Dispersed Liquid Crystalline Phases Research Articles

Crystallography of dispersed liquid crystalline phases studied by cryo‐transmission electron microscopy

Low molecular weight surfactants, for example monoglycerides and phospholipids, form a multitude of self-assembled structures, such as inverted cubic or hexagonal mesophases, if brought into contact with water/oil. These mesophases can be dispersed in water using adequate surface-active materials such as low molecular weight surfactants or surface active polymers. In order to use such mesophase particles for incorporating drugs and aromas, it is essential to determine their internal crystallographic structure and to understand their mechanism of stabilization. Cryo-transmission electron microscopy was used to investigate the internal structure of different dispersed particles at various temperatures and oil contents. It is shown here that cryo-transmission electron microscopy, in combination with fast Fourier transform and tilting experiments, is effective in obtaining information on crystallographic structure, space group and morphology of particles with reversed bicontinuous cubic and hexagonal structures. In particular, using the presence or the absence of the {111} reflections and viewing the same particle under different axes of observation allows one to discriminate between the Im3m and Pn3m space groups. A major advantage of cryo-transmission electron microscopy is the ability to analyse single particles. This allows the identification of particles present at very low concentrations and the coexistence of particles with different internal self-assembly structures. With this technique we have obtained strong evidence for the presence of two cubic internal self-assembly structures with different space groups within the same dispersion. In addition, we found that cryo-transmission electron microscopy combined with tilting experiments enables the analysis of internal particle morphology, allowing the discussion of mechanisms for hexosome stabilization.

Alexander Lecture 2003: Cubosomes, Vesicles, and Perforated Bilayers in Aqueous Systems of Lipids, Polymers, and Surfactants

AbstractFor Abstract see ChemInform Abstract in Full Text.

Alexander Lecture 2003: Cubosomes, Vesicles, and Perforated Bilayers in Aqueous Systems of Lipids, Polymers, and Surfactants

Cryo-Transmission Electron Microscopy (cryoTEM) is a method allowing visualization of many of the delicate structures that form by self-assembly of amphiphilic molecules in aqueous environments. The amphiphiles may be surfactants, lipids, or polymers, alone or in various mixtures. The distinctive feature of the method is that the objects are examined without staining or dehydration: This is achieved by capturing the structures in very thin aqueous films, that are subsequently vitrified at liquid nitrogen temperatures and examined using a microscope. Objects in the size range from 5 to 500 nm are well suited for the method. This includes various emulsion particles, such as liposomes, and more exotic cubosomes and hexasomes. In cryoTEM investigations perforated vesicles were found, an observation that triggered extensive studies of the nature and occurrence of such structures. As a complement to scattering methods, cryoTEM has proven its value in investigations of the size and morphology of various liposomal and vesicular systems. The microscopy studies show what type of structures that are present in the sample: uni- or multilamellar vesicles, open structures or closed defect-free vesicles, whether the form is spherical, tubular, or oblate, and so on. The scattering methods give good measures of size and polydispersity for defined systems.Three main themes are presented here. (a) Morphology of cubosomes and other emulsion particles from dispersed liquid-crystalline phases. (b) Perforated bilayers, their structure, nature, occurrence, and formation. (c) Spontaneous catanionic vesicles and their relationship to vesicles of zwitterionic lipids plus ionic surfactants.

Bioavailability of cyclosporine in rats after intragastric administration: a comparative study of the L 2-phase and two other lipid-based vehicles

The purpose of this study was to evaluate formulations based on surface active dietary lipids only as oral vehicles for cyclosporine. The absolute bioavailability of cyclosporine from two new lipid vehicles was determined in rats after intragastric administration and compared to that of Sandimmun® oral solution, which contain non-ionic surface active substances in addition to dietary lipids. In the new vehicles, cyclosporine was dissolved in two different mixtures of glycerides from long-chained fatty acids. One mixture forms as L 2-phase, an oil with very low interfacial tension towards water, and was administratered both as the oily L 2-phase and as a predispersed emulsion formulation. The other mixture forms a liquid crystalline phase and was administered only as an aqueous dispersion. The mean bioavailability of cyclosporine from Sandimmum® was 8% while it was 34% from the L 2-phase, 38% from the predispersed L 2-phase and 27% from the dispersed liquid crystalline phase. The coefficients of variation in area under the blood concentration curve after administration of the two formulations based on the L 2-phase were quite low (31% for the L 2-phase and 24% for the predispersed L 2-phase) and comparable to that after intravenous administration (24%), while the dispersed liquid crystalline phase gave a higher variability (91%), comparable to that of Sandimmum® oral solution (101%). The low variabilities found with the two L 2-phase vehicles suggest that this formulation is ‘self-emulsifying’ in the gastrointestinal tract. Since the L 2-phase is based on dietary lipids only, it is expected to be well tolerated and could prove to be a good vehicle for long-term clinical use of oral cyclosporine.

Dynamic surface tensions, foam and the transition from micellar solution to lamellar phase dispersion

If the packing behaviour of surfactants in close-packed layers is sufficiently modified then micelles are not formed. Solutions can become turbid at a critical aggregation concentration where a dispersion of liquid crystalline particles is found. Examples of this type of behaviour considered here include dodecyl polyethyleneglycol ethers with low degrees of ethoxylation. The precipitation of liquid crystalline particles either from mixtures of such compounds with micelle forming anionic surfactants or from mixtures of micelle forming zwitterionics with certain micelle forming anionics are also included. Rates of transport of surfactant to air-water surfaces may be monitored by measuring dynamic surface tensions. Solutions containing dispersed liquid crystalline phase, rather than micelles, exhibit diminished rates of transport to air-water surfaces which implies relatively low levels of dynamic adsorption at these surfaces. This leads to diminished foamabilities. However the stability of the resulting foam may be enhanced. This may be caused by diminished bulk foam drainage rates due to accumulation of liquid crystalline material in Plateau borders. It may also derive from diminished foam film drainage rates caused by suppression of marginal regeneration.

