Synthesis,thermal and lyotropic liquid crystalline properties of protic ionic salts

A series of protic ionic salts were synthesised by a simple acid–base reaction from various pyridine derivatives and dodecylbenzenesulfonic acid in a common organic solvent and characterised in terms of their thermal and lyotropic liquid crystalline properties using various experimental techniques. All of them exhibited lyotropic liquid crystalline phases in toluene, methanol, acetonitrile, dimethyl sulfoxide and water. Their critical concentrations for the formation of biphasic solutions and concentrations for the formation of lyotropic solutions were quite broad depending on the dielectric constants of the solvents. Their lyotropic phases were identified as lamellar phases, since their textures exhibited bâtonnets, oily streaks and mosaic textures. They can potentially be used for many organic transformations, which may have implications in green chemistry.     

A new urea gelator: incorporation of intra- and intermolecular hydrogen bonding for stable 1D self-assembly

A new bisurea gelator derived from 2,6-diaminopyridine has been developed. It efficiently gelates common organic and liquid crystalline (LC) solvents by forming elongated self-assembled fibres in solvents. X-Ray crystallography and 1H NMR measurements reveal that two urea groups in pyridine-based bisurea compounds form different hydrogen bonding patterns. One of two urea units is involved in intramolecular hydrogen bonding with the pyridyl nitrogen, while the other urea unit forms bifurcated intermolecular hydrogen bonding. This hydrogen-bonded structure is key for the fibrous self-assembly along with the efficient gelation. In addition, LC gels based on the pyridine-based gelator exhibit good electrooptic properties. These results indicate that the pyridine-based bisurea compound is a good gelator not only effective in gelation but also useful as a component of functional soft materials.     

Architectural diversity and elastic networks in hydrogen-bonded host frameworks: from molecular jaws to cylinders

Guest-free guanidinium organomonosulfonates (GMS) and their inclusion compounds display a variety of lamellar crystalline architectures distinguished by different "up-down" projections of the organomonosulfonate residues on either side of a two-dimensional (2D) hydrogen-bonding network of complementary guanidinium ions (G) and sulfonate moieties (S), the so-called GS sheet. Using a combinatorial library of 24 GMS hosts and 26 guest molecules, a total of 304 inclusion compounds out of a possible 624 possible host-guest combinations were realized, revealing a remarkable capacity of the GMS hosts to form inclusion compounds despite the facile formation of the corresponding guest-free compounds and the absence of "predestined" inclusion cavities like those in related guanidinium organodisulfonate host frameworks. The GS sheets in the inclusion compounds behave as "molecular jaws" in which organomonosulfonate groups projecting from opposing sheets clamp down on the guest molecules, forming ordered interdigitated arrays of the host organic groups and guests. Both the guest-free and inclusion compounds display a variety of architectures that reveal the structural integrity of two-dimensional GS sheet and the unique ability of these hosts to conform to the steric demands of the organic guests. Certain GMS host-guest combinations prompt formation of tubular inclusion compounds in which the GS sheet curls into cylinders with retention of the 2D GS network. The cylinders assemble into hexagonal arrays through interdigitation of the organosulfonate residues that project from their outer surfaces, crystallizing in high-symmetry trigonal or hexagonal space groups. This unique example of network curvature and structural isomerism between lamellar and cylindrical structures, with retention of supramolecular connectivity, is reminiscent of the phase behavior observed in surfactant microstructures and block copolymers. The large number of host-guest combinations explored here permits grouping of the inclusion compound architectures according to the shape of the guests and the relative volumes of the organomonosulfonate groups, enabling more reliable structure prediction for this class of compounds than for molecular crystals in general.     

吡嗪二羧酸-三聚氰胺双组分水凝胶的制备及性能

以3,6-二甲基-2,5-吡嗪二羧酸(P)和三聚氰胺(M)为组分,采用不同的摩尔比(1∶1,1∶2,1∶3)混合配制了3个样品PM11,PM12和PM13,并对其凝胶性能进行了测试.实验结果表明,PM能在水中及部分含水有机溶剂中形成稳定的凝胶,这些凝胶对酸碱具有良好的响应性能.采用扫描电子显微镜分析了3种水凝胶的微观形貌,均为纤维状的网络结构;红外光谱及紫外光谱测试结果表明氢键是形成凝胶的关键驱动力;XRD测试结果显示凝胶为层状结构.对PM12在不同pH值的水中的凝胶性能测试结果表明,在pH=3~11的范围内PM12均能形成凝胶.测试了PM12在混合溶剂中的凝胶性能,并将测试结果与混合溶剂的Hansen溶解度参数关联,以便用于分析溶剂与凝胶因子间的相互作用,所得结果亦表明氢键在凝胶形成的过程中起重要作用.     

