Local high-density distributions of phospholipids induced by the nucleation and growth of smectic liquid crystals at the interface

Amphiphilic molecules adsorbed at the interface could control the orientation of liquid crystals (LCs) while LCs in turn could influence the distributions of amphiphilic molecules. The studies on the interactions between liquid crystals and amphiphilic molecules at the interface are important for the development of molecular sensors. In this paper, we demonstrate that the development of smectic LC ordering from isotropic at the LC/water interface could induce local high-density distributions of amphiphilic phospholipids. Mixtures of liquid crystals and phospholipids in chloroform are first emulsified in water. By fluorescently labeling the phospholipids adsorbed at the interface, their distributions are visualized under fluorescent confocal microscope. Interestingly, local high-density distributions of phospholipids showing a high fluorescent intensity are observed on the surface of LC droplets. Investigations on the correlation between phospholipid density, surface tension and smectic LC ordering suggest that when domains of smectic LC layers nucleate and grow from isotropic at the LC/water interface as chloroform slowly evaporates at room temperature, phospholipids transition from liquid-expanded to liquid-condensed phases in response to the smectic ordering, which induces a higher surface tension at the interface. The results will provide an important insight into the interactions between liquid crystals and amphiphilic molecules at the interface.     

Self-assembling with formation of mesogenic polymer structures in non-mesogenic species

The trivial statement that liquid crystals are formed on the base of either mesogenic organic molecules or molecules with amphiphilic structure may be applied to polymers as well. But at the same time it became clear that polymer LC world in certain sense is richer than the low-molecular one and more and more one finds mesophase and LC polymers based on non-mesogenic macromolecules. Linear polyphosphazenes, polysiloxanes, some ladder polymers etc. exhibit mesophase behavior in absence of any mesogenic groups in their structure.     

Temperature and compression rate independent domain shape in Langmuir monolayers of di-n-dodecyl hydrogen phosphate at the air-water interface

Thermodynamic and morphological properties of Langmuir monolayers of di-n-dodecyl hydrogen phosphate (DDP) have been studied by film balance and Brewster angle microscopy (BAM) over a wide range of temperature between 5 and 40 degrees C. From pi-A isotherms, a generalized phase diagram consisting of gas (G), liquid expanded (LE) and liquid condensed (LC) phases is constructed for the DDP monolayers. The BAM images show the formation of gas bubble in the bright background of LE phase during G-LE phase transitions and fingering LC domains during LE-LC phase transitions. The shapes of these domains are independent of temperature, showing a sharp contrast to the temperature-dependent monolayer morphologies of amphiphilic systems where the shape of the LC domains changes either from compact circular to fingering or from irregular or spiral to compact patterns with increasing temperature. In addition, the domains do not show any change in their shapes with decreasing the compression rate. Since the two-alkyl chains are directly attached by covalent bonds to the phosphate group, the rearrangement of the molecules needs to move the whole molecules including the hydration sphere. The difficulty related to such a movement of the molecules causes the fingering domains, which are independent of external variables. Although the domains are formed in a fingering shape, the equilibrium shape can be attained by about 120 min at 15 degrees C indicating a rather slow relaxation rate.     

Self-organization of amphiphilic liquid-crystalline dendrimers in bulk and at the interface

A comparative analysis of the structure and phase behavior of synthesized carbosilane amphiphilic LC dendrimers of the third generation containing mesogenic phenyl and oligo(ethylene glycol) fragments is performed. When phenol groups are replaced with oligo(ethylene glycol) moieties, the temperature interval of the existence of the LC phase in the mesogen-containing dendrimers decreases. The chemical nature of hydrophilic terminal groups is found to control the organization of dendrimers in the smectic mesophase. Structural models for their packing are proposed. Amphiphilic dendrimers are shown to form stable Langmuir films at the water/air interface. Surface-pressure-surface area-isotherms are constructed. The effect of the chemical nature of hydrophilic groups on the formation of a monolayer at the interface and on the packing density of dendrimer molecules in the monolayer is discussed.     

