Influence of simple electrolytes on the orientational ordering of thermotropic liquid crystals at aqueous interfaces

We report orientational anchoring transitions at aqueous interfaces of a water-immiscible, thermotropic liquid crystal (LC; nematic phase of 4'-pentyl-4-cyanobiphenyl (5CB)) that are induced by changes in pH and the addition of simple electrolytes (NaCl) to the aqueous phase. Whereas measurements of the zeta potential on the aqueous side of the interface of LC-in-water emulsions prepared with 5CB confirm pH-dependent formation of an electrical double layer extending into the aqueous phase, quantification of the orientational ordering of the LC leads to the proposition that an electrical double layer is also formed on the LC-side of the interface with an internal electric field that drives the LC anchoring transition. Further support for this conclusion is obtained from measurements of the dependence of LC ordering on pH and ionic strength, as well as a simple model based on the Poisson-Boltzmann equation from which we calculate the contribution of an electrical double layer to the orientational anchoring energy of the LC. Overall, the results presented herein provide new fundamental insights into ionic phenomena at LC-aqueous interfaces, and expand the range of solutes known to cause orientational anchoring transitions at LC-aqueous interfaces beyond previously examined amphiphilic adsorbates.     

Influence of specific anions on the orientational ordering of thermotropic liquid crystals at aqueous interfaces

We report that specific anions (of sodium salts) added to aqueous phases at molar concentrations can trigger rapid, orientational ordering transitions in water-immiscible, thermotropic liquid crystals (LCs; e.g., nematic phase of 4'-pentyl-4-cyanobiphenyl, 5CB) contacting the aqueous phases. Anions classified as chaotropic, specifically iodide, perchlorate, and thiocyanate, cause 5CB to undergo continuous, concentration-dependent transitions from planar to homeotropic (perpendicular) orientations at LC-aqueous interfaces within 20 s of addition of the anions. In contrast, anions classified as relatively more kosmotropic in nature (fluoride, sulfate, phosphate, acetate, chloride, nitrate, bromide, and chlorate) do not perturb the LC orientation from that observed without added salts (i.e., planar orientation). Surface pressure-area isotherms of Langmuir films of 5CB supported on aqueous salt solutions reveal ion-specific effects ranking in a manner similar to the LC ordering transitions. Specifically, chaotropic salts stabilized monolayers of 5CB to higher surface pressures and areal densities (12.6 mN/m at 27 ?(2)/molecule for NaClO(4)) and thus smaller molecular tilt angles (30° from the surface normal for NaClO(4)) than kosmotropic salts (5.0 mN/m at 38 ?(2)/molecule with a corresponding tilt angle of 53° for NaCl). These results and others reported herein suggest that anion-specific interactions with 5CB monolayers lead to bulk LC ordering transitions. Support for the proposition that these ion-specific interactions involve the nitrile group was obtained by using a second LC with nitrile groups (E7; ion-specific effects similar to 5CB were observed) and a third LC with fluorine-substituted aromatic groups (TL205; weak dipole and no ion-specific effects were measured). Finally, we also establish that anion-induced orientational transitions in micrometer-thick LC films involve a change in the easy axis of the LC. Overall, these results provide new insights into ionic phenomena occurring at LC-aqueous interfaces, and reveal that the long-range ordering of LC oils can amplify ion-specific interactions at these interfaces into macroscopic ordering transitions.     

Synthesis and properties of hydroxy tail-terminated cyanobiphenyl liquid crystals

Two series of new hydroxy tail-terminated cyanobiphenyl compounds are described. The 4′-ω-hydroxyalkynyl-4-cyanobiphenyl compounds (1a–1g) were synthesised as the key intermediates to the 4′-?ω-hydroxyalkyl-4-cyanobiphenyl compounds (2a–2g) obtained upon reduction of the acetylenes. Many of these ω-hydroxyalkynyl and ω-hydroxyalkyl cyanobiphenyl compounds exhibit nematic mesophases and they also serve as precursors for the synthesis of other interesting materials. Using density functional theory, we calculate the dipole moment of all relevant ω-hydroxyalkynyl and ω-hydroxyalkyl cyanobiphenyl compounds and find a correlation between the calculated dipole moments and measured crystalline to nematic or isotropic liquid transition temperatures.     

