Calorimetric investigation of phase transitions in cholesteryl oleate

An adiabatic scanning calorimeter has been used to investigate the temperature dependence of the enthalpy and the heat capacity near phase transitions in the cholesteric liquid crystal cholesteryl oleate (CO). It is found that the blue phases in CO are thermodynamically stable and that the observed enthalpy differences between the phases are small.     

Calorimetric investigation of phase transitions in cholesteryl oleate

An adiabatic scanning calorimeter has been used to investigate the temperature dependence of the enthalpy and the heat capacity near phase transitions in the cholesteric liquid crystal cholesteryl oleate (CO). It is found that the blue phases in CO are thermodynamically stable and that the observed enthalpy differences between the phases are small.     

Reflectance properties of polymer-stabilised cholesteric liquid crystals cells with cholesteryl compounds of different functionality

In this paper, we describe the synthesis of cholesteryl compounds of different functionality. A series of polymer-stabilised cholesteric liquid crystal (PSCLC) cells are prepared by photo-polymerisation of a cholesteric liquid crystal (Ch-LC) mixture containing a non-reactive liquid crystal, a nematic diacrylate and the above cholesteryl compounds. The effect of cholesteryl compound functionality on the morphology and the reflectance properties of PSCLC cells is evaluated, as are parameters such as the polymerisation temperature. The results indicate that the higher functional cholesteryl compound is more effective for broadening the reflection band of the Ch-LC composites, which is speculated to be a result of the pitch differences in the local network environment. Scanning electron microscopy is used to examine the morphologies of the polymer network of the PSCLC cells. We have found that the morphologies of the polymer network are determined by the functionality of the cholesteryl compounds as well as polymerisation temperature, which further influence the reflectance properties of the composites.     

Force field studies of cholesterol and cholesteryl acetate crystals and cholesterol–cholesterol intermolecular interactions

To model the physical properties of sterols and related species, an all-atom Class II force field has been derived based on the recently reported CFF93 force field for hydrocarbons. It has been tested using both energy minimization and molecular dynamics (MD) simulations of the low-temperature neutron-diffraction structure of cholesteryl acetate crystals and the X-ray diffraction crystal structure of cholesterol. Thus these studies test the techniques and limitations of high-accuracy crystal simulations as well. Employing energy minimization, all cell vectors and volumes were reproduced to within 2.4% of experimental values. For cholesteryl acetate, the root mean square (rms) deviations between the calculated and experimental bond lengths, angles, and torsions of nonhydrogen atoms are 0.013 Å, 1.2°, and 2.4°, respectively. The corresponding maximum deviations are also very small: 0.027 Å for bond length, 3.2° for angle, and 7.6° for torsion. For cholesterol, good agreement between the calculated and experimental structures was found only when the comparison was limited to atoms with relatively small thermal factors (B_eq < 15 Ų). It was found that for both systems, the MD averaged structures were in better agreement with the experimental ones than the energy minimized structures, since the rms deviations in atom positions are smaller for the MD-averaged structures (0.064 Å for cholesteryl acetate and 0.152 Å for cholesterol) than those for the minimized structures (0.178 Å for cholesteryl acetate and 0.189 Å for cholesterol). The force field was then applied to isolated molecules focusing on the rigidity of the cholesteryl ring and cholesterol–cholesterol interaction energies. It is concluded that the cholesteryl ring is fairly rigid since no major conformational change was observed during an MD simulation of a single cholesterol molecule in vacuo at 500 K, in agreement with condensed phase experiments. Calculations of cholesterol–cholesterol pairs suggest that there are only four low-energy configurations and that it is more useful to describe each molecule as having a plane (flat face) and two grooves rather than as having two (one flat and one rough) faces. This provides some insight into the equilibrium crystal structures. Limited results from a modified Class I (CVFF) force field are presented for comparison. © 1995 by John Wiley & Sons, Inc.     

