Gel-liquid Crystalline Research Articles

Self-assembled gels of liquid crystals: hydrogen-bonded aggregates formed in various liquid crystalline textures

An azobenzene-containing gelator can gel the nematic liquid crystal BL006 through self-assembly of hydrogen-bonded aggregates. Due to the high nematic–isotropic transition temperature of BL006, the gelation occurs either in the isotropic or the nematic phase depending on the concentration of the gelator. Prior to the aggregation in the nematic phase upon cooling, the gelator can act as a chiral dopant and induce a chiral nematic phase for BL006 over a range of temperatures. This finding has been used to investigate the influence of the liquid crystal environment on the aggregation of the gelator. Making use of rubbed surfaces, electric field and different concentrations of the gelator, various liquid crystalline orientation states, referred to as planar, fingerprint, homeotropic and homogeneous textures, were obtained, and the aggregation of the gelator was allowed to proceed in these environments. It was found that a variety of fibrous and elongated aggregates, differing in morphology and alignment, could be formed, and that the self-assembled liquid crystalline gels with different aggregates could exhibit different electrooptic effects.

Effects of lysozyme and bovine serum albumin on membrane characteristics of dipalmitoylphosphatidylglycerol liposomes

The effects of adsorption of two kinds of proteins on the membrane characteristics of liposomes were examined at pH 7.4 in terms of adsorption amounts of proteins on liposomes, penetrations of proteins into liposomal bilayer membranes, phase transition temperature, microviscosity and permeability of liposomal bilayer membranes, using positively charged lysozyme (LSZ) and negatively charged bovine serum albumin (BSA) as proteins and negatively charged l- α-dipalmitoylphosphatidylglycerol (DPPG) liposomes. The saturated adsorption amount of LSZ was 720 g per mol of liposomal DPPG, while that of BSA was 44 g per mol of liposomal DPPG. The penetration of LSZ into DPPG lipid membranes was greater than that of BSA. The microviscosity in the hydrophobic region of liposomal bilayer membranes increased due to adsorption (penetration) of LSZ or BSA, while the permeability of liposomal bilayer membranes increased. The gel–liquid crystalline phase transition temperature of liposomal bilayer membranes was not affected by adsorption of LSZ or BSA, while the DSC peak area (heat of phase transition) decreased with increasing adsorption amount of LSZ or BSA. It is suggested that boundary DPPG makes no contribution to the phase transition and that boundary DPPG and bulk DPPG are in the phase-separated state, thereby increasing the permeability of liposomal bilayer membranes through adsorption of LSZ or BSA. A possible schematic model for the adsorption of LSZ or BSA on DPPG liposomes was proposed.

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.

Electro-optic effects and phase behavior of liquid-crystalline physical gels: self-assembly of hydrogen-bonded molecules for the formation of dynamically functional composites

Anisotropic physical gels of liquid crystals are obtained by the formation of non-covalently-bonded networks through self-organization of low molecular weight compounds in nematic solvents. They exhibit thermoreversible transitions between isotropic liquid and isotropic gel, and between isotropic gel and liquid-crystalline gel, whose temperatures are dependent on the components. Electro-optic properties of liquid-crystalline gels are examined with twisted nematic cells. A nematic liquid crystal in a gel structure can respond to electric fields twice faster than a single liquid-crystalline component.

Liquid Crystalline Anisotropic Gels Based on Azobenzene-Containing Networks

A method is described using optical alignment of azobenzene groups to prepare liquid crystalline anisotropic gels. We demonstrate that anisotropic gels with uniform liquid crystal orientation can be obtained by performing thermal polymerization of a homogeneous mixture composed of a nonreactive nematic liquid crystal BL006 and a reactive nonmesogenic diacrylate monomer bearing azobenzene groups while the mixture is exposed to linearly polarized light. Infrared dichroism measurements show that this liquid crystal orientation is recoverable upon repeated cycles of heating the sample into the isotropic phase followed by cooling it to the nematic phase. In this method, the optical alignment of azobenzene monomer and, as a result, of liquid crystal molecules before and during the polymerization replaces the surface alignment or electric and magnetic fields, which are usually used to prepare anisotropic gels. Anisotropic gels were obtained from two diacrylate monomers that lead to networks having azobenzene groups as part of main chain and side groups.