Surface tension of some liquid crystals in the cyanobiphenyl series

The temperature dependence of the surface tension was measured by the pendant drop method for four compounds from the homologous series of alkylcyanobiphenyls (nCB), in the nematic liquid crystal and isotropic phases. For 8CB (octylcyanobiphenyl) the temperature dependence was also measured in the smectic range. Not very close to the isotropic transition temperature, and with the exception of 8CB, the surface tension decreases with increase in temperature in the nematic range. A downward jump at the transition temperature was observed for all liquid crystals studied. The shape of the drop in the smectic A phase of 8CB gives indications of stratification in a system of terraces.     

Influence of electrostatic interactions on the properties of cyanobiphenyl liquid crystals predicted from atomistic molecular dynamics simulations

The influence of force field details in all-atom molecular dynamics (MD) simulations on the predicted thermodynamic, structural, and dynamic properties of bulk 4-cyano-4?-pentylbiphenyl (5CB) systems have been investigated in the 292–368 K temperature range. The effect of the molecular dipole moment and the details of dihedral potential for biphenyl unit were investigated using both polarisable (POL) and non-polarisable (NP) versions of the quantum chemistry-based force field. The predicted densities for the nematic and isotropic phases of bulk 5CB were found to be in excellent agreement with available experimental data. The nematic-isotropic transition temperature (T_NI) showed strong sensitivity to the force field details, MD simulations with partial atomic charge distributions and molecular dipole moment corresponding to high-level quantum chemistry calculations predicted an overestimation of the T_NI by about 30 K. Rescaling the charges to allow the molecular dipole to be closer to experimentally reported values of 5CB dipole in condensed phases, significantly improved the prediction of T_NI as well as other thermodynamic and dynamic properties of 5CB. We also discuss how the structural, thermodynamic, and dynamic properties of bulk 5CB are affected by the flexibility of the central biphenyl dihedral and the inclusion of induced polarisation effects.     

Surface tension of some liquid crystals in the cyanobiphenyl series

The temperature dependence of the surface tension was measured by the pendant drop method for four compounds from the homologous series of alkylcyanobiphenyls (nCB), in the nematic liquid crystal and isotropic phases. For 8CB (octylcyanobiphenyl) the temperature dependence was also measured in the smectic range. Not very close to the isotropic transition temperature, and with the exception of 8CB, the surface tension decreases with increase in temperature in the nematic range. A downward jump at the transition temperature was observed for all liquid crystals studied. The shape of the drop in the smectic A phase of 8CB gives indications of stratification in a system of terraces.     

向列相液晶4-氰基-4'-庚基联苯和4-氰基-4'-戊基联苯在准固态染料敏化太阳电池中的应用

将两种向列相液晶,4-氰基-4'-庚基联苯和4-氰基-4'-戊基联苯,引入到染料敏化太阳电池(DSC)用偏氟乙烯-六氟乙烯共聚物基准固态电解质中,以期提高DSC的光电转化效率.研究了液晶的引入对电解质中I-/I3-的氧化还原行为、DSC中TiO2/电解质界面的暗反应以及DSC光伏性能的影响.结果表明,尽管液晶的引入会降低电解质中I-/I3-的传输并且使得DSC中暗反应加快,但是DSC的短路电流密度却显著地提高,使DSC的光电转化效率增加.这可能是由于液晶的引入改善了DSC中的界面接触以及增加染料的光吸收引起的.     