Hierarchically structured free-standing hydrogels with liquid crystalline domains and magnetic nanoparticles as dual physical cross-linkers

Here we report a modular strategy for preparing physically cross-linked and mechanically robust free-standing hydrogels comprising unique thermotropic liquid crystalline (LC) domains and magnetic nanoparticles both of which serve as the physical cross-linkers resulting in hydrogels that can be used as magnetically responsive soft actuators. A series of amphiphilic LC pentablock copolymers of poly(acrylic acid) (PAA), poly(5-cholesteryloxypentyl methacrylate) (PC5MA), and poly(ethylene oxide) (PEO) blocks in the sequence of PAA-PC5MA-PEO-PC5MA-PAA were prepared using reversible addition-fragmentation chain transfer polymerization. These pentablock copolymers served as macromolecular ligands to template Fe(3)O(4) magnetic nanoparticles (MNPs), which were directly anchored to the polymer chains through the coordination bonds with the carboxyl groups of PAA blocks. The resulting polymer/MNP nanocomposites comprised a complicated hierarchical structure in which polymer-coated MNP clusters were dispersed in a microsegregated pentablock copolymer matrix that further contained LC ordering. Upon swelling, the hierarchical structure was disrupted and converted to a network structure, in which MNP clusters were anchored to the polymer chains and LC domains stayed intact to connect solvated PEO and PAA blocks, leading to a free-standing LC magnetic hydrogel (LC ferrogel). By varying the PAA weight fraction (f(AA)) in the pentablock copolymers, the swelling degrees (Q) of the resulting LC ferrogels were tailored. Rheological experiments showed that these physically cross-linked free-standing LC ferrogels exhibit good mechanical strength with storage moduli G' of around 10(4)-10(5) Pa, similar to that of natural tissues. Furthermore, application of a magnetic field induced bending actuation of the LC ferrogels. Therefore, these physically cross-linked and mechanically robust LC ferrogels can be used as soft actuators and artificial muscles. Moreover, this design strategy is a versatile platform for incorporation of different types of nanoparticles (metallic, inorganic, biological, etc.) into multifunctional amphiphilic block copolymers, resulting in unique free-standing hybrid hydrogels of good mechanical strength and integrity with tailored properties and end applications.     

Self-assembly of amphiphilic liquid crystal polymers obtained from a cyclopropane-1,1-dicarboxylate bearing a cholesteryl mesogen

We study the self-assembly of a new family of amphiphilic liquid crystal (LC) copolymers synthesized by the anionic ring-opening polymerization of a new cholesterol-based LC monomer, 4-(cholesteryl)butyl ethyl cyclopropane-1,1-dicarboxylate. Using the t-BuP(4) phosphazene base and thiophenol or a poly(ethylene glycol) (PEG) functionalized with thiol group to generate in situ the initiator during the polymerization, LC homopolymer and amphiphilic copolymers with narrow molecular weight distributions were obtained. The self-assemblies of the LC monomer, homopolymer, and block copolymers in bulk and in solution were studied by small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and transmission electron microscopy (TEM). All polymers exhibit in bulk an interdigitated smectic A (SmA(d)) phase with a lamellar period of 4.6 nm. The amphiphilic copolymers self-organize in solution into vesicles with wavy membrane and nanoribbons with twisted and folded structures, depending on concentration and size of LC hydrophobic block. These new morphologies will help the comprehension of the fascinating organization of thermotropic mesophase in lyotropic structures.     

