Hexagonal Liquid Crystalline Phase Research Articles

Supramolecular Tubular Structures of a Polymethacrylate with Tapered Side Groups in Aligned Hexagonal Phases

X-ray methods have been used to analyze the supramolecular tubular structures in the hexagonal solid state and columnar hexagonal liquid crystalline (O h ) phases of a polymethacrylate with tapered side groups: specifically, poly{2-{2-[2-(2-(methacryloyloxy)ethoxy)ethoxy]ethoxy)}ethyl 3,4,5-tris((p-(dodecyloxy)-benzyl)oxy)benzoate}, abbreviated as 12-ABG-4EO-PMA. Oriented fibers were drawn from the O h liquid crystalline phase at ∼ 60°C and annealed for several days at 4°C. The X-ray pattern for these fibers recorded at room temperature (25°C) contained eight equatorial reflections that are orders predicted for a hexagonal unit cell with a= b= 60.4 A

Studying the mechanism of Lα/HII and Lα/QII phase transmissions in phospholipid liquid crystals with temperature-jump cryo-TEM

The transitions between lamellar (Lα), inverted cubic (QII), and inverted hexagonal (HII) liquid crystalline phases can be very rapid, despite drastic changes in system topology. The Lα phase is a stack of flat lipid bilayers, the HII phase is a hexagonal array of inverted rod micelles, and the QII phase is a lipid bilayer labyrinth with the geometry of an infinite periodic minimal surface. The mechanism of the transitions from the Lα a phase are of particular interest: they start with interbilayer events that are related to the process of biomembrane fusion, a ubiquitous and essential process in cellular function. Previous theoretical work suggested two possible mechanisms for these transitions. We have studied these transitions using time-resolved cryo-TEM. Earlier, we used on-the-grid mixing techniques to trigger the transition and observe some of the intermediates that form within seconds. Here we use a novel temperature-jump cryo-TEM technique which triggers the phase transition several milliseconds before vitrification. We extend the initial results obtained with this apparatus and reinterpret them in terms of a new theory of the transition mechanism.

Dipyrenylphosphatidylcholine as a probe of bilayer pressures

We have used dipyrenylphosphatidylcholines (dipyPCs) to study the pressure in the fluid lamellar phase formed by mixtures of fully hydrated dioleoylphosphatidylcholine (DOPC) and dioleoylphosphatidylethanolamine (DOPE). As we increase the DOPE mole fraction at 25°C we observe a linear increase in the ratio of the excimer-to-monomer signal (E/M1). We argue that this observation can be understood in terms of an increase in the lateral pressure in the chain region, i.e., in the bilayer plane. This change itself is driven by the decrease in lateral pressure between headgroups as we add DOPE. We expect the lateral pressure to vary in magnitude as we probe the bilayer at different depths [1]. We have confirmed this by recording E/M1 using di[10-(pyren-1-yl)decanoyl]phosphatidylcholine (10dipyPC) and di[4-(pyren-1-yl)butanoyl]phosphatidylcholine (4dipyPC). We find that in 100% DOPC the E/M1 for 4dipyPC is 2.5 times greater than that for 10dipyPC. The above observations can all be rationalized in terms of changes in the lateral pressure profile. An inverse hexagonal liquid crystalline phase is found in the range 100-83% DOPE [2]. In this region of the phase diagram we observe a quadratic variation in E/M1, with a minimum at 95% DOPE. We hypothesize that this variation reflects the chain stretching that is necessitated by the geometrical packing constraints of the hexagonal phase [3]. Again, we find that the E/M1 for 4dipyPC is greater than that for 10dipyPC, but in this phase only by a factor of two.

Self-Assembly in Systems of Didodecyldimethylammonium Surfactants: Binary and Ternary Phase Equilibria and Phase Structures with Sulphate, Hydroxide, Acetate, and Chloride Counterions

