Maxwell displacement current allows to study structural changes of gramicidin A in monolayers at the air-water interface

We applied methods of measurement Maxwell displacement current (MDC) pressure-area isotherms and dipole potential for analysis of the properties of gramicidin A (gA) and mixed gA/DMPC monolayers at an air-water interface. The MDC method allowed us to observe the kinetics of formation of secondary structure of gA in monolayers at an air-water interface. We showed, that secondary structure starts to form at rather low area per molecule at which gA monolayers are in gaseous state. Changes of the MDC during compression can be attributed to the reorientation of dipole moments in a gA double helix at area 7 nm(2)/molecule, followed by the formation of intertwined double helix of gA. The properties of gA in mixed monolayers depend on the molar fraction of gA/DMPC. At higher molar fractions of gA (around 0.5) the shape of the changes of dipole moment of mixed monolayer was similar to that for pure gA. The analysis of excess free energy in a gel (18( ) degrees C) and in a liquid-crystalline phase (28( ) degrees C) allowed us to show influence of the monolayer structural state on the interaction between gA and the phospholipids. In a gel state and at the gA/DMPC molar ratio below 0.17 the aggregates of gA were formed, while above this molar ratio gA interacts favorably with DMPC. In contrast, for DMPC in a liquid-crystalline state aggregation of gA was observed for all molar fractions studied. The effect of formation ordered structures between gA and DMPC is more pronounced at low temperatures.     

Phase behaviour and electro-optical response of systems composed of nematic liquid crystals and poly (2-ethylhexylacrylate)

Experimental studies of phase diagram and morphology of mixtures composed of linear poly(2-ethylhexylacrylate) (poly-2EHA) of molecular weight M_w = 92000g/mol and E7, an eutectic blend of nematic liquid crystals (LCs), were performed. The latter consists of four cyanoparaphenylenes, i.e. 4-cyano-4?-n-pentyl-biphenyl (5CB), 4-cyano-4?-n-heptyl-biphenyl (7CB), 4-cyano-4?-n-oxyoctyl-biphenyl (8OCB) and 4-cyano-4?’-n-pentyl-p-biphenyl (5CT). Thermal properties of the poly-2EHA/E7 system were studied by differential scanning calorimetry and the sample morphologies were investigated by optical microscopy. Phase diagram analysis revealed an increase of the nematic–isotropic transition temperature when E7 was added to poly-2EHA, compared to pure E7. This effect, related to a preferential miscibility phenomenon of E7 components towards poly-2EHA, was evidenced by gas chromatography analysis coupled to mass spectrometry, allowing to qualify and quantify the content of phase separated LC domains. Thermograms of model LC mixtures including 5CB, 7CB, 8OCB and 5CT, which correspond to their composition in segregated LC domains of poly-2EHA/E7, present strong deviations from eutectic behaviour. In order to access the impact of such variations for practical applications, such a model LC mixture was introduced in photochemically crosslinked 2-EHA (polycross-2EHA). Surprisingly, the electro-optical response of the obtained polycross-2EHA/LC films was considerably improved compared to the original polycross-2EHA/E7 system.     

Wide-Line 1H NMR and DSC study of phase states of mesomorphic states of 4′-n-alkoxy-4-biphenylcarbonitrile mesogens (n = 3 and 7)

4′-(Propyloxy)-4-cyanobiphenyl (3OCB) and 4′-(heptyloxy)-4-cyanobiphenyl (7OCB) have been studied in the temperature range 20–80°C by wide-line ¹H NMR and DSC. The ¹H NMR line shapes, half-widths, and second moments, as well as the relaxation characteristics of separate lines, are discussed. The DSC data are compared with the specific features of mesogenic transformations of these homologues and ¹H NMR data. At certain temperatures depending on the composition of alkyl moieties, a dynamically disordered fraction of molecules appear in solid crystals. These molecules are involved in the thermotropic transitions of 3OCB to isotropic liquid and of 7OCB to nematic liquid crystal. The temperature ranges, stabilities, and other characteristics of the transitions of 3OCB to isotropic liquid and of 7OCB to nematic liquid crystal were determined.     