Dynamic ordering transitions of liquid crystals driven by interfacial complexes formed between polyanions and amphiphilic polyamines

We report the design of an amphiphilic polyamine (polymer 1) based on poly(2-alkenyl azlactone) that strongly couples the formation of polyelectrolyte complexes at aqueous/liquid crystal (LC) interfaces to ordering transitions in the LC. We demonstrate that the addition of a strong anionic polyelectrolyte to aqueous solutions in contact with polymer 1-laden LC interfaces (prepared by Langmuir-Schaefer transfer of monolayers of polymer 1 onto micrometer-thick films of nematic LC) triggers ordering transitions in the LCs. We further demonstrate that changes in the ordering of the LCs (i) are driven by electrostatic interactions between the polyelectrolytes, (ii) involve multivalent interactions between the polyelectrolytes, and (iii) are triggered by reorganization of the hydrophobic side chains of amphiphilic polymer 1 upon formation of the interfacial complexes. The results presented in this paper lead us to conclude that ordering transitions in LCs can be used to provide insights into the structure and dynamics of interfacial complexes formed between polyelectrolytes.     

Adsorption at liquid interfaces: the generalized Langmuir isotherm and interfacial structure

The thermodynamics of adsorption of amphiphilic surface-active compounds at the interface between two immiscible liquids is considered. At the interface, these molecules are supposed to replace a few of the adsorbed molecules of both solvents. Classical isotherms of adsorption (Frumkin, Langmuir, Henry) were based on the model of nonpenetrable interface, where an adsorbate can substitute only molecules of one solvent. However, at the interface between two immiscible electrolytes, like nonpolar oil-water interfaces, or liquid membrane amphiphilic molecules can substitute molecules of both solvents; therefore, classical isotherms are not applicable in these cases. The generalization of Langmuir and Frumkin isotherms for permeable and nonpermeable interfaces, known as the Markin-Volkov (MV) isotherm, gives the possibility to analyze adsorption and the interfacial structure in a general case. In the present paper, the adsorption isotherms of pentafluorobenzoic acid at the octane-water interface at various pH were measured by the drop-weight method. The thermodynamic parameters of pentafluorobenzoic acid (PFBA) adsorption at the octane-water interface were found. From the measurements of PFBA adsorption, the structure of the octane-water interface was determined. Substitution of one adsorbed octane molecule requires approximately three adsorbed PFBA molecules. This result shows that the orientation of solvent molecules at the interface is different from the bulk solution. Adsorbed octane molecules have a lateral orientation with respect to the interface. Gibbs free energy of adsorption equilibrium and thermodynamic parameters of PFBA adsorption show that the adsorption of PFBA at the octane-water interface is accompanied by a reduction in the attraction between adsorbed PFBA molecules as the pH decreases to the acidic region.     

Colloidal shape controlled by molecular adsorption at liquid crystal interfaces

The ability to control finely the structure of materials remains a central issue in colloidal science. Due to their elastic properties, liquid crystals (LC) are increasingly used to organize matter at the micrometer scale in soft composites. Textures and shapes of LC droplets are currently controlled by the competition between elasticity and anchoring, hydrodynamic flows, or external fields. Molecules adsorbed specifically at LC interfaces are known to orient LC molecules (anchoring effect), but other induced effects have been poorly explored. Using specifically designed amphitropic surfactants, we demonstrate that large-shape transformations can be achieved in direct LC/water emulsions. In particular, we focus on unusual nematic filaments formed from spherical droplets. These results suggest new approaches to design new soft LC composite materials through the adsorption of molecules at liquid crystal interfaces.     

Langmuir monolayers of bent-core molecules

A systematic study of five different, symmetric bent-core liquid crystals in Langmuir thin films at the air/water interface is presented. Both the end chains (siloxane vs hydrocarbon) and the core (more or less amphiphilic) are varied, to allow an exploration of different possible layer structures at the interface. The characterization includes systematic surface pressure isotherms, Brewster angle microscopy, and surface potential measurements. The properties of these layers are strongly dependent on the individual type of molecule: the molecules with amphiphilic end chains lie quite flat on the surface, while the molecules with hydrophobic end chains construct multilayer structures. In both cases, the three-dimensional collapse structure is reversible.     