The phase equilibria for four binary (temperature vs composition) surfactant systems didodecyldimethylammonium sulphate (DDAS), hydroxide (DDAOH), acetate (DDAAc), and chloride (DDAC) with water and one isothermal ternary system (DDAS-water-dodecane) have been studied by water deuteron NMR and polarizing microscopy methods. Both DDAS and DDAC surfactants are practically insoluble in water (≈3.5 × 10 -3 m DDAS and ≈ 1.4 × 10 -2 m DDAC) (m, moles of surfactant per kg of water) at 298 K, but they swell in water, each giving rise to a lamellar liquid crystalline phase. On the other hand, DDAOH and DDAAc are easily soluble in water (1.1 m surfactant) forming extended isotropic micellar solution phases which are followed by a lamellar liquid crystalline phase. The single lamellar phase formed in the surfactant system with divalent SO 2- 4 counterion can incorporate significantly smaller amounts of water ( n ≈ 14.4 moles of water per mole surfactant ion) between their lamellae compared to the corresponding system with monovalent Cl - ( n ≈ 102), OH - ( n ≈ 37), and CH 3COO - ( n ≈ 42) counterions, whereas the minimum amounts of water ( n = 3-5) necessary to form the lamellar phase does not differ significantly among the systems. Moreover, all systems yield only one lamellar liquid crystalline phase compared to the bromide (DDAB) system in which two lamellar phases are obtained in the aqueous system. Unlike other systems, the DDAS system, on heating, forms two isotropic solution phases: one, water-rich L 1 phase exists below ≈5 wt% of surfactant and the other, L 2 phase, above 45 wt% of surfactant and the two phases coexist above 353 K. Thermal stability of the lamellar phase increases in the order DDAAc ≈ DDAS < DDAC < DDAOH ≈ DDAB. The lamellar phase formed in the binary sulphate system has very limited swelling capability with dodecane (≈3%). In addition, in the ternary DDAS system three new homogeneous phases are found to exist; a cubic liquid crystalline phase in the oil-rich region (≈49% oil), a hexagonal liquid crystalline phase between ≈30-38% of water and ≈20-30% of oil and a narrow isotropic solution phase with few percent of oil and extending between ≈62-almost 100% of water. The phase behavior of the ternary DDAS system is almost identical to that of the ternary DDAB system, but the location of single phases in the triangle and microstructure of some phases are drastically different between the two systems. The phase behavior and phase stability may be understood in terms of electrostatic effects and surfactant packing parameters.

Phase behaviour of the lidocaine-monoolein-water system

The phase behaviour of the four-component system, lidocaine base-lidocaine HCl-monoolein-water, was investigated. It was found that the cubic phase formed in the monoolein-water system was transformed into a lamellar liquid crystalline phase on addition of lidocaine HC1, and to a reversed hexagonal liquid crystalline phase or a reversed micellar phase when the base form was added. With roughly equal amounts of the base and salt forms of lidocaine, the cubic phase persists. These findings strongly indicate not only that lidocaine in both forms participates in the lipid aggregation, but also that the interfacial curvature changes in the opposite direction in relation to the curvature of the cubic phase. These changes were rationalized by making use of the so-called packing concept of amphiphilic molecules. From a pharmaceutical point of view, the phase behaviour of the system shows a number of interesting properties. Some phases may be in equilibrium with excess water solutions, and temperature-induced transitions occur between phases of very different rheology.

Effect of head-group structure and counterion condensation on phase equilibria in anionic phospholipid-water systems studied by deuterium, sodium-23, and phosphorus-31 NMR and x-ray diffraction

The phase equilibria, hydration, and sodium counterion association for the systems DOPA-2H2O, DOPS-2H2O, DOPG-2H2O, and DPG-2H2O were investigated with 2H, 23Na, and 31P NMR and X-ray diffraction. The following one-phase regions were found in the DOPA-water system: a reversed hexagonal liquid-crystalline (HII) phase up to about 35 wt % water and a lamellar liquid-crystalline (L alpha) phase between about 55 and 98 wt % water. The area per DOPA molecule was 36-65 A2 in the HII phase (10-40 wt % water) and 69 A2 in the L alpha phase (60 wt % water). DOPS and DOPG with 10-98 wt % water, and DPG with 20-95 wt % water formed an L alpha phase at temperatures between 25 and 55 degrees C. At temperatures above 55 degrees C, DPG with 20 and 30 wt % water formed a mixture of L alpha, HII, and cubic liquid-crystalline phases, the mole percent of lipid forming nonlamellar phases being smaller at 30 wt % water than at 20 wt % water. DPG with 10 wt % water probably formed a mixture of an L alpha phase and at least one nonlamellar liquid-crystalline phase at 25 and 35 degrees C, and a pure HII phase at 45 degrees C and higher temperatures. At water concentrations above about 50 wt % the 23Na quadrupole splitting was constant for all four lipid-water systems studied, implying that the counterion association to the charged lipid aggregates did not change upon dilution. These experimental observations can be described with an ion condensation model but not with a simple equilibrium model. The fraction of counterions located close to the lipid-water interface was calculated to be greater than 95%. The 2H and 23Na NMR quadrupole splittings of 2H2O and sodium counterions, respectively, indicate that the molecular order in the polar head-group region decreases for the L alpha phase in the order DOPA approximately DPG greater than DOPS greater than DOPG.