Study of the surface and interfacial tensions in systems containing a low molar mass liquid crystal

The surface tension of a low molar mass liquid crystal (LMMLC), 4-cyano-4'-n-heptyloxybiphenyl (70CB), was measured as a function of temperature using the pendant drop method, forming drops of different volumes ranging from 5 to 11 mm³. Contact angles formed by drops of 70CB in the nematic and isotropic phases on plates of polystyrene (PS) and of a liquid crystal polymer (LCP), VECTRA A910, were also measured. Only large drops could be used for surface tension analysis. It was shown that in the nematic phase the surface tension of 70CB decreases with increasing temperature, and that in the isotropic phase the surface tension increases with increasing temperature. Using the values of contact angle and of surface tension of 7OCB it was possible to evaluate the interfacial energy between 7OCB and PS and between 7OCB and VECTRA. The interfacial energy between 7OCB and PS, and between 7OCB and VECTRA, decreased with increasing temperature for ranges of temperatures corresponding to both phases of 70CB.     

A comparison of the structure,flexibility and mesogenic properties of 4-methoxy-4′-cyanobiphenyl and the α,α,α-trifluorinated derivative

Abstract

The compound 4-cyano-4′-(α,α,α-trifluoromethoxy)biphenyl (1OCBF₃) has been synthesized. Unlike the fully protonated analogue, 4-cyano-4′-methoxybiphenyl (1OCB), it does not show a liquid crystalline phase on cooling from the melting point (51°C) to room temperature. The transition temperature to a monotropic nematic phase was obtained as approximately 0°C by determining the transition temperatures of mixtures with 1OCB. The structures, conformational properties and orientational ordering of both 1OCB and 1OCBF₃ as solutes in a nematic solvent ZLI 1132 have been investigated via the 17 dipolar couplings obtained by analysing the proton and fluorine NMR spectra of these solutions. It is concluded that the major difference between the two molecules lies in the potential, V(φ₂), governing rotation about the ring–oxygen bonds. In 1OCB the potential has the same form as in anisole, with a minimum when the C–O bond is in the plane of the attached ring (φ₂ = 0°), and a maximum of about 15 kJ mol^?1 when φ₂ is 90°. In 1OCBF₃ the barrier to rotation about the ring–O bond decreases substantially to being near zero.     

Study of the surface and interfacial tensions in systems containing a low molar mass liquid crystal

The surface tension of a low molar mass liquid crystal (LMMLC), 4-cyano-4'-n-heptyloxybiphenyl (70CB), was measured as a function of temperature using the pendant drop method, forming drops of different volumes ranging from 5 to 11 mm³. Contact angles formed by drops of 70CB in the nematic and isotropic phases on plates of polystyrene (PS) and of a liquid crystal polymer (LCP), VECTRA A910, were also measured. Only large drops could be used for surface tension analysis. It was shown that in the nematic phase the surface tension of 70CB decreases with increasing temperature, and that in the isotropic phase the surface tension increases with increasing temperature. Using the values of contact angle and of surface tension of 7OCB it was possible to evaluate the interfacial energy between 7OCB and PS and between 7OCB and VECTRA. The interfacial energy between 7OCB and PS, and between 7OCB and VECTRA, decreased with increasing temperature for ranges of temperatures corresponding to both phases of 70CB.     

Optical microscopy, DSC and X-ray diffraction studies in binary mixtures of 4-pentyloxy-4'-cyanobiphenyl with three 4,4'-di(alkoxy)azoxybenzenes

A large number of binary mixtures of 4-pentyloxy-4'-cyanobiphenyl (5OCB) and 4,4'-di(alkoxy)azoxybenzenes (nOAB) with n = 5, 6 and 7 have been studied by optical microscopy, DSC and X-ray diffraction methods. Over a wide concentration range the smectic A (SmA) phase is induced and the nematic (N) phase of the parent compounds is absent. Instead of the tilted smectic C phase of 7OAB an orthogonal SmA phase is observed when only 12% of the host molecules are replaced by 5OCB molecules. Moreover, in some mixtures the thermal stability of the induced SmA phase is found to be more than 1.6 times the stability of the mesophses in the pure compounds. The N-isotropic or Sm-I transition temperatures slowly increases with increasing concentrations, reaching a maximum at x _5OCB = 0.50 and then falling quite rapidly. In all the mixtures the enthalpy changes at the N-I transitions follow the simple additivity rule. In no mixture, except mixture C2, could the SmA-N transition be detected by DSC, although in all the mixtures the ratio T _NA/T _NI is found to be more than 0.95, which is in contradiction to McMillan's theory. Average intermolecular distances are found to have a minimum value near the equimolar concentration. From the concentration dependence of the smectic layer spacings it also appears that in all the mixtures the smectic A_d phase evolves from the smectic A₁ phase with increasing concentrations.     