Equation of state for the phase coexistence region of insoluble monolayers under consideration of the entropy nonideality

The equation of state for the monolayer with a fluid (G, LE)/condensed (LC) phase transition derived earlier (Fainerman, V.B.; Vollhardt, D. J. Phys. Chem. B 1999, 103, 145) in the framework of a quasichemical approach is generalized. A term is added that takes into account the entropy nonideality of mixing of the monomers and clusters of amphiphilic molecules. The results calculated from the proposed equations agree well with the experimental Pi-A isotherms obtained for various types of amphiphilic monolayers. The values of molecular areas of the amphiphilic molecules estimated from the fitting of experimental data to the proposed equation are quite similar to the real values. Another equation of state capable of describing the fluid state of insoluble monolayers and based on equations for the chemical potential of the solvent in the bulk phase and in the surface layer (Fainerman, V. B.; Vollhardt, D. J. Phys. Chem. B 2006, 110, 10436) is also generalized to be extended to the fluid/condensed phase transition region (A < A(c)), taking into account entropy nonideality for mixing solvent molecules, monomers, and clusters of amphiphilic molecules. The values calculated on this basis agree also well with the experimental data.     

Alignment of liquid crystals using Langmuir‒Blodgett films of unsymmetrical bent-core liquid crystals

ABSTRACT

The properties of the thin films of liquid crystal (LC) molecules can be governed easily by external fields. The anisotropic structure of the LC molecules has a large impact on the electrical and optical properties of the film. The Langmuir monolayer (LM) of LC molecules at the air–water interface is known to exhibit a variety of surface phases which can be transferred onto a solid substrate using the Langmuir?Blodgett (LB) technique. Here, we have studied the LM and LB films of asymmetrically substituted bent-core LC molecules. The morphology of LB film of the molecules is found to be a controlling parameter for aligning bulk LC in the nematic phase. It was found that the LB films of the bent-core molecules possessing defects favour the planar orientation of nematic LC, whereas the LB films with fewer defects show homeotropic alignment. The defect in LB films may introduce splay or bend distortions in the nematic near the alignment layer which can govern the planar alignment of the bulk LC. The uniform layer of LB film facilitates the molecules of nematic to anchor vertically due to a strong van der Waals interaction between the aliphatic chains leading to a homeotropic alignment.     

Adsorption at liquid interfaces: The generalized Frumkin isotherm and interfacial structure

The thermodynamics of adsorption of amphiphilic surface-active compounds at the interface between two immiscible liquids is considered. At the interface, these molecules are supposed to replace a few of the adsorbed molecules of both solvents. Classical isotherms of adsorption (Frumkin, Frumkin-Damaskin, Langmuir, Henry) were based on the model of non-penetrable interface, where an adsorbate can substitute only molecules of one solvent. At the interface between two immiscible electrolytes, nonpolar oil/water interfaces, and liquid membranes amphiphilic molecules can substitute molecules of both solvent and classic isotherms cannot be used. The generalization of Frumkin isotherm for permeable and non-permeable interfaces, known as the Markin-Volkov isotherm, gives the possibility to analyze adsorption in a general case. The adsorption isotherms of pentafluorobenzoic acid at the octane/water interface at different pHs were measured by the drop-weight method. The thermodynamic parameters of pentafluorobenzoic acid (PFBA) adsorption at octane/water interface were determined. From the measurements of PFBA adsorption, the structure of the octane/water interface was determined. Substitution of one adsorbed octane molecule requires approximately three adsorbed PFBA molecules. This result shows that the orientation of solvent molecules at the interface is different from the bulk. Adsorbed octane molecules have a lateral orientation with respect to the interface. Gibbs free energy of adsorption equilibrium and thermodynamic parameters of PFBA adsorption show that the adsorption of PFBA at the octane/water interface is accompanied by a reduction in the attraction between adsorbed PFBA molecules as the pH decreases to the acidic region. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 10, pp. 1194–1200. The text was submitted by the authors in English.     