Supramolecular organization of double-stranded DNA molecules in the columnar hexagonal liquid crystalline phase: An electron microscopic analysis using freeze-fracture methods

I present an electron microscopical analysis of the columnar hexagonal liquid crystalline phase of DNA. Freeze-fracture methods reveal that this phase is a lamellar structure, each layer (30 to 40 Å thick) composed of DNA molecules aligned in parallel. Numerous defects can be seen in the structure, and their nature is determined. I show that they are mainly screw dislocations of both handedness. By this method it is possible to follow individual double-stranded DNA molecules in this highly packed structure. I show, moreover, that there is a local twist between DNA molecules along the screw dislocation lines and that this twist can be either right-handed or left-handed. The interest of such ultrastructural analysis is discussed in relation to the understanding of chromatin structure.

Probing the microstructure of liquid crystalline surfactant systems; a physicochemical study using rheometry, DTA and 17O NMR

Mixtures of Brij 96 (decaethylene oxide oleyl ether) and water form liquid crystalline systems (creams) which have a potential as dosage forms for controlled dermal and transdermal drug delivery. By changing the water-surfactant mixing ratio in these systems, lamellar, viscous isotropic or hexagonal liquid crystalline phases can be obtained. First, a number of liquid crystalline creams were characterized using rheometry. Going from a lamellar cream (low water content) to a hexagonal (high water content) one, the macroscopic viscosity strongly increased. In Brij 96-water creams, the polyfethylene oxide) (PEO) chains of the surfactant molecules and the water form a continuous hydrophilic domain, the microstructure of which win be important for the diffusion of drugs through the creams. The hydrophilic domains in the liquid crystalline creams were investigated using 17O nuclear magnetic relaxation and differential thermal analysis and compared with those of PEO-water solutions. From these studies the conclusions were drawn that (i) the hydrophilic domains may be regarded as PEO solutions in water, and that (ii) the cream structure has no detectable influence on the properties of the microstructure of the hydrophilic domains. In the most concentrated creams and PEO solutions studied, having a water-EO ratio of 1.5, an 8-fold retardation of the rotational mobility of the water molecules was observed using 17O nuclear magnetic relaxation.

Self-association of an α,ω-type (bolaform) surfactant: Phase diagram, self—diffusion, and NMR studies of eicosane-1,20-bis(triethylammonium bromide)

The binary phase diagram (temperature vs composition) of the cationic bolaform (α,ω-type) surfactant eicosane-1,20-bis (triethylammonium bromide) has been determined by polarizing microscopy and water deuteron NMR methods. The surfactant is easily soluble in water giving an extended isotropic solution phase followed by a normal hexagonal liquid crystalline phase (E) at higher surfactant concentrations. On increasing the temperature the liquid crystalline phase extends to the surfactant-rich side. The 2H NMR quadrupolar splitting values in the E phase are found to increase at first with increasing surfactant concentration, then to decrease and go through zero splitting; then the splitting values increase again sharply with increasing surfactant concentration. The micellar phase has a very low capacity to solubilize hydrophobic substances. The narrow 1H and 14N NMR bandwidths as well as the NMR self-diffusion data for water and surfactant indicate the formation of small, closely spherical miceless in the entire isotropic micellar solution phase.

Liquid crystallinity of lecithin systems. Ternary phase diagrams of lecithin—water with triton X-100 and decanol

The isothermal phase diagrams for two ternary systems of soybean lecithin—water with Triton X-100 and decanol have been determined by water deuteron NMR and polarizing microscopy methods. The lamellar phase formed in the binary lecithin systems is found to swell and take up more water by the addition of both Triton X-100 and decanol, but the swelling is more pronounced in the presence of Triton X-100. Two isotropic solution phases, one water-rich (L 1) and the other rich in Triton X-100 (L 2), separated by a hexagonal liquid crystalline phase, form in the aqueous Triton X-100 system. The hexagonal, L 1, and L 2 phases can solubilize 5, 10, and 5 wt% lecithin, respectively, and they form mixed aggregates. The solubility of lecithin in pure Triton X-100 is limited. On the other hand, lecithin is highly soluble (62%) in decanol in which about 18% water can also be dissolved. The decanol system also yields a cubic liquid crystalline phase with both high lecithin and water contents and this phase occupies a very narrow region in the triangle. The swelling of the lamellar liquid crystalline phase can be understood with the hydration force based on the electrostatic continuum theory. The calculated phase diagram is found to agree fairly well with that constructed from the experimental data.