Effects of nonmesomorphic substituted benzonitriles on the dielectric properties of cyanobiphenyl liquid crystals

The effects of the cyano-substituted aromatic nonmesomorphic additives N-(3,4-dicyanophenyl)-4′-nonyloxybenzamide (I), 3,4-dicyanophenyl-4′-nonyloxybenzoate (II), 3,4-dicyanophenyl-3′,4′,5′-tris-(4″-undecyloxyphenylbenzoate)benzoate (III), and 4-(6-hydroxyhexyloxy)benzonitrile (IV) on the dielectric properties of 4-hexyloxy-4′-cyanobiphenyl and a liquid-crystal mixture E3 were studied. Dielectric anisotropy, birefringence, and orientational ordering were studied to show that monocyano derivative IV most strongly increased the dielectric anisotropy of mesomorphic compositions because of its effective incorporation into the liquid crystal matrix.     

A database for the determination of orientational ordering of nine classes of liquid crystals using carbon-13 chemical shifts

The orientational ordering of nine classes of liquid crystals, namely 4-n-alkyl-413 cyanobiphenyls (nCB), 4-n-alkoxy-4-cyanobiphenyls (nOCB), 4-n-alkoxybenzilidene-4-nalkylanilines (nO.m), 4-cyanophenyl 4-alkylbenzoates (nCPB), 4-n-alkylphenyl 4-n-alkoxycinnamates (Cin-n-m), esters of alpha-chloro carboxylic acids and 4-n-alkyl-4-hydroxybiphenyls (An, Bn and Cn), 4,4-di-n-alkyldiphenyldiacetylenes (PTTP), 4-n-alkylphenyl 4-n-alkoxybenzoates (nOm), and 4-n-alkoxyphenyl 4-n-alkoxybenzoates (nOm), have been investigated by the use of 13C NMR. The order parameters of the phenyl rings were determined by using a 2D 13C NMR technique known as separated local field (SLF) spectroscopy in combination with variable angle spinning (VAS). The order parameters obtained were then correlated with their corresponding anisotropic 1D C chemical shifts in their nematic and smectic phases to give a linear relationship in each case. The parameters obtained in the linear correlations provide a convenient way to obtain the order parameters from direct measurement of the 13C chemical shifts for other homologous members of these classes of liquid crystal.     

Surface ordering of diskotic liquid crystals

We present Monte Carlo simulations of diskotic molecules using the Gay-Berne potential in a slab geometry. The disk-wall interaction is described by two different functions according to whether or not the equilibrium distance is dependent on the relative orientation of the disk to the wall. Furthermore, by changing the parameters of these potentials, we model either homeotropic (face-on) or planar (edge-on) anchoring of the disks. We have found that the isotropic-nematic transition does not change in comparison with the bulk situation. The temperature of the nematic-columnar transition, on the contrary, is found to increase for homeotropic anchoring, and decrease for planar anchoring, independently of the details of the potential. We explain the decrease of the transition temperature in the planar anchoring situation as the result of an induced frustration, due to the competition between the two orientations induced independently by the upper and lower walls.     

Phase effect on formation of intermediates in the pulse radiolysis of 4-cyano-4'- octylbiphenyl liquid crystals

It is shown that the reaction intermediates were formed by pulse radiolysis of 4-cyano-4'- octylbiphenyl liquid crystals. The remarkable phase effect was observed in the pulse radiolysis of the liquid crystals. The optical absorption spectrum obtained in the smectic A phase of 4-cyano-4'-octylbiphenyl had no absorption maximum in the visible region, but had an absorption increasing toward the shorter wavelengths below 400 nm, while a broad absorption band with a maximum around 550 nm was observed in the nematic phase. In the isotropic liquid phase an absorption band with a maximum at 480 nm was observed, but it was observed neither in the smectic A phase nor in the nematic phase. The absorption below 400 nm was observed in the liquid phase as well as in the smectic A phase. The assignment of the absorptions observed for each phase is discussed. Then it is suggested that the structural properties could be related to the formation and behavior of reaction intermediates.     