Liquid-Crystalline Physical Gels: Self-Aggregation of a Gluconamide Derivative in Mesogenic Molecules for the Formation of Anisotropic Functional Composites

Liquid-crystalline physical gels have been obtained by the self-aggregation of a gluconamide derivative through intermolecular hydrogen bonding in liquid crystals. These liquid-crystalline gels are microphase-separated composites consisting of the solid fibrous aggregates of the gelling agent and the liquid-crystalline phase of mesogenic molecules. Thermoreversible three states, isotropic liquid, normal (isotropic) gel, and liquid-crystalline gel, have been achieved for the mixtures of liquid crystals and the gelling agent. As the concentration of the gelling agent increases, the sol−gel transition temperature and the enthalpy change of the transition from isotropic to anisotropic increase. The increase of the enthalpy change suggests that some mesogenic molecules are anchoring strongly to the surface of the fibrous gelling agents. Variable-temperature infrared spectra show that the association and the dissociation of intermolecular hydrogen bonding of the gelling agent occur in the process of the sol−gel transition. Liquid-crystalline gels are responsive to electric fields in a twisted nematic (TN) cell. These anisotropic gels would be applicable to new dynamically functional materials.

Physical Gelation of Nematic Liquid Crystals with Amino Acid Derivatives Leading to the Formation of Soft Solids Responsive to Electric Field.

4′-heptyloxybiphenyl (7OCB), have been physically gelled due to the formation of hydrogen-bonded networks comprised of amino acid derivatives. The mixtures of nematic liquid crystals and amino acid derivatives exhibit thermoreversible three states, isotropic, isotropic gel, and nematic liquid-crystalline gel, through the phase change of each of components. The gel states are induced by the formation of microphase-separated structures and transition properties of liquid-crystalline solvents in the network structures are maintained. The anisotropic gels of 5CHB can respond to electric fields in polyimide-coated liquid crystal cells. The existence of a small amount of gelling agents has great effects on the electro-optic properties. Acceleration of response behavior of the nematic liquid crystal has been achieved by such physical gelation.

Stabilization of insulin against agitation-induced aggregation by the GMO cubic phase gel

The main objective of the study was to evaluate if the liquid crystalline cubic phase gel of glyceryl monooleate (GMO) protects insulin from agitation induced aggregation. The aggregation of Humulin®, Regular Iletin I® and Regular Iletin II®, in cubic phase GMO gels at 30 U/g of gel was compared with that in PBS at 100 oscillations/min at 37°C using optical density at 600 nm. The effect of agitation on the secondary structure of insulin in solution and in the gels was determined with circular dichroism (CD) spectroscopy, and the time course of aggregation was also followed by HPLC. A sigmoidal increase in optical density of solution with time indicated formation of increasing amounts of insoluble insulin aggregates. However, in the gels, optical density values stayed at, or around, the initial optical density value, comparable with that of a blank gel suggesting that insulin had not aggregated in the gel. CD spectroscopy of the soluble insulin showed a total loss of native conformation upon aggregation of insulin in solution. In contrast, CD spectra of insulin in the gel were unaltered suggesting protection from aggregation during agitation. Furthermore, agitation of insulin in gels for a duration as long as 2 months at 37°C, had very little adverse effect on the native conformation of insulin, as indicated by the lack of a significant change in its CD spectrum. Therefore, the cubic phase gel was indeed able to protect insulin from agitation-induced aggregation and subsequent precipitation. Although the majority of insulin in solution appeared to have aggregated and precipitated after 8 days by UV and CD spectroscopy, RP-HPLC results indicated the presence of some soluble aggregates of insulin. In summary, the liquid crystalline cubic phase gel of GMO protects peptides, like insulin, from agitation-induced aggregation.

Influence of inorganic salts on optical transmission behaviour of thermotropic hydrogels