Some observations on phase diagrams and structure in binary and ternary systems of sodium and calcium dodecyl(monooxyethylene) sulfate

The phase equilibria of sodium (SDE 1S) and calcium (CDE 1S) dodecylmonooxyethylene sulfate in two- and three-component systems are studied by water deuteron NMR and polarizing microscopy methods. In the binary systems with water both SDE 1S and CDE 1S give micellar solutions, followed by the normal hexagonal liquid crystalline phases at 293 K. At high temperatures and with low water content, the lamellar liquid crystalline phases are also formed. The micellar aggregates with Ca 2+ as counterion start to grow at very low concentration whereas with Na + counterion, the micellar growth is observed only at very high surfactant concentration. The micellar solution and hexagonal liquid crystalline phases solubilize very small amounts of decanol which also increases the stability of the liquid crystalline phases toward the water corner. In the ternary system at 293 K, the coexistence of two lamellar liquid crystalline phases, one at high water content and the other at low water content, is detected with Ca 2+ as counterion whereas with Na + ion, there exists only one lamellar liquid crystalline phase with an extensive stability range. The lyotropic nematic liquid crystalline phases are found to form in the ternary systems with high water and very low decanol contents, and the structure of the phases are of lamellar-nematic type with Na + and hexagonal-nematic type with Ca 2+ as counterion. The calcium system also yields reverse hexagonal liquid crystalline phase with high both surfactant and decanol contents. The micellar growth and the coexistence of two lamellar liquid crystalline phases are explained qualitatively by recent theoretical models of attractive double-layer forces and image-charge interactions.

Phase diagrams of alkyltrimethylammonium surfactants in some polar solvents

Phase diagrams of alkyltrimethylammonium surfactants and different polar solvents were determined or collected from the literature. Hexadecyltrimethylammonium bromide forms with water a solution phase L, a hexagonal liquid crystalline phase E, a cubic phase I, and a lamellar phase D, with increasing surfactant concentration. In glycerol, the same aggregation sequence was found. With ethylene glycol, the existence regions of the liquid crystalline phases were found to be considerably smaller. The alkyl chain length of the surfactant also influences the extensions of the liquid crystalline phases. With a shorter alkyl chain the size of the existence regions was found to decrease, in particular with ethylene glycol as solvent. The size of the existence regions for the liquid crystalline phases with hexadecyltrimethylammonium bromide and water, glycerol, formamide, ethylene glycol, and N-methylformamide, respectively, was correlated to the solvent/hydrocarbon interfacial tension. A high interfacial tension suggests a considerable solvophobic interaction which favors extensive surfactant aggregation. The interfacial tension decreases in the order water > formamide ≈ glycerol > ethylene glycol > N-methylformamide, the same sequence as for the size of the existence region of the hexagonal phase E.

Structure and phase behavior of hydrated mixtures of L-dipalmitoylphosphatidylcholine and palmitic acid. Correlations between structural rearrangements, specific volume changes and endothermic events

