Light scattering study of the isotropic-to-blue phase transition in cholesteryl oleyl carbonate (COC) and its mixtures with a nematogen

Abstract

Light scattering experiments have been performed on cholesteryl oleyl carbonate (COC) and its mixtures with a nematogen in order to investigate pretransition phenomena accompanying the transition from the isotropic to the blue phase. Estimates of the critical temperature for the flat-spiral (m = 2) mode have been obtained in each case. The critical temperatures approach the transition temperature, T _c, in the mixtures, probably due to an impurity-induced blurring of the phase transition. Measurements of the optical rotatory power have also been carried out on pure COC as well as its mixtures with the nematogen and possible origins for the pretransition phenomena observed are discussed.     

Pressure-induced re-entrant cholesteric phase behaviour of non-polar liquid crystals

In a previous paper we reported the existence of a pressure-induced re-entrant cholesteric phase in mixtures of non-polar liquid crystals. Now the influence of the mixing ratio on this behaviour has been studied up to 3000 bar and 190°C and the phase boundaries based on light reflection measurements have been confirmed by transmission and texture observations in a diamond anvil cell. Additional thermodynamic investigations show that when the cholesteric/smectic A phase transition line possesses a maximum temperature the pretransition enthalpy and volume disappear.     

Audio to microwave frequency dielectric study of the pretransition region in DPL-water systems

Abstract

The phospholipid–water mixtures are used as model systems to investigate the structure and function of biological membranes. The dielectric behaviour of DPPC–water and DPPE–water systems was explored at audio (≥ 2kHz) and microwave (10 GHz) frequencies versus temperature. Particular care was devoted to the pretransition below the main gel-liquid-crystalline phase transition. A possible interpretation of the pretransition mechanism is given in terms of capillary waves.     

磷脂DHPC和短杆菌肽D相互作用的DSC研究

Changes of the behavior of the thermotropic phase transitions of the phospholipid DHPC by the action of miniprotein gramicidin D (G.D) have been studied by differential scanning calorimetry. The experimental results demonstrated that G.D could not remove the pretransition of DHPC, but the pretransition peak and the main transition peak of DHPC were incorporated into a broad peak. The pretransition temperature and the main transition temperature of DHPC decreased to different ex-tent. The sums of the pretransition and main transition enthalpies of DHPC showed a lineally decreasing relationship with increasing the mole fraction x_g of gramicidin D.     

Computer simulation of molecular organization in a nematogen. The role of thermodynamic parameters

Computer simulations of molecular organization in a nematogen, alkenyl bicyclohexylnitrile (ALBCHNL), have been carried out with respect to translational and orientational motions. The atomic net charge and dipole moment at each atomic center has been evaluated using the complete neglect of differential overlap (CNDO/2) method. The modified Rayleigh-Schroedinger perturbation theory, along with multicentered-multipole expansion method, has been employed to evaluate long-range intermolecular interactions, while a “6-exp” potential function has been assumed for short-range interactions. The total interaction energy values obtained through these computations have been used as input to calculate the thermodynamical parameters such as entropy and Helmholtz free energy of each configuration at room temperature (300 K), nematic-isotropic transition (364.7 K) and above transition temperature (450 K). An attempt has been made to understand the molecular organization, to develop a new and interesting model for nematogen based on the thermodynamic parameters introduced in this article.     

A mode coupling theory description of the short- and long-time dynamics of nematogens in the isotropic phase

