattering. Above a critical shear stress vesicles were degraded and perpendicularly aligned lamellae (i.e. with their normal along the vorticity direction) were obtained. A comparison of experiments performed at constant stress and constant rate revealed that the vesicle to planar lamellae transition occurred above a critical shear stress. The behavior of the polysoap lyotropic mesophases under shear, i.e. the strain dependent alignment in the hexagonal phase, the shear induced formation of vesicles, and a transition to planar lamellae in the lamellar phase, is very similar to the behavior of lyotropic mesophases formed by low molar mass surfactants or amphiphilic block copolymers. The geometrical constraints that are introduced when amphiphilic side groups are fixed to a polymer backbone do not significantly alter the response of the mesophase to a shear deformation.

Bolaamphiphilic Phosphocholines: Structure and Phase Behavior in Aqueous Media

The phase behavior of 1,1‘-di-O-eicosamethylene-2,2‘-di-O-decylbis(glycero-3-phosphocholine) (1) and 1,1‘-di-O-hexadecamethylene-2,2‘-di-O-octylbis(glycero-3-phosphocholine) (2) was studied at the air−water interface and in aqueous dispersions using monolayer film balance, small-angle X-ray scattering, Raman spectroscopy, and cryo-TEM techniques. The limiting molecular areas (20 °C, pH 6.8) were observed to be 172 Å2 for 2 and 105 Å2 for 1, indicating a U-shaped configuration of the long polymethylene chains at the air−water interface. The larger molecular area of 2 relative to 1 at the air−water interface suggests that the alkyl chains of 2 are less ordered than those of 1 at similar pressures. The extrapolated d spacings for 1 from SAXS experiments were 32 Å in the liquid crystalline lamellar phase and 40 Å in the gel phase where the hydrocarbon chains are in their all trans configuration. The extrapolated d spacings for 2 were 27 and 33 Å in the liquid crystalline and gel phases, respectively. DSC analysis revealed phase transition temperatures of 1 and 2 in excess water at 15.3 and −23.5 °C, respectively, with observed transition enthalpies of 9.1 and 2.9 kJ/mol, respectively. These values are in good agreement with Raman spectroscopy experiments that detect alkyl melting transitions near these temperatures. Addition of 30 mol % cholesterol to both bolaamphiphile samples lowered the observed transition temperatures (1, 9.5 °C; 2, −25.1 °C) and enthalpies (1, 5.7 kJ/mol; 2, 2.5 kJ/mol), suggesting that cholesterol reduces the transition cooperativity. Extruded samples of pure 1 in buffer (30 mg/mL in 20 mM Tris, pH 8) produced vesicle structures that were variable in size (0.15−1.5 μm), with multilamellar and nonspherical morphologies apparent. Incorporation of cholesterol in the extruded samples (7:3 1/Chol), however, gave stable spherical unilamellar vesicles with a mean diameter of 857 ± 237 Å. Similar results were observed for dispersions of 2 and 7:3 2/Chol. These results suggest that stable, unilamellar monolayer membrane vesicles can be formed from bolaamphiphiles with relatively short (C16 and C20) membrane-spanning alkyl chains.

Molecular Dynamics Study of the Poly(oxyethylene) Surfactant C12E2 and Water

Constant pressure and temperature (NPT) molecular dynamics (MD) simulations have been carried out to investigate the properties of a binary mixture of the poly(oxyethylene) surfactant C12E2 and water in its liquid crystalline lamellar phase (Lα). The calculated interlamellar spacing and the area per surfactant were found to be in reasonable agreement with X-ray diffraction results. The water molecules were observed to form hydrogen bonded bridged structures linking the oxygen atoms of the same surfactant chain. This interaction leads to a strong preference for the C−C bonds in the headgroup to attain a gauche conformation.

X-ray Diffraction Study of the Effect of the Detergent Octyl Glucoside on the Structure of Lamellar and Nonlamellar Lipid/Water Phases of Use for Membrane Protein Reconstitution

The phase behavior and structural parameters of dioleoylphosphatidylethanolamine (DOPE), monoolein (MO), and dipalmitoylphosphatidylcholine (DPPC) hydrated in excess aqueous phase containing micellar octyl glucoside (OG) were investigated by means of synchrotron X-ray diffraction in a wide range of temperatures. The lipid-to-detergent molar ratios were selected to be equal to those in a recent membrane-protein-reconstitution study. It was established that lipids displaying either nonlamellar (DOPE and MO) or lamellar propensities (DPPC) form mixed supramolecular structures with the detergent OG. The phase states and structures of these mixed assemblies are essentially different from those of the pure lipid components. At full hydration and at the investigated lipid-to-detergent molar ratios, OG destabilizes the inverted-hexagonal (HII) phase organization of DOPE and the inverted cubic (Q224) structure of MO, transforming them into a lamellar liquid crystalline (Lα) phase. The fluid DOPE/OG and MO/OG bilay...

Effects of solvents interacting favorably with hydrophilic segments of the membrane surface of phosphatidylcholine on their gel-phase membranes in water

We have investigated the effects of two kinds of solvents forming the lamellar liquid-crystalline (L α) phase in phosphatidylcholine (PC) membranes in neat condition, such as formamide and 1,3-propanediol, on phase behaviors of multilamellar vesicle (MLV) of DPPC (DPPC-MLV). These solvents induced the interdigitated gel (L βI) phase in DPPC-MLV in excess water above their critical concentrations. Solubility measurement indicates that these solvents interact favorably with the hydrophilic segment of the PC membrane but interact unfavorably with the alkyl chains. Based on these results, we propose the mechanism of the induction of the L βI phase by these solvents.

Phase and aggregation behaviour of double-chain cationic surfactants from the class of N -alkyl- N -alkyl?- N , N -dimethylammonium bromide surfactants

We present the phase diagrams and the properties of newly synthesised double-chain cationic N-alkyl-N-alkyl′-N,N-dimethylammonium bromide surfactants [CxCyDMABr (x = 12, 14 and 16; y = 10, 11, 12, 14 and 16)]. All the systems studied form liquid-crystalline lamellar phases but with different morphologies: unilamellar vesicles at low surfactant concentrations, multilamellar vesicles and tubular aggregates for surfactant concentrations between 2 and 10 wt% and at even higher concentrations planar bilayers of surfactant molecules in the classical Lα phase. The phase diagrams were determined with macroscopic and microscopic methods (polarisation microscopy, freeze-fracture transmission electron microscopy, scanning electron microscopy and differential interference contrast microscopy). The properties of the surfactant solutions were determined with differential scanning calorimetry measurements for Krafft point determination and small-angle neutron scattering measurements for interlamellar spacing and bilayer thickness. Finally, conductivity and viscosity measurements for phase characterisation were carried out.