A study of polycarbonate‐liquid crystal polymer blends

AbstractThe morphology and the thermal and mechanical properties of blends of bisphenol‐A polycarbonate (PC) with a wholly aromatic liquid crystal copolyester‐amide (Vectra‐B of Hoechst‐Celanese) are discussed with reference to the conditions used for their preparation. The blends made by extrusion behave as mixtures of incompatible polymers: the dispersed liquid crystalline phase is more or less oriented in the extrusion direction and acts as a reinforcing filler for the PC matrix. The behaviour of the blends prepared in a batch mixer depends strongly on mixing time. A slight decrease in the mechanical properties (especially tensile strength and elongation) accompanies the morphological changes associated with a progressive improvement in the compatibility of the two polymers, which finally leads to an apparently homogeneous material after 45 min mixing at 290°C and 150 rpm. These effects are interpreted in terms of chemical interactions taking place between the two components of the blends.

On Interfacial Tensions Measured With Alkyl Aryl Sulfonate Surfactants

Abstract Tensions of aqueous solutions and dispersions of sodium p (1-heptylnonyl/benzene sulfonate against decane are not ultralow in the absence of salt, but can fall below 0.01 dyne/cm (0.01 mN/m) with 0.3 wt% (3 g/kg) NaCl, depending on order of mixing, preparation age, and decane drop size. A dispersed preparation age, and decane drop size. A dispersed liquid crystalline phase revealed by spectroturbidimetry and microscopy appears responsible for ultralow tensions. Results with petroleum sulfonate Witco TRS 10-80 support this conclusion. Introduction The importance of ultralow [less than 0.01 dyne/ cm (0.01 mN/m)] interfacial tensions between oil and water as the means of achieving high capillary-number displacement in one mechanism of enhancing oil recovery is well recognized. Low tension is related closely to the phase behavior of suitable surfactant with oil, water, and often an alcohol, as well as other additives. Information in the technical and patent literature concentrates on commercial sulfonate surfactant systems. Few studies of the relationship between tension and phase behavior with pure surfactants have appeared. Since the pioneering work of Hartley on molecular structure dependence of interfacial tension, the influences of WOR, salinity, temperature, surfactant molecular-weight distribution, and other factors have been investigated. However, the origins of ultralow interfacial tensions are not yet known.This paper deals with the low-tension regime associated with low surfactant concentrations, i.e., no more than 1 wt% (10 g/kg). In contrast to the second regime, in which microemulsion plays the central role, this regime does not require the presence of a suitable alcohol or other cosurfactant. presence of a suitable alcohol or other cosurfactant. In this regime it is not clear whether ultralow tensions are equilibrium or nonequilibrium properties. Even with compositions capable of producing ultralow tensions, the measured tensions depend on the way the system is prepared, and time effects are seen. Nevertheless, reproducible measurements can be made by observing a fixed experimental protocol. Tensions measured this way with a given surfactant correlate strongly with semiempirical assignment of carbon numbers to a wide range of hydrocarbons. The correlation extends across related sulfonate surfactants and is a potentially useful means of characterizing low-tension formulations. Understanding this correlation may shed light on the mechanism of ultralow tension in the low-surfactant concentration regime and thus may aid process studies. Understanding the factors responsible for nonequilibrium effects is crucial to interpret laboratory and field data and to design surfactant flooding processes rationally. Commercial surfactants are complex mixtures and are characterized inadequately for basic physicochemical study of order-of-mixing effects physicochemical study of order-of-mixing effects and time-varying interfacial tensions. Experience suggested that a representative pure surfactant would display much of the behavior of commercial mixtures, thereby exposing the practically important aspects to systematic scientific study. The U. of Texas group synthesized an agreed-on alkyl aryl sulfonate that is the subject of extensive studies in this laboratory and elsewhere. Results on its phase behavior with brine and with decane are phase behavior with brine and with decane are reported elsewhere. Parallel studies are under way with a petroleum sulfonate surfactant to establish the extent to which a pure surfactant can mimic the behavior of a commercial mixture. Companion investigations are aimed at explaining the unknown microstructures of ultralow tension interfaces. SPEJ p. 71

Studies on lecithin-cholesterol-water interactions by differential scanning calorimetry and X-ray diffraction

Addition of cholesterol to dipalmitoyl- l-lecithin in water lowers the transition temperature between the gel and liquid crystalline phase, and decreases the heat absorbed at the transition. No transition is observed with an equimolar ratio of the lecithin with cholesterol. This ratio corresponds to the maximum amount of cholesterol which can be introduced before cholesterol precipitation occurs. Unsaturated lecithins and the lipid extract of human erythrocyte ghosts exhibit similar behaviour. The cholesterol controls the fluidity of the hydrocarbon chains of the phospholipid by disruption of the crystalline chain lattice of the gel phase, and by inhibiting the flexing of chains in the dispersed liquid crystalline phase. The possible implications of this behaviour to the role of cholesterol in biological membranes are discussed.