Vibrational spectra and structure of 4-cyano-4′-pentalkoxybiphenyl in different physical states

The polymorphism, conformation mobility and structure of 4-cyano-4′-pentalkoxybiphenyl (5OCB) in different physical states are studied by IR spectroscopy. The spectra were measured in the frequency range 400–4000 cm⁻¹ at temperatures from 300 to 350 K. The IR spectra of 5OCB are modeled using the concept of conformational mobility of these molecules. An analysis of the experimental and theoretical spectra reveals absorption bands whose spectroscopic parameters are sensitive to variations of temperature (experiment) and conformation (theory); a relationship between these changes is established. It is concluded that the polymorphism of 5OCB is of conformation type. In the solid crystalline state, 5OCB molecules have conformations with a planar biphenyl fragment; the angle of orientation of the plane of the carbon framework of the alkyl radical relative to the biphenyl fragment decreases as the temperature increases from 35° in the solid crystalline state to 10° in the liquid crystalline and isotropic liquid states. In both of these states the biphenyl fragment becomes nonplanar. The angle between the phenyl rings is up to 30°. Saratov State University. Institute of Physics, Ukrainian Academy of Sciences. Samarkand State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 1, pp. 55–60, January–February, 1998. This work was supported by RFFR grant No. 97-03-32175a.     

巯基取代噻二唑自组装铂电极测定对苯二酚

用自己合成的巯基取代噻二唑试剂对铂电极进行了首次自组装,用电化学方法研究了该电极的电化学性质.研究了对苯二酚在该电极上的电化学行为.实验结果表明,在0.1mol/LKCl溶液中,对苯二酚表现出一种准可逆行为,峰电流对称且峰电位差为80mV,峰电位较之裸铂电极上有较大的降低,峰电流较裸电极大大提高.利用该电极DPV法进行了对苯二酚的电化学测定,峰电流对其浓度在1×10-7~5×10-4mol/L之间成良好的线性关系,检出限为4×10-8mol/L.对对苯二酚模拟品进行了测定,回收率在96.6%-99.6%之间.     

Detailed study of a two component smectic 4TPB-8OCB system with a nematic gap. Phase diagram and viscosity study

The phase diagram of a two component system composed of two smectic compounds: 4-octyloxy-4'-cyanobiphenyl (8OCB) and 4-isothiocyanatophenyl 4-butylbenzoate (4TPB) was investigated. Three Miesowicz viscosity coefficients η₁, η₂, η₃ and the refractive indices at different temperatures as well as the enthalpies of the phase transitions were measured. It was stated, that the properties of the induced nematic phase, for example, the nematic phase existing between two smectic regions, are the same as the ones observed in the case of low viscosity nematic mesogens.     

Second-harmonic generation and the influence of flexoelectricity in the nematic phases of bent-core oxadiazoles

Second-harmonic generation (SHG) in the nematic phase of bent-core oxadiazole-based liquid crystals (LCs) was studied and compared to that for the rod-like compound 4-cyano-4?-n-octylbiphenyl (8CB). Weak, isotropically scattered second-harmonic (SH) light was observed for all materials, consistent with SHG by nematic director fluctuations. The SH intensity produced by the bent-core materials was found to be up to ~ 3.4 times that of 8CB. We discuss this result in terms of the dependence of SH intensity on temperature, elastic constants and flexoelectric coefficients. We have calculated the latter by using a molecular field approach with atomistic modelling, thus demonstrating how molecular parameters contribute to the flexoelectric coefficients and illustrating the potential of this method for predicting the flexoelectric behaviour of bent-core LCs. We show that the increased SH signal in the bent-core compounds is partly due to their nematic phases being at a much higher temperature, and also potentially due to them having greater flexoelectric coefficients, up to ~1.5 times those of 8CB. These estimates are consistent with reports of increased flexoelectric coefficients in bent-core compounds in comparison to rod-like compounds.     