Lyotropic liquid crystalline gels based on a polyalkoxy derivative with a middle molecular weight of 4800 can be used for the development of smart windows. Inorganic salts added to the polymer/water mixture have a strong influence on temperature-transmission behaviour. Changes in transmission are based on phase transitions as well as on phase separations. The results of our studies show that the detailed knowledge of the influence of salt concentration on the temperature of the phase separation permits researchers to develop systems which change their transmission from clear to cloudy at any desired temperature. Based on the gel systems studied, it is possible to develop intelligent windows which change their transmission values from ≥ 95% to almost 0% in the temperature range between 25 and 50 °C, thus allowing them to work as intelligent shadow-givers. Einfluß von anorganischen Salzen auf das optische Transmissionsverhalten von thermotropen Hydrogelen Lyotrope flüssigkristalline Gele auf der Basis eines Polyalkoxyderivates mit einer mittleren Molmasse von 4800 eignen sich für die Entwicklung von intelligenten Fenstern. Der Zusatz von anorganischen Salzen zur Polymer/Wasser-Mischung hat einen starken Einfluß auf das temperaturabhängige Transmissionsverhalten der Gele. Die Transmissionsänderungen basieren sowohl auf Phasenumwandlungen als auch auf Phasenseparationen. Die detaillierte Kenntnis des Einflusses der Salzkonzentration auf die Temperatur der Phasenseparation gestattet es, Systeme zu entwickeln, die bei einer gewünschten Temperatur ihre Transmission von klar nach trüb ändern. Auf der Basis der vorgestellten Gelsysteme ist es möglich, intelligente Fenster zu entwickeln, die im Temperaturbereich zwischen 25 und 50 °C ihre Transmissionswerte von ≥ 90% auf fast 0% ändern und somit als intelligente Schattenspender wirken.

Shape transformations of giant unilamellar vesicles induced by ethanol and temperature variations

The shape transformations of giant unilamellar vesicles (GUVs) of dimyristoylphosphatidylcholine (DMPC) in water/ethanol solutions were investigated as a function of temperature (18–43°C) and ethanol content (0–40 vol%). GUVs were prepared by electroformation and their shapes were monitored by phase-contrast microscopy furnished with an image-recording set-up. Two systems were compared: (1) GUVs formed in presence of ethanol; and (2) GUVs preformed in distilled water with ethanol added subsequently. In the first case, GUVs in the gel state are typified by a multifaceted, irregular shape that transforms into a smooth, rounded shape with increase of temperature. This transformation is reversible and takes place at about or below 24°C. It coincides in temperature with the DMPC gel–liquid crystalline phase transition in the respective water/ethanol solutions. The effect of ethanol added to GUVs preformed in distilled water is strikingly different. The temperature of the multifaceted(folded)-to-smooth shape transformation first increases from 24 to 37°C with increase of ethanol content up to 10 vol%, then decreases back to 24°C upon further increase of ethanol up to 40 vol%. The reversal point of this biphasic effect at 10 vol% of ethanol coincides with that for formation of DMPC interdigitated phase. The shape transformations are reversible with a temperature hysteresis of 2–3°C. They are rather cooperative and take place in a narrow range of less than 0.2°C at temperatures that remain constant for times of up to 15 h.

Supramolecular liquid crystals and liquid-crystalline gels. Molecular self-organization through the formation of hydrogen bonding

Supramolecular liquid crystals and liquid-crystalline gels. Molecular self-organization through the formation of hydrogen bonding

Capillary gel electrophoresis of nucleic acids in pluronic F127 copolymer liquid crystals

The liquid crystalline gel phases of solutions of Pluronic F127, a triblock copolymer, were recently introduced as an alternative to disordered solutions of random coil polymers as replaceable media for capillary gel electrophoresis (CGE). Pluronic F127, from BASF, is a copolymer of poly(ethylene oxide) and poly(propylene oxide) with the approximate formula (EO)106 (PO)70 (EO)106. Polymer chains aggregate into spherical micelles in aqueous solutions, with poly(propylene oxide) chains creating a hydrophobic core surrounded by brushes of hydrated poly(ethylene oxide) tails. Crowding at high concentrations promotes ordering of micelles. Solutions in the range of about 14–24 % polymer are self-supporting, gel-like cubic liquid crystals at 25–30°C, but when cooled they become low viscosity liquids that are easily loaded into capillaries. This article reviews applications of Pluronic F127 media for capillary gel electrophoresis separations of nucleic acids of several types including oligonucleotides, double stranded DNA fragments, and supercoiled plasmid DNAs.

Structure and Optical Properties of a Thermoresponsive Polymer-Grafted, Lipid-Based Complex Fluid

We report herein an example of a self-assembling, stimulus-responsive complex fluid that switches between two distinct structural (two-dimensional (2-D) to one-dimensional supramolecular architecture) and functional (fluid to gel and nonbirefringent to birefringent) states in response to modest temperature changes. This complex fluid is formed by the noncovalent self-assembly of a quaternary mixture of a phospholipid, dimyristolyphosphatidylcholine (DMPC), a lipopolymer consisting of poly(ethylene oxide) terminally grafted onto the phosphate headgroup of DMPE (DMPE-EO45), and a cosurfactant, lauryldi-methylamine N-oxide (LDAO), dispersed in water. The supramolecular assembly formed by this mixture has been found to undergo a reversible transformation from a liquid-crystalline gel to a nonbirefringent fluid upon reduction in temperature. The onset of the thermotropic phase transition has been determined to be 16 °C by differential scanning calorimetry (DSC). The structural organization (i.e., the supramole...