Optical heterodyne-detected optical Kerr effect (OHD-OKE) experimental data are pre-sented on nematogens 4-(trans-4-n-octylcyclohexyl)isothiocyanatobenzene (8-CHBT), and 4-(4-pentyl-cyclohexyl)-benzonitrile (5-PCH) in the isotropic phase. The 8-CHBT and 5-PCH data and previously published data on 4-pentyl-4-biphenylcarbonitrile (5-CB) are analyzed using a modification of a schematic mode coupling theory (MCT) that has been successful in describing the dynamics of supercooled liquids. At long time, the OHD-OKE data (orientational relaxation) are well described with the standard Landau-de Gennes (LdG) theory. The data decay as a single exponential. The decay time diverges as the isotropic to nematic phase transition is approached from above. Previously there has been no theory that can describe the complex dynamics that occur at times short compared to the LdG exponential decay. Earlier, it has been noted that the short-time nematogen dynamics, which consist of several power laws, have a functional form identical to that observed for the short time behavior of the orientational relaxation of supercooled liquids. The temperature-dependent orientational dynamics of supercooled liquids have recently been successfully described using a schematic mode coupling theory. The schematic MCT theory that fits the supercooled liquid data does not reproduce the nematogen data within experimental error. The similarities of the nematogen data to the supercooled liquid data are the motivation for applying a modification of the successful MCT theory to nematogen dynamics in the isotropic phase. The results presented below show that the new schematic MCT theory does an excellent job of reproducing the nematogen isotropic phase OHD-OKE data on all time scales and at all temperatures.     

Electric field N.M.R. of pretransitional effects in liquid crystals A solute study

Pretransitional effects in the isotropic phase of liquid crystals are manifest in the N.M.R. spectra of solutes recorded in the presence of an applied electric field. The temperature dependence of the quadrupolar splittings of p-xylene-d₁₀ in the isotropic phase of the nematogen 4-n-pentyl-4'-cyanobiphenyl is investigated, and proves to be as expected from the Landau-de Gennes theory for pretransitional effects. The influence of temperature in the biphasic region of the phase diagram is very striking: the orientational ordering of the solute is temperature independent. The applied electric field not only determines the ordering of the solute, but also has a clear effect on the transition temperature. Electric field N.M.R. of solutes to study pretransitional effects is an alternative to the investigation of the nematogen itself.     

Phase and interfacial behavior of partially miscible symmetric Lennard-Jones binary mixtures

We have carried out extensive equilibrium molecular-dynamics simulations to study quantitatively the topology of the temperature versus density phase diagrams and related interfacial phenomena in a partially miscible symmetric Lennard-Jones binary mixture. The topological features are studied as a function of miscibility parameter, alpha = epsilonAB/epsilonAA. Here epsilonAA = epsilonBB and epsilonAB stand for the parameters related to the attractive part of the intermolecular interactions for similar and dissimilar particles, respectively. When the miscibility varies in the range 0 < alpha < 1, a continuous critical line of consolute points Tcons(rho)--critical demixing transition line--appears. This line intersects the liquid-vapor coexistence curve at different positions depending on the values of alpha, yielding mainly three different topologies for the phase diagrams. These results are in qualitative agreement to those found previously for square-well and hard-core Yukawa binary mixtures. The main contributions of the present paper are (i) a quantitative analysis of the phase behavior and (ii) a detailed study of the liquid-liquid interfacial and liquid-vapor surface tensions, as function of temperature and miscibility as well as its relationship to the topological features of the phase diagrams.     

Temperature-responsive ionic liquid/water interfaces: relation between hydrophilicity of ions and dynamic phase change

Phase separation between ionic liquids (ILs) and molecular liquids is of interest physico-chemically, and also has industrial relevance. IL/water mixtures are of great interest in many fields. Unlike static phase separation between IL and water, dynamic shifts of IL/water mixtures between a homogeneous mixture and separate phases have a wide variety of applications. The miscibility of ILs with water generally increases upon heating, and a few ILs undergo a lower critical solution temperature (LCST)-type phase transition with water in which the separated biphases become miscible upon cooling. As the phase transition is controlled by changing the temperature by a few degrees, the LCST-type phase response of IL/water mixtures makes it possible to use ILs as solvents in various energy-saving processes. Since many hydrophilic ILs do not undergo phase separation with water, we aim to determine the necessary conditions under which hydrophobic ILs undergo the phase transition. Based on physico-chemical analysis of many hydrophobic ILs that undergo a phase separation after mixing with water, we find there is a particular range of "hydrophilicity" of these hydrophobic ILs within which the LCST-type phase transition is possible. Accordingly, a hydrophilicity index (HI) of ILs is proposed, in terms of the number of water molecules in the separated IL phase. The HI value proves to be a good indicator of the phase behaviour of IL/water mixtures, as well as their phase transition temperature. Potential application of the LCST-type phase change to the selective extraction of water-soluble proteins is also summarised.     