Depletion at solid/liquid interfaces: flowing hexadecane on functionalized surfaces

We present a neutron reflectivity study on interfaces in contact with flowing hexadecane, which is known to exhibit surface slip on functionalized solid surfaces. The single crystalline silicon substrates were either chemically cleaned Si(100) or Si(100) coated by octadecyl-trichlorosilane (OTS), which resulted in different interfacial energies. The liquid was sheared in situ and changes in reflectivity profiles were compared to the static case. For the OTS surface, the temperature dependence was also recorded. For both types of interfaces, density depletion of the liquid at the interface was observed. In the case of the bare Si substrate, shear load altered the structure of the depletion layer, whereas for the OTS covered surface no effect of shear was observed. Possible links between the depletion layer and surface slip are discussed. The results show that, in contrast to water, for hexadecane the enhancement of the depletion layer with temperature and interfacial energy reduces the amount of slip. Thus a density depletion cannot be the origin of surface slip in this system.     

Dependence of molecular polarizability on the orientational order in nematic liquid crystals

The dispersion dependences of refractive indices in the visible range were used to obtain experimental values of the Lorentz tensor components L _j and the mean molecular polarizability $ \bar \gamma $ \bar \gamma for five nematic liquid crystals belonging to two homological series. The dependence of L _j components on the homologue number, mesophase temperature, birefringence value, and the orientational order of molecules in the nematic phase and upon a nematic-smectic A phase transition was revealed. The effect of the isotropization of the Lorentz tensors and the local field tensor with decreasing birefringence and molecular polarizability anisotropy Δγ was confirmed. The quadratic dependence $ \bar \gamma $ \bar \gamma (S) on the molecular orientational order parameter S in the nematic phase was found. It was invariant with respect to the nematic-smectic A transition. The dependences $ \bar \gamma $ \bar \gamma (S) and Δγ(S) are explained within molecular statistical theory as consequences of the correlation between orientational and conformational degrees of freedom of molecules. These conformational degrees of freedom are related to the internal rotation of molecular fragments, which affects the electronic conjugation of the fragments and the oscillator strengths of molecular transitions.     

Mechanisms of solute orientational order in nematic liquid crystals

The order parameters of a number of different solutes dissolved in a number of different nematic liquid-crystal solvents are measured and compared. It is shown that the order parameters can all be rationalized with the ansatz that there exist two independent ordering mechanisms operating in the liquid crystals employed. With this ansatz it is possible to fit the experimental order parameters to better than 5%. This opens the possibility of the accurate prediction of order parameters in ordered liquids.     

Gravitational field-induced orientational transition of aligned nematic liquid crystals

We demonstrate induced orientational transition of liquid crystal (LC) E7 between two optical fibre tips in a gravitational field as a result of minimising the free energy. The LC orientational transition is from a homeotropic state, with respect to fibre tip, to a tilted state as the spacing between two fibre tips is increased. The orientational distortion introduces birefringence in the optical fibre that changes the polarised direction of the output light. At short spacing, the undistorted homeotropic orientation of LC is preferred. However, at longer spacing, a distorted orientation is preferred. Once the LC director profile is known, a Finite Difference Time Domain method is used to calculate the optical properties, which agree well with the experimental results.     

Influence of spacer and terminal group lengths on the smectic ordering of cholesterol-containing dimer liquid crystals

The smectic properties of four series of dimer liquid crystals containing cholesteryl and biphenylyl groups were investigated by polarization microscopy, DSC and X-ray diffraction. Compounds that contain the strongly dipolar cyanobiphenylyl group exhibit a smectic layer spacing that is about 1.7 times the length of the molecule. Three series of alkoxybiphenylylcontaining dimers exhibit two other smectic modifications. For short spacers, a smectic layer spacing is observed that is about the same length as the molecule and for long spacers the smectic layer spacing is about half the length of the molecule. In the latter cases the entropy change at the SmA-N* transition is clearly larger than for the compounds with the other smectic modifications. One or two compounds in each alkoxybiphenylyl-containing series, that have similar spacer length and terminal group length and have an odd number of flexible units in the spacer, exhibit only a chiral nematic phase, and no smectic phase.     