Viewing angle characteristics of a homogeneously aligned two-domain liquid crystal cell with an inter-digitated electrode

The in-plane switching (IPS) mode in liquid crystal displays is known to exhibit a wide viewing angle. However, since the LC director rotates in one direction in the plane, devices with a single domain exhibit both a colour shift depending on the viewing angle, and greyscale inversion at specific angles especially at low grey levels. This has been improved by wedge shaped electrodes so that fields in two directions exist inside a pixel, causing the LC molecules to rotate in opposite directions to compensate each other; this acts as a virtual two domains structure. Nevertheless, the colour shift still exists to some extent, especially at low grey levels. In this paper, we propose a realistic two-domain IPS mode that exhibits a minimized colour shift at all grey levels on changing the viewing direction. In this device, the LC molecules are initially aligned in two directions orthogonal to each other, and two field directions exist perpendicular to each other. We have performed device simulations with respect to viewing angle characteristics, and found that IPS devices with a real two-domain structure reduce the variation of the retardation more effectively, when the viewing direction changes.     

π–π Interactions Between Aromatic Groups in Amphiphilic Molecules: Directing Hierarchical Growth of Porous Zeolites

The development of hierarchical macro- or mesoporous zeolites is essential in zeolite synthesis because the size of the micropores limits mass transport and their use as industrial catalysts for bulky molecules. Although major breakthroughs have been achieved, fabricating crystallographically ordered mesoporous zeolites using a templating strategy is still an unsolved challenge. This minireview highlights our recent efforts on the self-assembly of amphiphilic molecules to obtain ordered hierarchical MFI zeolites by introducing aromatic groups into the hydrophobic tail of the amphiphilic molecules. Owing to the geometric matching between the self-assembled aromatic tails and the MFI framework, a) single-crystalline mesostructured zeolite nanosheets (SCZNs), b) SCZNs with a 90° rotational intergrowth structure, c) a hierarchical MFI zeolite with a two-dimensional square P4mm mesostructure, and d) a single-crystalline mesoporous ZSM-5 with three-dimensional pores and sheetlike mesopores layered along the a-axis were successfully synthesized.     

Microstructuring of a polymer globule in solution in the presence of an amphiphilic substance

Microstructuring in the bulk of a polymer globule in a solution that contains dimeric amphiphilic molecules, in particular, surfactants, is studied in terms of the weak-segregation theory. An inhomogeneous structure can result from a decrease in free energy with the orientation of amphiphilic molecules in the region of inhomogeneity owing to the interaction of hydrophobic and polar parts of the molecules with the solvent. For the sake of simplicity, we discuss the case of identical second virial coefficients of the interaction of monomer units and amphiphilic molecules with different energies of interaction of the hydrophobic and polar parts of the molecule with the solvent. By comparing the free energy for different types of microstructures, we predict that, with deterioration in the quality of the solvent, there is an initial formation of a homogeneous globule followed by formation of a body-centered cubic structure; a hexagonal cylindrical structure; and, finally, a lamellar structure. For a low degree of amphiphilicity, the transition from a homogeneous globule to only a lamellar structure occurs. An increase in the concentration of the amphiphilic substance in the surrounding solution hinders the formation of a globule but facilitates its microstructuring, which is also promoted by an increase in the volume of the amphiphilic molecule and the difference in the interaction energies of its hydrophobic and polar parts with the solvent. Phase diagrams of a globule??s state at different values of model parameters are plotted.     

Lyotropic liquid crystals from designed helical beta-peptides

We demonstrate that 14-helical beta-peptides can self-assemble to form lyotropic liquid crystalline (LC) phases in water. beta-Peptides 1-4 were designed to form globally amphiphilic 14-helices of increasing length. Optical microscopy showed that several of these beta-peptides formed LC phases in aqueous solutions at concentrations as low as 2.5 wt % (15 mM). Liquid crystallinity appears to require the adoption of a globally amphiphilic conformation because a scrambled sequence, 5, does not display LC behavior. Thermal stability and reversibility of LC phase formation were assessed by variable temperature 2H NMR spectroscopy and optical microscopy. The LC phase formed by beta-peptide 3 at 10 wt % is disrupted above 40 degrees C in D2O and re-forms within minutes upon cooling. LC phase behavior for solutions of 3 is influenced by concentration and net charge. These studies demonstrate that highly folded 14-helical beta-peptides can produce LC phases at shorter lengths than do alpha-helical alpha-peptide mesogens and can provide a basis for tailoring properties of LC phases for future applications.     