Activity coefficients of butanol isomers in liquid crystalline cholesteryl tridecylate according to data of reverse phase gas chromatography and tensiometry

The reverse phase gas chromatography technique was used to determine the activity coefficients of butanol isomers at infinite dilution in the smectic, cholesteric and isotropic phases of cholesteryl tridecylate. Experimental parameters with which the chromatographic process may be viewed as balanced and proceeding within the bulk of the liquid crystal were defined. Alcohol activity coefficients in binary systems with cholesteryl tridecylate determined by the gas chromatographic and tensiometric techniques were found to be in good agreement. A conclusion is reached on the suitability of the reversed-phase gas chromatographic technique for studying smectic and cholesteric liquid crystalline phases.     

Thermal bulk polymerization of cholesteryl acrylate

The thermal bulk polymerization of cholesteryl acrylate was carried out in the solid phase, the mesomorphic phase, and the liquid phase to study the effect of monomer ordering on polymerization rate and polymer properties. The rate increased with decreasing ordering (or enhanced mobility) of the monomer. Formation of inhibitive by-products during the polymerization limited conversions to 35%. The sedimentation constant S₀ = 6.2 S was the same for the polymers obtained in the three phases. The weight-average molecular weight (M?_w) was 480,000 as determined by ultracentrifugation. Poly-(cholesteryl acrylate) formed in bulk is randomly coiled when dissolved in tetrahydrofuran. The thermal properties of the monomer are given.     

Effect of Cholesteric Liquid Crystalline Compounds on the Radical Polymerization of Methyl Methacrylate

Polymerization of methyl methacrylate was carried out at 55 and 65°C using benzoyl peroxide as initiator in the presence of the fol-lowing additives: (a) cholesteric compounds like cholesterol, cho-lesteryl acetate, cholesteryl caprylate, cholesteryl stearate, choles-teryl chloride, cholesteryl laurate, cholesteryl oleyl carbonate, and cholesteryl 2-ethylhexyl carbonate, (b) a nematic liquid crystal, N-(p-methoxy benzylidene)-p-butyl aniline, and (c) rodlike molecules like 2-butynediol and diacetylene diol. The rates of polymerization, activation energies, molecular weights, and tacticities of the poly-mer are discussed in the light of monomer + additive interactions. It is found that monomer-additive complexes affect the polymerization rates.     

Synthesis and properties of non-symmetric liquid crystal dimers containing a cholesteryl moiety

A series of non-symmetric liquid crystal dimers having cholesteryl and 4-trans-(4-n-alkylcyclohexyl)phenoxy groups were synthesized by condensation of cholesteryl ω-bromoalkanoates with appropriate 4-trans-(4-n-alkylcyclohexyl)phenols. The structures and thermal phase behaviour of the dimers were characterized using IR, ¹H NMR and mass spectroscopy, elemental analysis, differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction measurements. Their thermal phase behaviour is significantly different to that of other cholesterol-based liquid crystal dimers. All of these liquid crystal dimers exhibit low phase transition temperatures. The relationships between their properties and chemical structures of these new dimers are discussed.     

Liquid-crystalline behaviours of novel chitosan derivates containing singular and cholesteryl groups

A series of esters of chitosan with cholesteryl hexanoate and cholesteryl decanoate side chains were synthesised. These compounds had higher solubilities than chitosan itself and all formed cholesteric lyotropic liquid crystalline phases. They had enhanced mesogenic properties as compared with the parent polymer. We consider that these compounds may prove to be of value as vehicles for drug delivery.     

Thermal properties of systems containing cholesteryl esters and triglycerides.

Binary and ternary systems of the three cholesteryl esters, linoleate, oleate, and stearate and the two triglycerides, triolein and tristearin were studied in order to determine the phase transitions and the conditions for the cholesteric and smectic mesophases. Phase transitions were determined using differential thermal analysis, melting point determination, and polarizing microscopy. Of the cholesterol esters the linoleate-oleate system showed complete miscibility in both the liquid and solid phases. The linoleate-stearate and oleate-stearate systems are of the eutectic type with limited solid solubility. The mesophases are monotropic as to the crystalline state and exist over the entire composition interval in all cholesteryl ester systems studied.     