Temperature-induced structural and permeability changes in dioctadecyldimethylammonium bromide (DODAB) vesicles: a fluorescence investigation using 2-[(p-methylamino)phenyl]-3,3-dimethyl-5-carboethoxy-3H-indole as a probe

An investigation of the temperature dependence of the fluorescence spectral characteristics of 2-[(p-methyl-amino)phenyl]-3,3-dimethyl-5-carboethoxy-3H indole (I) in aqueous micelles(sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB)) and surfactant vesicles (dioctadecyldimethylammonium bromide (DODAB)) is presented. The gel-to-liquid crystalline phase transition temperature T_c, determined to be approximately 309 K (36°C) for DODAB vesicles, is in close agreement with the value reported previously. A pretransition at 294 K (21°C) has also been obtained. The blue shift in the fluorescence maximum and the increase in bandwidth are accounted for by the displacement of molecule I towards the interior of the bilayer as a function of temperature. Arrhenius plots for the non-radiative decay processes competing with fluorescence as a function of temperature show relatively high values for the activation energy above the phase transition temperature, indicating the displacement of molecule I to a new, more viscous, less polar site compared with its initial location in DODAB. The highest value of the activation energy in water indicates that the decay dynamics of this molecule are different in water than in the organized media studied here.     

Molecular interactions and crystal packing in nematogen: Computational thermodynamic approach

A computational thermodynamic approach of molecular interactions in a nematogen p-n-alkyl benzoic acid (nBAC) molecule with an alkyl group butyl (4BAC) has been carried out with respect to translational and orientational motion. The atomic net charge and dipole moment at each atomic center were evaluated using the complete neglect differential overlap (CNDO/2) method. The modified Rayleigh-Schrödinger perturbation theory along with multicentered-multipole expansion method were employed to evaluate long-range intermolecular interactions, while a 6-exp potential function was assumed for short-range interactions. Various possible geometrical arrangements of molecular pairs with regard to different energy components were considered, and the energetically favorable configuration was found to understand the crystal packing picture. Furthermore, these interaction energy values are taken as input to calculate the configurational entropy at room temperature (300 K), nematic-isotropic transition temperature (386 K) and above transition temperature (450 K) during different modes of interactions. An attempt has been made to describe interactions in a nematogen at molecular level, through which one can simplify the system to make the model computationally feasible in understanding the delicate interplay between energy and entropy, that accounts for mesomorphism and there by to analyze the molecular structure of a nematogen.     

Anomalous selective reflection in cholesteryl oleyl carbonate – nematic 5CB mixtures and effects of their doping by single-walled carbon nanotubes

Liquid crystalline (LC) mixtures of cholesteryl oleyl carbonate (COC) and 4-pentyl-4'-cyanobiphenyl (5CB) as well as dispersions of single-walled carbon nanotubes (NTs) in these mixtures were studied by means of selective reflection measurements, differential scanning calorimetry (DSC) and optical microscopy. The relative mass of COC in a mixture X was varied between 0.4 and 1.0, the temperature range of measurements was between 284 and 314 K, and concentration of NTs was fixed at 0.1%. Two important anomalies were noted: (1) the cholesteric to smectic-A transition temperature increased on dilution of COC by non-smectogenic 5CB in the concentration range 0.8 < X < 1 and (2) the reciprocal pitch versus 5CB concentration dependence was essentially linear, in contrast to behaviour commonly observed in nematic-cholesteric mixtures. A model of molecular arrangement in the mixtures, accounting for the possibility of integration of 5CB dimers and monomers between COC molecules and presumably explaining the experimental data, was proposed. The helical pitch of the cholesteric mixtures remained practically unchanged upon doping by NTs, and only slight widening of the selective reflection peaks was noted. The obtained results allow considering the COC + 5CB mixtures as promising matrices for composite materials on the basis of liquid crystals and NTs.     