Effects of chemical structure and composition of the polymer matrix on the morphology and electro-optical performance of polymer-dispersed liquid crystal films

Polymer-dispersed liquid crystal (PDLC) systems on the basis of nematic liquid crystal E7 and amphiphilic binary copolymers of acrylic acid (AA) with such acrylates as 2-ethylhexyl acrylate (EHA), n-butyl acrylate (BA), and methyl acrylate (MA) are investigated. It is shown that the liquid crystal (LC) drops in the copolymer EHA–AA have submicrometre sizes, and their dependence on the composition of the photo-curable monomer mixture is described by a parabolic curve. The highly oriented domain structure in the same system is first revealed when electric field is applied. The threshold voltage for all systems begins to increase with some critical composition of a monomer mixture in which the longer the hydrocarbonic radical in an acrylate molecule, the higher the content of AA. The PDLC system based on the BA–AA copolymer with 30 wt% LC exhibits the least value of the driving voltage, 1 V μm^–1, and the lowest memory effect.     

Molecular dynamics simulation of polymer dispersed liquid crystal droplets under competing boundary conditions

This paper is devoted to the molecular dynamics simulation of structural organization inside a polydispersed liquid crystal (LC) droplet under competing boundary conditions. The droplet is assumed to be placed at the liquid crystal interface between two different regions of the solid polymer matrix, which accordingly separates the droplet into two hemispheres: the first of these is under radial boundary conditions; the second hemisphere is under bipolar boundary conditions. The droplet is considered as a jagged sphere filled with LC molecules, modelled as classical spins (unit vectors), whose centres of mass are associated with sites of a cubic lattice inside the cavity. The orienting action of the polymer matrix, and hence the resulting boundary conditions, are modelled by the interaction between the internal LC molecules (possessing only orientational degrees of freedom), and those of a delimiting surface layer (a jagged spherical shell), whose orientations are fixed, radial or bipolar, respectively. All interactions are modelled by the short range McMillan pair potential. The molecular orientation inside the LC droplet has been determined for various anchoring strengths of the interaction between internal spins and boundary layers. We have investigated the structure of the spherical defect resulting in the central region of the droplet, as well as of the boojum - like defects existing near the poles of the droplet. It has been found that a change of relative radial and bipolar anchoring strengths can affect both central and boojum - like defects. The effect of an external field on the molecular orientation inside the droplet has also been investigated. It has been found that a sufficiently strong external field increases the radius of the spherical defect placed in the central region of the droplet.     

Fabrication and properties of fullerodendron thin films

Thin films of fullerodendron (C(60)(Gn-COOMe) (n = 0.5, 1.5, 2.5)), which was synthesized from fullerene and anthracenyl poly(amido amine) dendron with methyl ester terminals and different generations (G), were fabricated by the Langmuir-Blodgett (LB) and adsorption techniques. It was characterized by X-ray reflectometry that the LB films possessed well-ordered structure, although the adsorption method led to random orientation of molecules. As to C(60)(G0.5-COOMe) and C(60)(G1.5-COOMe), the LB films took a four-layer structure consisting of a double layer of molecules, and fullerene moieties exist in the interior of the LB films. On the other hand, C(60)(G2.5-COOMe) led to a two-layer structure in which the fullerene moieties were at the air side and the dendron moieties were at the substrate side. With increasing generation of dendron, the monolayer formation ability at the air/water interface as amphiphilic molecule strengthens and the amphiphilic property becomes superior to the fullerene-fullerene attractive interaction that prevents the monolayer formation. Furthermore, in the case of C(60)(G0.5-COOMe) and C(60)(G1.5-COOMe), the reduction peak in cyclic voltammetry of the LB film remained even after UV light irradiation. On the contrary, the peak of the C(60)(G2.5-COOMe) LB film disappeared, indicating that molecular arrangement in the films affects electrochemical properties.