Water–mucin phases: conditions for mucus liquid crystallinity

Previous studies of mucin conformation in aqueous media have focussed on the polymer backbone, on dilute concentrations, and on ambient temperatures. Using differential scanning calorimetry and transmitted polarised light microscopy, we demonstrate that commercially available pig gastric mucin forms a liquid crystalline gel at concentrations above approximately 26% w/w in water. Solvated mucin exhibits a glass transition at approximately 24.8°C, and the liquid crystalline phase is only fluid above this temperature. We associate the glass transition with the onset of flexibility in the backbone of glycosylated molecular segments. Consideration of molecular geometry suggests that mucin liquid crystallinity is governed by interactions between side chains, not main chains. For mucin from a given source, neither the conditions required for liquid crystallinity nor the glass transition temperature should be sensitive to the mucin molecular weight, as long as the integrity of the glycosylated segments is maintained. We consider some physiological implications of our results, including the impact of hypothermic conditions on the functionality of mucus, and we emphasise the need to characterise mucin and mucus structure at physiological temperatures.

Catalytic hydrogenation of polyunsaturated biological membranes: effects on membrane fatty acid composition and physical properties

The relationship between phospholipid saturation and membrane physical structure in a complex, highly polyunsaturated biological membrane (trout liver microsomes) has been studied by the graded and specific hydrogenation of polyunsaturated fatty acids. The homogeneous catalyst Pd(QS) 2 caused rapid and effective hydrogenation, increasing the proportion of saturated fatty acids from 20–30% up to 60%, without loss or fragmentation. Long chain, polyunsaturated fatty acids (20:5 ω3, 22:6 ω3) were rapidly converted to a large number of partially hydrogenated isomers, and ultimately to the fully saturated C20 or C22 fatty acids. C18 mono- and di-unsaturates showed slower rates of hydrogenation. Increased saturation was closely associated with an increased membrane physical order as determined by the fluorescence anisotropy probe, 1,6-diphenyl-1,3,5-hexatriene. However, extensive hydrogenation led to highly ordered membranes exhibiting a gel-liquid crystalline phase transition between 30 and 60°C. Polyunsaturated membranes can thus be converted into partially or substantially saturated membranes with measurable phase structure without direct alteration of other membrane components. This offers a less equivocal means of assessing the influence of polyunsaturation upon membrane structure and function.

Liquid crystalline phase metastability of phosphatidylglycerols

Aqueous dispersions of dilauroylphosphatidylglycerol (DLPG) and dimyristoylphosphatidylglycerol (DMPG) form stable subgel and liquid crystalline lamellar phases, Lc and Lα, and also metastable gel phases, Lβ′ and Pβ′, at physiologically relevant conditions (neutral pH, <0.2 M salt). The metastable gel phase Lβ′ relaxes to the stable subgel phase upon low-temperature storage, with at least one metastable intermediate in the case of DMPG. The gel–liquid crystalline phase transition takes place at temperature with 15–30°C lower as compared to that of the subgel–liquid crystalline phase transition in these phosphatidylglycerols. Therefore, the lamellar liquid crystalline phase Lα is expected to exhibit metastability in the temperature range between the gel–Lα and subgel–Lα phase transitions, in which the subgel phase is the equilibrium one. Here we demonstrate by using differential scanning calorimetry that crystallisation from the liquid crystalline phase in this temperature interval takes place in DMPG and DLPG. The kinetics of the observed Lα-to-Lc phase relaxation is sensitive to the thermal prehistory of the lipid dispersion. This is possibly related to the different morphologies of the lipid aggregates in the subgel and liquid crystalline phases (Epand et al., 1992) and the slow interconversion between them.