Phase behavior of binary mixture of 1-dodecyl-3-methylimidazolium bromide and water revealed by differential scanning calorimetry and polarized optical microscopy

Phase behavior of aqueous mixture of imidazolium ionic liquid, 1-dodecyl-3-methylimidazolium bromide, was investigated by means of differential scanning calorimetry and polarized optical microscopy. The mixture forms two types of lyotropic liquid-crystalline gels, one is composed of lamellar phase and the other is of hexagonal phase. T-X phase diagram of the mixture was constructed, which defines the regions of various phases appearing in this mixture.     

High-resolution NMR characterization of a gel-like surfactant mesophase

The addition of phosphatidylcholine to AOT water-in-oil microemulsions leads to the formation of a rigid gel as the water content is increased above a specific threshold. This system is a gel-like crystalline phase where the microstructure evolves from reverse hexagonal to lamellar with increasing water content and/or temperature. Couette sheared (1)H and (31)P NMR experiments carried out at varying temperature and water content show distinct signatures with microstructure evolution. Because the system has been fully characterized through small-angle neutron scattering, it is possible to relate the NMR signatures to the microstructure. The NMR technique therefore complements scattering techniques but is additionally useful because the technique also picks up isotropic signatures from concurrently occurring noncrystalline phases. The use of NMR to identify such lyotropic gel-like crystalline phases allows easy correlation between templated materials synthesis in these phases and phase microstructure. NMR can therefore be used as a probe to understand microstructure in specific surfactant systems and to characterize the retention of microstructure during materials synthesis.     

Bis(amino acid) oxalyl amides as ambidextrous gelators of water and organic solvents: supramolecular gels with temperature dependent assembly/dissolution equilibrium

Bis(LeuOH) (1a), bis-(ValOH) (2a) and bis(PhgOH) (5a) (Phg denotes (R)-phenylglycine) oxalyl amides are efficient low molecular weight organic gelators of various organic solvents and their mixtures as well as water, water/DMSO, and water/DMF mixtures. The organisational motifs in aqueous gels are dominated primarily by lipophilic interactions while those in organic solvents are formed by intermolecular hydrogen bonding. Most of the gels are thermoreversible and stable for many months. However, 2a forms unstable gels with organic solvents which upon ageing transform into variety of crystalline shapes. For some 1a/alcohol gels, a linear correlation between alcohol dielectric constants (epsilon) and gel melting temperatures (Tg) was found. The 1H NMR and FTIR spectroscopic investigations of selected gels reveal the existence of temperature dependent network assembly/dissolution equilibrium. In the 1H NMR spectra of gels only the molecules dissolved in entrapped solvent could be observed. By using an internal standard, the concentration of dissolved gelator molecules could be determined. In FTIR spectra, the bands corresponding to network assembled and dissolved gelator molecules are simultaneously present. This enabled determination of the Kgel values by using both methods. From the plots of InKgel versus 1/T, the deltaHgel values of selected gels have been determined (-deltaHgel in 10-36 kJ mol(-1) range) and found to be strongly solvent dependent. The deltaHgel values determined by 1H NMR and FTIR spectroscopy are in excellent agreement. Crystal structures of 2a and rac-5a show the presence of organisational motifs and intermolecular interactions in agreement with those in gel fibres elucidated by spectroscopic methods.     

Effect of pharmaceutically acceptable glycols on the stability of the liquid crystalline gels formed by Poloxamer 407 in water

The ability of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers (Poloxamers) to form "gels" (lyotropic liquid crystalline structures) in water is of interest to pharmaceutical applications. In such applications the presence of polar organic solvents is often desirable or required. The effect of such solvents on the stability of lyotropic liquid crystalline gels formed by PEO-PPO-PEO block copolymers was assessed by studying the phase behavior and structure in ternary isothermal (25 degrees C) systems of pharmaceutical interest consisting of Poloxamer 407 (EO(100)PO(70)EO(100)), water, and one of the following solvents (referred to here collectively as "glycols"): glycerol, propylene glycol, ethanol, polyethylene glycol 400, and glucose. Small-angle X-ray scattering was employed to establish the structure of the liquid crystals obtained and to determine their characteristic length scales. The stability range of the liquid crystalline gel phases in the systems studied was found to vary with the glycol type. For example, the micellar cubic structure can accommodate about 0.85:1 parts glucose per part water (in terms of weight) and up to as much as 5.5:1 parts propylene glycol per part water. A correlation between the glycol effects on the stability of the liquid crystalline phases and glycol physiochemical characteristics such as octanol/water partition coefficient or solubility parameter is proposed.     