Several new features of the phase diagram of L-dipalmitoylphosphatidylcholine (DPPC)/palmitic acid mixtures in excess water were established by means of static and time-resolved X-ray diffraction, densitometry and differential scanning calorimetry (DSC). At low temperatures, palmitic acid has a biphasic effect on the lamellar subgel phases: at concentrations below 5–6 mol%, it prevents formation of the DPPC subgel phase (L c ), while at higher contents (between about 40 and 90 mol%) another subgel phase (L c com) is formed as a result of lipid co-crystallization at 1 DPPC : 2 palmitic acid stoichiometry. A crystalline palmitic acid phase separates from L c com above 70–80 mol% of fatty acid. The L c com phase transforms into a lamellar gel phase (L β) in an endothermic transition centered at 38°C. At high temperatures, the mixtures form hexagonal liquid-crystalline phase (H II) in the region of 60–70 mol% and an isotropic phase (I) at 90–100 mol% of palmitic acid. No coexistence of H II phase with the fluid lamellar phase of DPPC was observed at intermediate compositions (20 and 50 mol% of palmitic acid) but rather formation of a complex phase with non-periodic geometry characterized by molten chains and a broad, continuous small-angle scattering band. No evidence for fluid phase coexistence was found also at compositions between H II and I phases. The L β-H II transition at 60–70 mol% of palmitic acids is readily reversible and two-state in both heating and cooling modes. It is characterized by the coexistence of initial and final phases with no detectable intermediates by time-resolved and static X-ray diffraction. The crystalline-isotropic transition in palmitic acid is two-state only in heating direction. On cooling, it is characterized by strong undercooling and gradually relaxing lamellar crystalline structures. The slowly reversible L c com-L β transition proceeds continuously through intermediate states. Although clearly discernible by both DSC and X-ray diffraction, it is not accompanied by specific volume changes.

Magnesium and calcium surfactants Ternary phase diagrams of magnesium and calcium dodecylsulphate with decanol and water

The isothermal ternary phase diagrams for the systems magnesium dodecylsulphate-decanol-water at 40 °C and calcium dodecylsulphate-decanol-water at 50 °C are determined by water deuteron NMR and polarizing microscopic studies. In the magnesium system, three liquid crystalline phases (lamellar and normal and reverse hexagonal) and two isotropic (normal and reverse) solution phases are characterized and their ranges of existence are obtained. The calcium system yields the same liquid crystalline phases, but only the lamellar liquid crystalline phase is investigated in detail. The important observations made are: (i) The lamellar liquid crystalline phase for the magnesium and calcium systems can incorporate, respectively, a maximum of 22.5 and 14.3 mole water per mole surfactant ion against 139 mole water for the corresponding sodium system. (ii) The reverse hexagonal liquid crystalline phase is formed for both the magnesium and calcium systems while no such liquid crystalline phase exists for the corresponding sodium system. (iii) The2H NMR quadrupole splittings obtained in the liquid crystalline phases for C8SO 4 − and C12SO 4 − surfactant systems with different counterions (Ca2+,Mg2+,Be2+,Na+) reveal that surfactant hydration is almost independent of alkyl chain length and counterions.

Phase behaviour and structure of polymer surfactants in aqueous solution. The occurrence of lyotropic nematic phases

AbstractThe binary phase diagrams are analyzed in aqueous solutions of monomeric, non‐ionic surfactants and the corresponding polysiloxanes having attached the surfactants as side chains via the hydrophobic parts of the amphiphiles. In contrast to common surfactants these hydrophobic parts additionally contain a rigid, mesogenic biphenyl moiety. While the polymeric surfactants exhibit hexagonal and lamellar lyotropic liquid crystalline phases, similarly to the low‐molar‐mass surfactants, two important results have to be emphasized: (i) The liquid crystalline phase regions are markedly extended with respect to concentration and temperature compared with the corresponding monomers, as shown in a similar way for thermotropic polymer liquid crystals. (ii) The phase sequence of the polymorphic liquid crystalline phases can be changed by polymerization. While for the systems under investigation the monomeric surfactant exhibits a cubic phase, the polymeric surfactant shows no cubic but a nematic phase in front of the hexagonal phase at low surfactant concentration. The nematic phase of the polymeric surfactant is the first example for a nematic phase, which is formed for binary mixtures of non‐ionic surfactants and water.

Molecular organization in phases of lecithin-cholate-water as studied by nuclear magnetic resonance.

The molecular organization in the liquid crystalline phases and the micellar solution phase has been investigated using numerous nuclear magnetic resonance techniques. A brief review of previous studies on the lamellar, hexagonal and cubic liquid crystalline phases is given. Mixed micelles were studied by measurements of 2H T1 and T2 nuclear magnetic resonance relaxation times of 2H-labeled phosphatidylcholine. Using simple and rough models, the size and shape of the micellar aggregate were estimated.