Influence of spacer and terminal group lengths on the smectic ordering of cholesterol-containing dimer liquid crystals

The smectic properties of four series of dimer liquid crystals containing cholesteryl and biphenylyl groups were investigated by polarization microscopy, DSC and X-ray diffraction. Compounds that contain the strongly dipolar cyanobiphenylyl group exhibit a smectic layer spacing that is about 1.7 times the length of the molecule. Three series of alkoxybiphenylylcontaining dimers exhibit two other smectic modifications. For short spacers, a smectic layer spacing is observed that is about the same length as the molecule and for long spacers the smectic layer spacing is about half the length of the molecule. In the latter cases the entropy change at the SmA-N* transition is clearly larger than for the compounds with the other smectic modifications. One or two compounds in each alkoxybiphenylyl-containing series, that have similar spacer length and terminal group length and have an odd number of flexible units in the spacer, exhibit only a chiral nematic phase, and no smectic phase.     

Parameters influencing the templated growth of colloidal crystals on chemically patterned surfaces

The influence of various experimental parameters on the vertical deposition and structure formation of colloidal crystals on chemically patterned surfaces, with hydrophilic and hydrophobic areas, was investigated. The pattern dimensions range from about 4 to 400 microm, which is much larger than the individual particle size (255 nm), to control the microscopic crystal shape rather than influencing the crystal lattice geometry (as achieved in colloidal epitaxy). The deposition resolution and selectivity were tested by varying the particle concentration in the suspension, the substrate withdrawing speed, pattern size and orientation, and wetting contrast between the hydrophilic and hydrophobic regions. The evolution of colloidal crystal thickness with respect to the pattern dimensions and deposition parameters was further studied. Our results show that the pattern size has a rather strong influence on the deposited number of colloid layers and on the crystal quality. Better results are obtained when the lines of a stripe pattern are oriented parallel to the withdrawing direction rather than perpendicular. The deposition resolution (defined as the minimum feature size on which particles can be deposited) depends on the wetting contrast and increases with lower average hydrophobicity of the substrate.     

Conformational ordering of chain molecules in liquid crystals

Deuterium NMR studies have been performed to elucidate orientational characteristics of some ether-type main-chain liquid crystals. Spacers used are of the type -O(CH₂)_nO- with n = 9 and 10. Dimers, homopolymers as well as copolymers in which spacers n = 9 and 10 are arranged in an alternative fashion were investigated. The quadrupolar splitting data obtained from the deuterium-labeled mesogenic core and spacer have been studied within the rotational isomeric state (RIS) approximation. The ordering characteristics thus estimated were found to be consistent with the magnetic susceptibility data obtained by using superconducting quantum interference device (SQUID) for the same polymers.     

Nematic ordering and crystal structure of liquid crystals containing a 4-chlorophenyldifluoroethylene unit

Partially fluorinated liquid crystals have many desirable properties such as high impedance and low viscosity. The compounds trans -1-( trans -4-alkylcyclohexyl)-2-(4-chlorophenyl)difluoroethylene have very low viscosities and reasonable nematic ranges, and are very suitable for electro-optic displays. The crystal structure of a compound in this homologous series has been determined by X-ray diffraction. The orientational ordering for this compound and for another homologue in their nematic phases has been studied by the use of one and two dimensional 13C NMR. The proton-decoupled 13C spectra are complex due to splittings from the two F-19 nuclei, but they have been analysed by combining chemical shift anisotropy and variable angle spinning studies. Carbon-proton and carbon-fluorine dipolar coupling constants were obtained, and the order parameters for the two conjugated molecular segments (the chlorophenyl ring and the difluoroethylenic link) were calculated from the dipolar coupling constants. The temperature dependences of the local order parameters of each fragment were studied.