Liquid crystals derived from multi-cationic azamacrocyclic alkylsulphates

Two homologous series of azamacrocyclic n-alkylsulphates were synthesized and characterized. Their thermotropic liquid crystalline behaviour was studied by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. At room temperature both series exhibited lamellar crystalline phases. For the tetraazacyclotetradecane alkylsulphate salts a highly ordered smectic phase was observed following their melting. The triazacyclododecane derivatives however melted into disordered smectic A phases, apparently due to the less symmetric polar group which does not favour in-layer organization. Extensive hydrogen bonding was observed in the crystalline phases of both series of compounds as well as above their melting into smectic phases, albeit rather weak in the case of the triaza derivatives.     

Lyotropic liquid crystalline phases formed in ternary mixtures of 1-cetyl-3-methylimidazolium bromide/p-xylene/water: a SAXS, POM, and rheology study

The phase behavior of ternary mixtures of 1-cetyl-3-methylimidazolium bromide (C(16)mim-Br)/p-xylene/water is studied by small-angle X-ray scattering (SAXS), polarized optical microscopy (POM), and rheology measurements. Two types of lyotropic liquid crystalline phases are formed in the mixtures: hexagonal and lamellar. The structural parameters of the lyotropic liquid crystalline phases are calculated. Greater surfactant content in the sample leads to denser aggregation of the cylindrical units in the hexagonal liquid crystalline phase. The increase in lattice parameter and thickness of the water layer in lamellar phase are attributed to the increase of water content, and the area per surfactant molecule at the hydrophobic/hydrophilic interface for lamellar phase is found to be larger than that for hexagonal phase. The structural parameters of the liquid crystalline phases formed from the cetyltrimethylammonium bromide (CTAB) system are larger than those for the C(16)mim-Br system. The rheological properties of the samples are also found to be related to the structure of the liquid crystalline phases.     

Chelating DTPA amphiphiles: ion-tunable self-assembly structures and gadolinium complexes

A series of chelating amphiphiles and their gadolinium (Gd(iii)) metal complexes have been synthesized and studied with respect to their neat and lyotropic liquid crystalline phase behavior. These amphiphiles have the ability to form ion-tunable self-assembly nanostructures and their associated Gd(III) complexes have potential as magnetic resonance imaging (MRI) contrast enhancement agents. The amphiphiles are composed of diethylenetriaminepentaacetic acid (DTPA) chelates conjugated to one or two oleyl chain(s) (DTPA-MO and DTPA-BO), or isoprenoid-type chain(s) of phytanyl (DTPA-MP and DTPA-BP). The thermal phase behavior of the neat amphiphiles was examined by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and cross polarizing optical microscopy (POM). Self-assembly of neat amphiphiles and their associated Gd complexes, as well as their lyotropic phase behavior in water and sodium acetate solutions of different ionic strengths, were examined by POM and small and wide angle X-ray scattering (SWAXS). All neat amphiphiles exhibited lamellar structures. The non-complexed amphiphiles showed a variety of lyotropic phases depending on the number and nature of the hydrophobic chain in addition to the ionic state of the hydration. Upon hydration with increased Na-acetate concentration and the subtle changes in the effective headgroup size, the interfacial curvature of the amphiphile increased, altering the lyotropic liquid crystalline structures towards higher order mesophases such as the gyroid (Ia3d) bicontinuous cubic phase. The chelation of Gd with the DTPA amphiphiles resulted in lamellar crystalline structures for all the neat amphiphiles. Upon hydration with water, the Gd-complexed mono-conjugates formed micellar or vesicular self-assemblies, whilst the bis-conjugates transformed only partially into lyotropic liquid crystalline mesophases.     