Liquid crystallinity in systems of magnesium and calcium surfactants: Phase diagrams and phase structures in binary aqueous systems of magnesium and calcium di-2-ethylhexylsulfosuccinate

The two binary systems of water and calcium or magnesium di-2-ethylhexylsulfosuccinate were investigated in the temperature range 298–423 K (25–150°C) concerning phase behavior, in particular, and also as regards to water binding and thermodynamics of phase transitions. The results are compared with those of the corresponding sodium system (Aerosol OT). In all three systems, the sequence of homogeneous phases with increasing surfactant concentration is: isotropic solution (L 1, lamellar (D), cubic (I 2), and reversed hexagonal (F) liquid crystalline phases. The single-phase regions are separated by two-phase regions. The Ca 2+ and Mg 2+ systems give closely the same phase diagrams which differ considerably in two respects from the sodium system phase diagram. First, the aqueous solubility is lower by orders of magnitude with the divalent counterions. Second, the lamellar phase can swell and incorporate water to a much larger extent with Na + than with Ca 2+ or Mg 2+ (difference in molar ratio of water to surfactant ion by more than a factor of 10). The gross features of the phase behavior are in qualitative agreement with electrostatic theory. Minor differences between Ca 2+ and Mg 2+ as regards water content of phases and thermal stability of phases are discussed in terms of ion specificity effects arising from differences in hydration, and the hydration aspects were also investigated directly from 2H nuclear magnetic resonance quadrupole splittings.

Aqueous lipid phases of relevance to intestinal fat digestion and absorption.

The phase behavior of monoglyceride/water systems, with oleic and linoleic acid as the dominating fatty acid residues, was investigated. Increased solubilization of triglycerides (oil) or oleic acid in the cubic liquid-crystalline phase formed by monoglyceride and water resulted in the formation of a reversed hexagonal liquid-crystalline phase followed by an L2-phase. The liquid-crystalline phases have different dispersion properties compared to each other in dilute micellar bile salt solutions. The cubic phase is found to be easily dispersed. The relevance of aqueous lipid phases other than micellar is discussed in relation to intestinal lipid digestion and absorption.

The effect of cholesterol on the phase structure of glucolipids from Acholeplasma laidlawii membranes.

1. The packing properties in lipid mixtures containing cholesterol and membrane glucolipids from Acholeplasma laidlawii are modified by varying the amounts of cholesterol, diacylmonoglucosylglycerol and diacyldiglucosylglycerol in the mixtures as well as the temperature and the degree of acyl chain unsaturation. These changes affect both the proportions of different lipids having dissimilar molecular geometries and the geometry of the lipid molecules themselves. 2. All mixtures containing glucolipids with equal amounts of palmitoyl and oleoyl chains formed a lamellar liquid-crystalline phase in the growth temperature range of A. laidlawii, while a reversed cubic liquid-crystalline phase dominated in mixtures containing dioleoyl glucolipids. These lipids formed a reversed hexagonal phase together with 27% cholesterol. Mixtures with lipid compositions occurring in the membranes of living Acholeplasma cells formed a lamellar liquid-crystalline phase. 3. Large amounts of cholesterol and diacylmonoglucosylglycerol, high temperatures and a high degree of cis unsaturation favoured the formation of cubic or hexagonal liquid-crystalline phase structures of the investigated lipid mixtures. Diacylmonoglucosylglycerol and cholesterol are both wedge-shaped. Temperature and cis unsaturation accentuate the wedge-shape properties of the glucolipid molecules. 4. The changes in the lipid composition of A. laidlawii membranes as a response to cholesterol incorporation can be explained by the geometry and packing characteristics of the sterol molecule and the concept of 'fluidity' does not need to be involved.

Ultrasonic Relaxation Measurements in Lyotropic Liquid Crystals

AbstractUltrasonic relaxation measurements covering the frequency range 5–100 MHz have been carried out in the isotropic as well as the hexagonal and lamellar liquid crystalline phases of aqueous solutions of the surfactants: caesium perfluorooctanoate, mono‐olein, mono‐caprylin, tetraethylene glycol dodecyl ether (CE04), pentaethylene glycol dodecyl ether (CEO) and di‐(2‐ethylhexyl) sodium sulfosuccinate (aerosol OT). The absorption data obtained in concentrated isotropic solution and also the hexagonal and lamellar phases can be fitted into an equation involving a single relaxation time in the frequency range 25–100 MHz. The concentration and temperature dependence of the relaxation times and amplitudes have been determined. A discussion of the likely mechanisms causing the relaxation will be given. These measurements also show that there is a pronounced increase in the quantity as the different phase boundaries are approached. This is very close to the behaviour of absorption data near the critical point in liquid mixtures.