Predicted stability and structure of (HXeCCH)n (n=2 or 4) clusters and of crystalline HXeCCH

Ab initio calculations predict the existence of the dimer and tetramer of HXeCCH. The interaction energies are -6.66 and -19.40 kcal mol-1 for the dimer and tetramer, respectively. For both complexes, larger blue shifts of the Xe-H stretching mode are found, while the Xe-C stretching modes are slightly redshifted. The stability and structure of HXeCCH crystals is predicted by density functional theory calculations with periodic boundary conditions. Strong electrostatic interactions are found between the monomers in the crystal. The results are first evidence for the existence of crystalline materials made of a novel class of noble gas molecules.     

High-performance liquid chromatographic determination of cholesteryl esters in the blood of obese children.

The serum of obese children and adolescents was analyzed for cholesteryl esters. The test substances were first separated from the sample matrix by solvent extraction and thin-layer chromatography and then resolved in a reversed-phase high-performance liquid chromatographic system involving a Separon SGX C18 column and a mobile phase of 2-propanol-acetonitrile (40:60, v/v), with ultraviolet detection at 206 nm. Cholesterol and 10-cholesteryl esters could be separated and determined within ca. 25 min at a flow-rate of 1 ml/min. The method was applied to a study of the effect of external conditions (physical stress, diet) on the content of cholesteryl esters in a test group of obese boys and girls aged from 13 to 16 years. The analyses have demonstrated that the above conditions do not affect the concentrations of the individual cholesteryl esters, although the total cholesterol concentration decreased significantly after spa treatment.     

Infrared analysis of phase transitions in liquid crystal acrylate monomers bearing urethanic groups and cholesteryl as mesogen

An infrared analysis of phase transitions in three enantiotropic liquid crystalline acrylate monomers with different spacer lengths bearing urethane groups and cholesteryl as mesogen was carried out. Through heating and cooling, spectral modifications especially at the level of carbonyl, –NH– and urethane alkoxyl oxygen groups were found. These findings reveal the influence of hydrogen bonding on mesomorphic properties of the studied compounds mentioning that both liquid crystal transitions are evidenced by the spectral changes. For all the compounds studied, the values of the transition temperatures deduced from the spectral modifications are in good agreement with those obtained from DSC measurements.     

Matrix isolation and ab initio study of the HXeCCH...CO2 complex

The HXeCCH...CO2 complex is studied experimentally and computationally. The complex is prepared in a low-temperature xenon matrix using UV photolysis of propiolic acid (HCCCOOH) and thermal mobilization of H atoms at 45 K. Photolysis of propiolic acid leads to the HCCH...CO2 complex as one of the photolysis products. The HCCH...CO2 complex is further photolyzed to the HCC...CO2 complex. Thermal annealing leads to the formation of HXeCCH complexed with CO2. The H-Xe stretching absorption of the HXeCCH...CO2 complex is blueshifted (+31.9 and +5.8 cm(-1)) from the value of the HXeCCH monomer in a xenon matrix. In the calculations, three HXeCCH...CO2 structures were found (one parallel and two linear structures) corresponding to the true energy minima on the potential energy surface. For the H-Xe stretching mode, the calculations give blueshifted values of +19.2 or +19.5 cm(-1) depending on the computational level [MP2/6-311++G(2d,2p) and MP2/aug-cc-pVDZ] for the parallel structure and +19.4 or +27.9 cm(-1) for one linear structure. For the second linear structure, the H-Xe stretching frequency is redshifted by -8.6 or -9.4 cm(-1) at these levels of theory. Based on the calculations, the experimental band shifted by +5.8 cm(-1) (1492.2 cm(-1)) most likely corresponds to the HXeCCH...CO2 parallel structure. The band with larger blueshift of +31.9 cm(-1) (1518.3 cm(-1)) can be due to another matrix site of the same structure or to the blueshifting linear structure.     