Thermotropic and lyotropic liquid crystalline behaviour of 4-alkoxyphenyl beta-D-glucopyranosides

The liquid crystalline properties of a series of 4-alkoxyphenyl beta -D-glucopyranosides (methoxy to decyloxy and dodecyloxy) were studied using polarized light microscopy and differential scanning calorimetry. The compounds with the shortest alkoxy substituents are not liquid crystalline. The butoxy derivative displays a monotropic smectic A phase and the higher homologues display enantiotropic smectic A phases. The lyotropic behaviour was studied as a function of concentration and temperature. Hexagonal, cubic and lamellar phases were observed for compounds with alkoxy chains longer than butoxy. The nonyloxy derivative forms long ribbons in dilute solution as revealed by electron microscopy.     

Nanostructured nonionic thymidine nucleolipid self-assembly materials

Three nucleoside lipids have been synthesized: 3'-oleoylthymidine, 3',5'-dioleoylthymidine, and 3'-phytanoylthymidine. Differential scanning calorimetry and X-ray diffraction have been employed to characterize the physical properties of these neat lipids. Polarizing optical microscopy, small-angle X-ray scattering, and cryo-transmission electron microscopy techniques have been used to investigate the phase behavior in aqueous systems. Both oleoyl-based nucleoside lipids adopted a lamellar crystalline phase in the neat form at room temperature, and the phytanoyl derivative exhibited a fluid isotropic phase. Under excess water conditions, the presence of one branched (phytanoyl) or one unsaturated (oleoyl) chain promoted the formation of a liquid-crystalline lamellar phase at physiological temperatures. In contrast, the 3',5'-dioleoylthymidine derivative is nonswelling and does not exhibit lyotropic liquid-crystalline phase behavior. The nucleolipids' propensity for DNA-type binding and recognition has been evaluated by using a monolayer system to measure surface pressure-area isotherms in a Langmuir trough and indicates that the nucleoside base is available for nonspecific hydrogen bonding in the monolayer liquid expanded state for the single-chain nucleolipids but not for the dual-chain amphiphile.     

Nonionic urea surfactants: influence of hydrocarbon chain length and positional isomerism on the thermotropic and lyotropic phase behavior

The thermotropic and lyotropic phase behavior of 1- and 5-decyl urea, and 1-, 2-, 4-, and 6-dodecyl urea have been studied. This allowed the effect of positional isomerism to be examined. Intermolecular hydrogen bonding by the urea moiety is the dominant factor in determining the solid-state thermal behavior and crystal solubility boundary of these linear nonionic surfactants. The positional isomers where the urea moiety was not situated at the terminus of the hydrocarbon chain exhibited higher melting points than the 1-alkyl ureas. This has been rationalized by postulating interdigitated chains in the solid state. In the urea surfactant-water systems, three phases are observed, viz. crystalline solid, a dilute aqueous solution of the alkyl urea, and an isotropic liquid. The last two phases coexist in the low-surfactant, high-temperature region of the binary phase diagram. An overview of structure-property correlations for linear nonionic urea surfactants is presented in light of the new physicochemical data obtained for the decyl urea and dodecyl urea positional isomers.     

The remarkable role of hydrogen bond,halogen, and solvent effect on self-healing supramolecular gel

Self-healing supramolecular gels of low-molecular-weight (LMW) molecules are smart soft materials; however, the development of self-healing LMW gelator is still a challenging task because of the lack of in-depth studies about self-healing mechanisms of LMW gels and the solvent effect on gel properties. Therefore, herein a different perspective was used to study a family of D-gluconic acetal-based gelators with variable structural fragments in 14 different solvents, and a more detailed understanding of self-assembly and self-healing mechanism of supramolecular gels was attained. Based on the critical gelation concentration, phase transition temperature, and rheological data, A8 bearing an amide group in side chain and two chlorine atoms linked to benzene ring was found to be an outstanding gelator, which could form gels with good self-healing ability in a variety of solvents. Interestingly, A8 gel formed in n-BuOH demonstrates high transparency, good mechanical strength, self-supporting behavior, and great self-healing ability from mechanical damage. Based on the Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and theoretical calculation analysis, the self-assembly and self-healing mechanisms of A8 gel were proposed, indicating that a combination of hydrogen bonding and halogen effect was responsible for the efficient self-healing behavior of supramolecular gel. Furthermore, the analysis of solvent parameters indicated that the dispersion force of solvent favored gelators to self-assemble, hydrogen bonding donor ability of solvent mainly affected the formation of one-dimensional assembly, and hydrogen bonding receptor ability and polarity of solvent mainly influenced the supramolecular interactions among assemblies, significantly intervening the self-healing ability of gels. Overall, this study provides a new perspective to the understanding of gelator structure–property correlation in solvents and sheds light for future development of self-healing supramolecular gels.     