Charge Measurement in the Modified Maxwell Displacement Current Method

A modified Maxwell displacement current (MDC) measurement is described. The distance between water surface and a suspended electrode is changed sinusoidally at an angular frequency omega=omega(t), hence the MDC and the induced charge on the suspended electrode are modulated. The dc MDC current and the induced charge could be detected simultaneously and independently. This simplifies considerably charge measurements and will be helpful in the study of monolayers, e.g., the determination of the dipole moment. Copyright 2000 Academic Press.     

Cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase

ABSTRACT

The synthesis and characterisation of several members of the 1,ω-bis(4-cyanobiphenyl-4′-yl) alkane (CBnCB) and the 1-(4-cyanobiphenyl-4′-yloxy)-ω-(4-cyanobiphenyl-4′-yl) alkane (CBnOCB) homologous series are reported. The new odd members described CB5CB, CB13CB, CB4OCB, CB8OCB and CB10OCB all exhibit twist-bend nematic and nematic phases. The members of these series already reported in literature, CB7CB, CB9CB, CB11CB and CB6OCB, were also prepared in order to allow for a direct comparison of their transitional properties. The properties of these dimers are also compared to those of the corresponding members of the 1,ω-bis(4-cyanobiphenyl-4,-yloxy) alkanes (CBOnOCB). For any given total spacer length, for odd members of these series, the nematic–isotropic transition temperatures and associated entropy changes are greatest for the CBOnOCB dimer and lowest for the CBnCB dimer. These trends are understood in terms of molecular shape. For short spacer lengths, the twist-bend nematic–nematic transition temperature (T_NTBN) is higher for the CBnOCB series than for the CBnCB series but this is reversed as the spacer length increases. Of the CBOnOCB dimers, a virtual value of T_NTBN was estimated for CBO3OCB and T_NTBN was measured for CBO5OCB. These values are considerably lower than those observed for the corresponding members of the CBnCB or CBnOCB series. The dependence of T_NTBN on molecular structure is discussed not only in terms of the molecular curvature but also in the ability of the molecules to pack efficiently. As the temperature range of the preceding nematic phase increases, so the twist-bend nematic–nematic transition entropy change decreases and the transition approaches second order for the longer spacers. For comparative purposes, the transitional behaviour of the even-membered dimers CB6CB, CB5OCB and CBO4OCB is reported and differences accounted for in terms of molecular shape.     

Assignment of the quadrupolar splittings in fully deuteriated alkyl chains of liquid crystalline compounds The case of 4-n-hexyloxy-4'-cyanobiphenyl

There have been disagreements about the assignment of the quadrupolar splittings in the deuteriated n-hexyl chain in 4-n-hexyloxy-4'-cyanobiphenyl (6OCB); these are resolved by comparing the splittings of the deuteron resonances in the fully deuteriated chain compound with those in the material with a selectively deuteriated chain. The methods available for assigning the quadrupolar splittings in fully deuteriated chains are then applied to 6OCB and it is concluded that measurement of spin-lattice relaxation times, T₁, is the best method.     

Continuum modelling of hybrid-aligned nematic liquid crystal cells: optical response and flexoelectricity-induced voltage shift

A continuum model is employed to study systematically the optical response of hybrid-aligned nematic (HAN) liquid crystal cells under the application of an external electric field. The influence of the flexoelectric effect is discussed for a large range of anchoring strengths at the homeotropic alignment layer. It is shown that the optical response of HAN cells is governed by a complicated interplay between the flexoelectric coefficient and homeotropic anchoring strength. In particular, the calculations reveal that, for weak homeotropic anchoring, the flexoelectric effect leads to a non-linear voltage shift of the optical transmittance as a function of flexoelectric coefficient, and gives rise to an asymmetry in the transmittance–voltage curve. Finally, a comparison of the continuum-model simulations with recent experimental observations indicates that both the flexoelectric coefficient and the anchoring strength of the nematic liquid crystal MBBA on a homeotropic polyimide alignment layer are significantly lower than previously reported.     