Vibrational spectroscopic studies and DFT calculations on tribromoacetate and tribromoacetic acid in aqueous solution

Aqueous solutions of sodium tribromoacetate (NaCBr3CO2) and its corresponding acid (CBr3COOH) have been studied using Raman and infrared spectroscopy. The spectra of the species in solution were assigned according to symmetry Cs. Characteristic bands of CBr3CO2-(aq) and the tribromoacetic acid, CBr3COOH(aq), are discussed. For the hydrated anion, the CO2 group, the symmetric CO2 stretching mode at 1332 cm(-1) and the asymmetric stretching mode at 1651 cm(-1) are characteristic while the CO mode at 1730 cm(-1) is characteristic for the spectra of the acid. The stretching mode, νC-C at 912cm(-1) for CBr3CO2-(aq) is 10 cm(-1) lower in the anion compared with that of the acid. These characteristic modes are compared to those in acetate, CH3CO2-(aq). Coupling of the modes are fairly extensive and therefore DFT calculations have been carried out in order to compare the measured spectra with the calculated ones. The geometrical parameters such as bond length and bond angles of the tribromoacetate, and tribromoacetic acid have been obtained and may be compared with the ones published for other acetates and their conjugated acids. CBr3COOH(aq) is a moderately strong acid and the pKa value derived from quantitative Raman measurements is equal to -0.23 at 23°C. The deuterated acid CBr3COOD in heavy water has been measured as well and the assignments were given.     

Effect of film thickness and supercooling on the performance of cholesteryl cinnamate liquid crystal capillary columns

Improved separations of the isomers of olefinic aliphatic insect pheromones were obtained on cholesteryl cinnamate glass capillary columns by operating in the supercooled temperature range of the liquid crystal. Capillary columns were prepared with varying film thickness of the stationary phase; choice of the correct film thickness ensured optimum retention for a wide range of compounds within the most effective temperature range of the liquid crystal. The deactivation procedures described made the liquid crystal columns suitable for separation of the geometric isomers of polar and nonpolar compounds.     

Synthesis and characterization of non-symmetrical liquid crystals containing a cholesteryl ester moiety

In this paper,a series of chiral non-symmetrical liquid crystals(nBA-chol)consisting of a cholesteryl ester moiety as chiral entity and a biphenyl aromatic core with different terminal alkyl chain has been synthesized and investigated for their liquid crystalline properties.Effects of numbers of methylene units in the terminal alkyl chain on the phase transition temperatures and on the temperature-dependent pitch lengths of the chiral liquid crystals have been studied.The long terminal alkyl chain tends ...     

The influence of substituents on the carbonyl stretching frequency; CNDO/2 force constant and localized orbital calculations

The influence of the substituents to the carbonyl group on the CO stretching frequency of organic carbonyl compounds was investigated by CNDO/2 calculation and analysis of the force constant and localized orbitals of the CO bond. The substituent-induced changes in the CO force constant could be split up in two additive contributions: an indirect, geometry-dependent influence and a direct, geometry-independent influence. From localized orbital calculations a relationship was found between the ionic characters of the carbonyl σ- and π-bonds and the CO bond length.     

Ferrocene-containing liquid crystals bearing a cholesteryl unit

The synthesis and characterization of new liquid crystalline compounds containing ferrocene azo or imino aromatic and cholesteryl units is reported. The purpose of this research was to combine the properties of metals (colour, electronic density, magnetism and polarizability) with those of chiral nematics (high birefringence and presence of physical colours) and correlate relationships between chemical structures and properties in the condensed state.     

First-principles study of CO adsorption and vibration on Au surfaces

A CO stretching frequency analysis is presented for the adsorption of CO on various Au(110) surfaces from density functional theory calculations. The structure sensitivity of the adsorption has been studied by considering the unreconstructed (1 x 1) surface, the missing-row reconstructed (1 x 2) surface, the vicinal stepped (20) surface, and the adsorption on adatoms deposited on the (110)-(1 x 2) surface. The calculated CO stretching frequencies are compared with infrared reflection-absorption spectroscopy (IRAS) measurements carried out at room temperature and pressure below 1 atm. The overall stability of the systems is discussed within the calculations of surface free energies at various coverages. At room temperature, the adsorption of CO on the ridge of the missing-row reconstructed surface competes in the high pressure regime with more complex adsorption structures where the molecule coadsorbs on the ridge and on adatoms located along the empty troughs of the reconstruction. This result is supported by the CO stretching frequency analysis.