Intermolecular interactions in mixtures of dichroic dyes with n-alkyl-4-(4-isothiocyanatophenyl)bicyclo[2,2,2]octanes

On the basis of the polarized absorption spectra, the guest order parameter as a function of temperature for five dichroic azo dyes dissolved in four members (pentyl to octyl) of the homologous series of n-alkyl-4-(4-isothiocyanatophenyl)bicyclo[2,2,2]octanes has been evaluated. Moreover, the influence of the dichroic dye addition on the nematic-isotropic phase transition temperature has been examined. The relation between the molecular geometry of the guest, its orientation efficiency in the nematic host and the clearing temperature of the azo dye-liquid crystal mixtures has been determined. The experimental results have been compared in some detail with calculations performed on the basis of the mean field theory for binary mixtures.     

Unbinding transition in lipid multibilayers induced by copper(II) ions

We describe our observations on an unbinding transition in a multilamellar dispersion of phosphatidylcholine (PC) vesicles induced by copper(II) ions. The small-angle X-ray measurements clearly show that the increasing amount of CuCl2 in the millimolar concentration range continuously increases the amount of the unbound bilayers in the gel phase. Moreover, this phenomenon becomes more pronounced when the samples are heated above the so-called pretransition temperature between the gel and the ripple gel phase. The proposed reason for the latter is the increased repulsive electrostatic interaction due to the appearance of the surface modulation in the ripple gel phase. The observed effects reveal a new aspect of the unbinding phenomena since only the transition induced by the steric repulsion due to the layer fluctuations has been considered so far. Here, we show that the unbinding can also be triggered by the change in the electrostatic interactions. These findings are connected to the physical basis of the crucial role of copper(II) ions in biological processes such as neurodegenerative diseases and cell evolution.     

Molecular dynamics simulations of nanoparticles in dense isotropic nematogens: the role of matrix-induced long-range repulsive interactions

We have carried out molecular dynamics simulation studies of binary mixtures of spherical nanoparticles (NPs) in a matrix of dense isotropic rod-shaped nematogens, with the size of the nematogen length being similar to that of the NP diameter. NPs at even low concentrations were found to shift the isotropic-nematic (I-N) transition significantly to higher pressure at a given temperature, indicative of long-range perturbation of the nematogenic matrix by the NPs. The NPs were found to be dispersed in the dense isotropic nematogenic matrix over a wide range of NP concentrations due to long-range (compared with the molecular size of the nematogens) repulsion caused by NP-induced local order fluctuations and reduced local orientational correlation in the isotropic nematogenic matrix, in contrast to the phase separation predicted and observed in other studies where the particles were much larger or smaller than the nematogens. Furthermore, since the repulsion observed in the NP-nematogen mixtures is only microscopically long range (on the order of about ten molecular lengths of the nematogens), globally ordered clustering observed in mixtures of colloidal particles in nematic matrices resulting from macroscopically long-range interaction is not observed in our simulations.     

Density functional theory study of the nematic-isotropic transition in an hybrid cell

We have employed the density functional theory formalism to investigate the nematic-isotropic capillary transitions of a nematogen confined by walls that favor antagonist orientations to the liquid crystal molecules (hybrid cell). We analyze the behavior of the capillary transition as a function of the fluid-substrate interactions and the pore width. In addition to the usual capillary transition between isotropiclike to nematiclike states, we find that this transition can be suppressed when one substrate is wet by the isotropic phase and the other by the nematic phase. Under this condition the system presents interfacelike states which allow us to continuously transform the nematiclike phase to the isotropiclike phase without undergoing a sharp phase transition. Two different mechanisms for the disappearance of the capillary transition are identified. When the director of the nematiclike state is homogeneously planar-anchored with respect to the substrates, the capillary transition ends up in a critical point. This scenario is analogous to the observed in Ising models when confined in slit pores with opposing surface fields which have critical wetting transitions. When the nematiclike state has a linearly distorted director field, the capillary transition continuously transforms in a transition between two nematiclike states.     