Continuum modelling of hybrid-aligned nematic liquid crystal cells: optical response and flexoelectricity-induced voltage shift

A continuum model is employed to study systematically the optical response of hybrid-aligned nematic (HAN) liquid crystal cells under the application of an external electric field. The influence of the flexoelectric effect is discussed for a large range of anchoring strengths at the homeotropic alignment layer. It is shown that the optical response of HAN cells is governed by a complicated interplay between the flexoelectric coefficient and homeotropic anchoring strength. In particular, the calculations reveal that, for weak homeotropic anchoring, the flexoelectric effect leads to a non-linear voltage shift of the optical transmittance as a function of flexoelectric coefficient, and gives rise to an asymmetry in the transmittance-voltage curve. Finally, a comparison of the continuum-model simulations with recent experimental observations indicates that both the flexoelectric coefficient and the anchoring strength of the nematic liquid crystal MBBA on a homeotropic polyimide alignment layer are significantly lower than previously reported.     

Gold nanoparticles-enhanced bisphenol A electrochemical biosensor based on tyrosinase immobilized onto self-assembled monolayers-modified gold electrode

A novel electrochemical sensor based on the immobilization of tyrosinase(tyr) onto gold nanoparticles(nano-Au) and thioctic acid amide(T-NH2) self-assembled monolayers(SAMs)-modified gold electrode has been developed for the determination of bisphenol A(BPA).It was found that the nano-Au could significantly enhance the electrochemical response of tyr/nano-Au/T-NH2/Au electrode to BPA,and the enhancement effect of nano-Au on the current response was also related to the enzyme.The results indicated that the biosensor could be used as a detector for BPA determination with a linear range from3.99 ×10-7mol/L to 2.34 ×10-4mol/L and a detection limit of 1.33×10-7mol/L.In addition,this biosensor showed good reproducibility.     

Dielectric and orientation effects of ‘classical’ and supramolecular liquid crystals self-assembly

Fragments of phase diagrams of binary systems containing 4-pentyloxy-4?-cyanobiphenyl (5OCB) and 4-(ω-hydroxyalkyloxy)-4?-cyanobiphenyl (HO3OCB, HO6OCB and HO9OCB) were obtained. The temperature range expansion of the nematic phase at mixing of the components was established. The dielectric and optical properties of binary mixtures were studied. A significant increase in the dielectric anisotropy was observed when the ‘classical’ liquid crystal (LC) was mixed with supramolecular HOnOCB. From the data on birefringence and the ¹H NMR spectra, temperature dependences of the orientational order parameter of the ‘classical’ LC + supramolecular LC systems were obtained. There is a good agreement between S values obtained by independent methods. The invariability of the order parameter at mixing 5OCB with HO3OCB and HO9OCB and some growth in systems containing HO6OCB were observed. On the basis of quantum chemical calculations, the analysis of the most probable types of components association in the systems 5ОСВ + HOnOCB was carried out. From the analysis of Kirkwood correlation factors, the main role of competition between the two mechanisms of intermolecular interaction was revealed.     

Determination of the difference of flexoelectric coefficients in a nematic liquid crystal using a conoscopic technique

The conoscopic images of twisted nematic liquid crystal devices filled with E7 are analysed under the application of in-plane electric fields. The differences observed between the images for positive and negative applied fields are attributed to the flexoelectric effect. By comparison of the conoscopic images with theoretical predictions made using an extended Jones technique, the sign and magnitude of the difference between the splay and bend flexoelectric coefficients is determined for E7.     

Correlated chain dynamics in the nematic phase of 4-cyano-4'-hexyloxybiphenyl

We report in this paper measurements of the Zeeman and Quadrupolar spin-lattice relaxation times at two different deuteron Larmor frequencies for the nematic phase of the perdeuteriated nematogen 6OCB. A model of correlated internal motion is used to account for both the quadrupolar splittings and the spectral densities of motion. The nematic mean field is constructed using the additive potential method, while the conformational transitions among the allowed configurations are described by a master equation. For modelling the quadrupolar splittings, we used 729 conformations, although this seemed impossible when the spectral densities were fitted by minimizing the sum of squares of per cent errors. The pentane effect has, therefore, been used to limit the size of the transition rate matrix in the master equation. We found that the dynamic model for liquid crystals proposed by one of us (1991, Phys. Rev. A, 43, 4310) is essentially correct for 6OCB.