Synthesis of inorganic gels in a lyotropic liquid crystal medium. I. Synthesis of silica gels in lamellar phases obtained from non-ionic surfactants

Some lamellar phases made with aqueous lyotropic liquid crystals were used as templates for the gelation of a silica inorganic network from tetramethylorthosilicate (TMOS). The aim was to synthesize materials with an anisotropic texture.Lamellar phases were obtained by using non-ionic surfacants. At first, structural, textural and rheological properties of the lamellar phases were studied. Then, the evolution of the system after introduction of the alkoxide, i.e., during the sol-gel transition, was followed by low angle X ray diffraction and rheological measurements. Finally, a textural study of dried gels was carried out.The formation of gels with a lamellar structure was analysed by compressing the total phase diagram (quaternary system) to a ternary system. The hydrolysis and condensation reactions of the TMOS are strongly influenced by the presence of the structured lamellar phase. Gelation seems to happen around the liquid crystal microdomains. A schematic model of gelation is proposed based on experimental observations.     

Templating Polymerization of Dodecylammonium Surfactants with Polymerizable (Meth)acrylate Counter Ions

Both dodecylammonium acrylate (I) and dodecylammonium methacrylate (II) are reactive surfactants in which the polymerizable group is the organic counterion. Templating polymerization of I and II from the lyotropic liquid crystalline (LLC) lamellar phase was successfully performed with conservation of the supramolecular structure. A 20 wt% aqueous solution of II formed cubic bicontinuous phases, even upon addition of divinyl benzene (DVB). Polymerization from the cubic phases was attempted without and with DVB (3 wt%). Bicontinuous cubic phases always evolved toward lamellar structures upon polymerization.     

Switchable fluorescent organogels and mesomorphic superstructure based on naphthalene derivatives

Bisurea-functionalized naphthalene organogelators via cooperative hydrogen bonding and pi-pi stacking interaction were designed and synthesized. The gelators showed excellent gelling capability in various solvents and performed switchable fluorescence in the gel state. The fluorescent emission of these compounds strongly depends on the aggregation of the fluorophore and is very sensitive to the temperature and chemical stimuli. A stronger and red-shifted emission was found in the gel state compared with the original solution. The gel-sol transition of the systems, as well as the fluorescent emission, is reversibly controlled by a change of the temperature or upon alternative addition of fluoride anions and protons. The influence of fluoride anions on the fluorescence and gel-sol processes is a result of the dissociation of intermolecular hydrogen bonds by bonding of fluoride anions with urea groups of the gelator. The obtained sol is turned to the gel state again upon addition of trifluoroacetic acid. Furthermore, polarizing optical microscopy and small-angle X-ray scattering indicated that the gelator exhibited the liquid crystalline property and displayed the column phase.     

Self-assembly and luminescence of oligo(p-phenylene vinylene) amphiphiles

We have synthesized a series of amphiphilic molecules consisting of oligo(phenylene vinylene) (OPV) asymmetrically end-substituted with a hydrophilic poly(ethylene glycol) (PEG) segment and a hydrophobic alkyl chain. This amphiphilic structure induces self-assembly into both thermotropic and lyotropic lamellar liquid crystalline (LC) phases. The molecules form strongly fluorescent, self-supporting gels in both water and polar organic solvents, even at high concentrations on the order of 30 wt %. These self-assembled structures have been characterized by small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and polarized optical microscopy (POM). Photoluminescence (PL) is influenced by the structure of the material, with enhanced emission in the LC state due to assembly of the chromophore in confined two-dimensional layers. Self-assembly controlling molecular aggregation at the nanoscale could significantly improve the performance of OPV-based materials in optoelectronic devices.     

A novel low-molecular-mass gelator with a redox active ferrocenyl group: tuning gel formation by oxidation

A novel low-molecular-mass gelator containing a redox-active ferrocenyl group, cholesteryl glycinate ferrocenoylamide (CGF), was intentionally designed and prepared. It was demonstrated that the gelator gels 13 out of the 45 solvents tested. Scanning electron microscopy (SEM) measurements revealed that the gelator self-assembled into different supramolecular network structures in different gels. Chemical oxidation of the ferrocenyl residue resulted in phase transition of the gel from gel state to solution state. FTIR and (1)H NMR spectroscopy studies revealed that hydrogen bonding between the gelator molecules in the gel was one of the main driving forces for the formation of the gels.