Calorimetric study of the effects of 2,4-dichlorophenol on the thermotropic phase behavior of DPPC liposomes

The effect of 2,4-dichlorophenol (DCP) on the main transition and pretransition of fully hydrated (20 mass%) dipalmitoylphosphatidylcholine (DPPC) multilamellar liposomes has been studied by differential scanning calorimetry (DSC). It was observed that an increase in the molar ratio of DCP/DPPC (from 4·10^-5 up to 2·10^-2) causes progressive reductions in the temperature and enthalpy of the pretransition. The higher concentration of DCP eliminates the pretransition. The influence of DCP on the main transition in this molar ratio range is not drastic, but a decrease in temperature and in the enthalpy values was observed. In the molar ratio range (from 2·10^-1 up to 4·10^-1) the DSC scans show multiple main transition peaks instead of the characteristic single peak of the main transition. Above a DCP/DPPC molar ratio of 0.6 a new peak appears at 25°C having about the same transition enthalpy as the main transition of the pure system.This revised version was published online in November 2005 with corrections to the Cover Date.     

The Twist Elastic Constant and Anchoring Energy of the Nematic Liquid Crystal 4-N-Octyl-4-Cyanobiphenyl

The twist elastic constant of the nematic liquid crystal 4-n-octyl-4'-cyano-biphenyl (8CB) and the azimuthal anchoring energy at the SiO-nematic interface have been measured by using the torsion pendulum technique. The twist elastic constant of 8CB is found to be systematically larger than that measured by the Freedericksz transition technique. The azimuthal anchoring energy is found to decrease rapidly as the nematic-isotropic transition temperature is approached. This behaviour is analogous to that already reported by us for the nematogen 5CB and can be interpreted by extending the Berreman model of the anchoring energy at a grooved interface.     

Thermophysical Properties of Cholesteryl Oleyl Carbonate Determined with Microscopic Ftir/Dsc System

A microscopic Fourier transform infrared spectrometer (Micro FTIR) equipped for differential scanning calorimetry (DSC) was used to measure simultaneously the chemical structural variation and the thermal response of phase transition of cholesteryl oleyl carbonate (COC). The differential scanning calorimeter served also to determine the phase transitions of COC during heating or cooling. Two endothermic features due to phase transition were found in the thermogram: 18. 3 °C for a smectic-cholesteric transition and 37.5 °C for a cholesteric-isotropic transition during heating; 35.1 °C for an isotropic-cholesteric transition and 15.8 °C for a cholesteric-smectic transition during cooling. The breadth of the feature indicated sluggish phase transitions. The three-dimensional plot indicated that the intensities of lines due to the C-H stretching and scissoring deformation modes and C=O stretching mode of COC decreased suddenly near the temperature of phase transition during heating but the intensity of the line dues to the C-O stretching mode of carbonate ester of COC increased. Lattice vibrations or coupling of vibrational modes might be responsible for the result. These intensities in the cooling process varied inversely to those in heating process.     

LCST and UCST behavior of poly(N-isopropylacrylamide) in DMSO/water mixed solvents studied by IR and micro-Raman spectroscopy

Temperature-responsive phase separations of poly(N-isopropylacrylamide) (PNiPAm)/dimethylsulfoxide (DMSO)/water mixtures have been investigated by infrared and confocal micro-Raman spectroscopy. The ternary mixtures exhibited lower critical solution temperature (LCST) and upper critical solution temperature (UCST) phenomena at low and high DMSO concentrations, respectively. The amide I band of PNiPAm consists of two components; the intensity of the 1650 cm-1 component increased, and that of the 1625 cm-1 component decreased with increasing temperature during both LCST and UCST phase transitions. Gradual red shifts of the C-H stretching and the amide II bands with increasing temperature or increasing DMSO concentration indicate a removal of water molecules from the alkyl and N-H groups. Raman microscopic measurements showed that DMSO is excluded from the polymer-rich phases upon both LCST and UCST phase separation. On the basis of the experimental results and the quantum chemical calculations, a model that explains the solvation change of the polymer during phase transitions was proposed.