Supramolecular events modulate flunitrazepam partitioning into natural and model membranes

In the present paper we investigate the effects of chemical and physical variables which modulate supramolecular membrane organization on the partitioning of FNTZ into natural and model membranes. In dpPC the value of P dpPC-B increased almost discontinuously at a temperature close to the gel-liquid crystalline phase transition temperature of dpPC (41.5°C). In natural membranes, P m b remained constant upto 37°C and increased significatively at higher temperatures. However, the ascending slope was less sharp than in liposomes, reflecting a lower cooperativity of the change due to the higher complexity of the natural membrane compared with that of the pure lipid. As a function of pH, P values increased slightly in dpPC, increased markedly in dpPE, decreased in dpPS and, in dpPA, P decreased up to pH 8 and, at higher pH values, it tended to increase again. The variation in P as a function of pH resulted from the balance between two opposite tendencies: (1) changes in the viscosity of phospholipid bilayers as a function of pH due to modification of the surface net charge leading to changes the packing of the lipid molecules, and (2) the reduction of interface hydration induced by the decreasing pH which enhances its hydrophobicity. The former membrane property is favored up to pH 7. Partitioning of FNTZ is favored in less hydrated and less ordered interfaces. At pH < 3, FNTZ stablishes with the lipid-interface attractive (dpPS and dpPA) or repulsive (dpPE) electrostatic interactions, which can be screened by the ions arising from salt dissociation over a certain concentration level. The increase in the proportion of cholesterol in dpPC-cholesterol mixed liposomes induces a decrease in the value of P. This result may be explained by the assumption that possible vacancies which might incorporate FNTZ are already filled with cholesterol, thus inhibiting further FNTZ incorporation. The membrane-buffer partition coefficient of FNTZ is strongly influenced by the physical state of the bilayer membrane. Particular interactions may lead to a strong local accumulation of FNTZ within the membrane due to a coupling to lateral density fluctuations.

An investigation of the parameters influencing the bioadhesive properties of Myverol 18–99/water gels

An in vitro assessment was undertaken of the parameters influencing the bioadhesive strength of lyotropic liquid crystalline gels formed by the monoglyceride blend Myverol 18–99 and water. The gels were found to bind more strongly to a dry Perspex surface than to moist mucosal tissue and were shown to be weaker bioadhesives than Carbopol 934P and sodium alginate. The works of adhesion and forces of detachment were independent of contact time, were reduced by the presence of surface water and increased with an increase in the compression force, suggesting that interpenetration was not the mechanism of bioadhesion. The bioadhesion of Myverol 18–99/water gels appeared to be due to secondary chemical bonds, such as van der Waals forces, but was limited by their cohesive strength. An in vivo gel retention assessment by gamma scintigraphy showed that the gels were retained within the vaginal cavity for a period of at least 6 h.

Molecular mechanics simulation studies of dienoic hydrocarbons: From alkenes to 1-Palmitoyl-2-linoleoyl-phosphatidylcholines

Although no crystal structures of mixed-chain phosphatidylcholines with unsaturated sn-2 acyl chains exist, the force field method in conjunction with the experimentally determined structure of saturated identical-chain phosphatidylcholine can be applied to simulate molecular structure for mixed-chain phosphatidylcholines. In this study, the packing models of mixed-chain 1-palmitoyl-2-linoleoyl-phosphatidylcholines in bilayers at temperatures below the gel-liquid crystalline phase transition temperature or T < Tm are simulated by using Allinger's MM3(92) force field. Our results indicate that the unsaturated sn-2 acyl chains of the mixed-chain lipid can fold into two energy-minimized topologies: the crankshaftlike and the U-shaped motifs. The folded region in the crankshiftlike sn-2 acyl chain is characterized by a sequence s− Δs+s+Δs−, and the U-shaped chain arises from the characteristic sequence s−Δs+s−Δs+, where s± denotes the ± skew conformation and Δ the cis carbon-carbon double bond. These modeled structures of 1-palmitoyl-2-linoleoyl-phosphatidylcholines in the bilayer at T < Tm should not be regarded as highly rigid structures, since torsion angles of carbon-carbon bonds associated with sequences s Δs+s+Δs and s Δs+s−Δ s+ can fluctuate somewhat without appreciably affecting the steric energy of the corresponding lipid bilayer. © 1996 by John Wiley & Sons, Inc.

The optical behavior of lyotropic liquid crystalline polymer gel networks: Dependence on temperature

Gel networks with interesting optical properties have been prepared on the basis of a highly ordered aqueous polymer lyotropic liquid crystalline phase, embedded in a thermally stable polymer network. The optical transparency changes reversibly with an increasing or decreasing temperature of a few degrees up to a contrast ratio of 85:1. Different lyotropic phases exist in close proximity in the system, for example a schlieren texture (see Figure) and a radial droplet structure. magnified image