Small angle X-ray scattering from lamellar phase for β-3,7-dimethyloctylglucoside/water system: comparison with β-n-alkylglucosides

Small angle X-ray scattering (SAXS) is measured for the lamellar phase in aqueous systems of 1-o-β-3,7-dimethyoctyl-D-glucopyranoside (β-Glc(Ger)), which has recently been prepared by us, 1-o-β-decyl-D-glucopyranoside (β-GlcC₁₀), and 1-o-β-octyl-D-glucopyranoside (β-GlcC₈). The repeat distance d obtained from the position of the diffraction peak does not follow the swelling law d = 2δ_hc/_hc, where δ_hc and _hc are the thickness and the volume fraction of the hydrophobic layer, respectively. This may result from the fact that δ_hc increases and, equivalently, the surface area per surfactant molecule (a_s) decreases with increasing concentration. So we calculate δ_hc and a_s from the observed d value at each concentration using the above swelling law. The half-thickness δ_hc increases in the order β-GlcC₈ < β-Glc(Ger) < β-GlcC₁₀ at a fixed concentration. On the other hand, the data on a_s for β-GlcC₁₀ and β-GlcC₈ lie on the same line and the data for β-Glc(Ger) lies above this line. These results suggest that the cross-sectional area of the geranyl chain is larger than that of the glucose headgroup. Existence of water filled defects in bilayer sheets is also discussed based on the SAXS pattern and the concentration dependence of d.     

Mercury-sulphur stretching frequencies, synthesis and x-ray crystal structure of catena-μ-(3-dimethylammonio-1-propanethiolato)dichloromercury(II)

The crystal structure of the title compound has been determined. Crystals of [HgC1₂{μ-S(CH₂)₃NH(CH₃)₂}] are monoclinic, space group P2₁/n, with a = 10.136(2), b = 6.519(1), c = 15.940(6) Å and β = 97.20(3)°. The structure consists of (---Hg---S---)_n helicoidal chains linked by hydrogen bonding, which give rise to chemically unconnected layers along the ( 02) planes. Each mercury is tetrahedrally coordinated to two terminal chlorine atoms and two bridging sulphur atoms. Assignments of ν_as(SHgS) and ν_s(SHgS) for this complex and its isostructural bromine analogue, and of ν_s(C1HgC1) and ν_s(BrHgBr) from IR and Raman spectroscopy are reported. Comparison of Hg---S frequencies with those reported for closely related compounds as well as correlation with Hg---S bond distances are made.     

High pressure studies of the static permittivity tensor components in the nematic phase of 6CB

The dielectric permittivity tensor components, ε_II and ε_⊥, in the nematic phase of 6CB (4-n-hexyl-4'-cyanobiphenyl) were measured in the pressure range 0.1-130 MPa and the temperature range 12-58°C. The dielectric anisotropy, Δε(p, V, T) = ε_II - ε_⊥, was analysed in isothermal, isobaric and isochoric conditions taking into account the pVT data and the well known Maier and Meier equation. On that basis the nematic order parameter S(p, V, T) was determined. This was used to calculate the parameter Γ relating the interaction potential with the volume (density). Its value Γ = 4.1 agrees very well with other estimates.     

Effect of molecular rotational hindrance on the spontaneous polarization in the S*C phase of a ferroelectric liquid crystal

The spontaneous polarization, P_s, and the tilt angle, θ, have been measured simultaneously in the S*_C phase of binary mixtures of two ferroelectric liquid crystals as a function of temperature and concentration. A molecular rotational model was used in order to calculate the temperature dependence of the P_s/θ ratio. A two parameter fit of the experimental data was carried out which showed good agreement. The molecular rotational potentials have been determined from the fit parameters and are presented as a function of concentration. It is shown that the chiral part of the potential depends very weakly on the composition in contrast to the non-chiral part which shows a strong negative non-linear deviation with a minimum corresponding to the concentration at which the smectic C* phase width is maximal.     

Photomechanically induced ferroelectricity in smectic liquid crystals

We report new photomechanical effects in the ferroelectric liquid crystal SCE13 doped with a photoisomerizing guest azo dye. Low concentrations of dye (∼5 per cent wt:wt) are shown to cause an isothermal, reversible disruption of smectic phases when the system is illuminated with low power density (∼ 1 mW cm^-2) UV light. In the case of a sample initially in the S*_c phase, this results in a fall in the magnitude of spontaneous electrical polarization (P_s) and changes in electro-optic switching characteristics. If the sample is illuminated in the S_A phase, the electroclinic switching decreases. In contrast to this, when systems containing higher concentrations of dye (≥ 10 per cent wt: wt) are UV illuminated in the S_A phase, a reversible, isothermal transition to a biphasic S*_c/isotropic state occurs. In this case, the P_s is seen to rise from zero in the S_A phase to a finite value(∼2 nC cm^-2) in the biphasic mixture and hysteresis occurs in the electro-optic switching. When these higher dye concentration mixtures are held initially in the S*_c phase and UV illuminated, a more complicated variation of P_s occurs with the sample again undergoing a transition to a biphasic S*_c/isotropic state. Possible mechanisms for the transition are discussed.     

Electroabsorption study of metal-to-ligand charge transfer in an organic complex of iridium (III)

The dipole moment and polarizability changes have been determined from electroabsorption (EA) spectroscopy of solid films of fac tris(2-(phenyl)pyridinato,N,C^2′)iridium (III) [Ir(ppy)₃]. The maximum changes in the dipole moment |Δμ|_S=(5.0±0.5) D/f (f is the local field correction factor: 1.3–1.7) accompany ground state to the lowest singlet, and |Δμ|_T=(1.7±0.5) D/f ground state to the lowest triplet metal-to-ligand charge transfer (MLCT) excited states formation, while the average polarizability change ų/f² follows from the fitting procedure throughout the visible absorption spectrum range. The experimental values of |Δμ| as well as energy positions of the MLCT states correlate with the literature results of time-dependent density functional theory.     

Calculation of the surface charge and potential and of the structural component of disjoining pressure in an interlayer in KCl solutions between glass samples

The dependence of the Stern potential, ψ₁, of glass samples on the distance between these, H, has been theoretically calculated, while taking into account the Stern isotherm and the electroneutrality equation. Comparison of the theoretical dependences ψ₁(C)_H→∞ with those previously experimentally obtained enables one to calculate the energy of adsorption of OH ions on glass and, further, the dependence ψ₁(H). It has been shown that for pH 4–6 and C_KCl = 10^-2-10^-5 mol/L, the value of ψ₁ practically does not depend on H. The result obtained was used to calculate theoretically the ionic-electrostatic forces and to compute (from the experimental values of the interaction forces) structural forces U_s(H). The dependence thus obtained, U_s(H), is of exponential character.     

Novel frequency dependence of dielectric biaxiality of surface stabilized ferroelectric liquid crystals

The frequency dependence of the dielectric biaxiality of surface stabilized ferroelectric liquid crystals (SSFLCs) was studied. The principal values of the dielectric tensor ε₁, ε₂ and ε₃ were measured by the MOM (molecular orientational model) method. Three dielectric permittivities were measured for each of two samples. These were the permittivity of the homeotropic cell and the permittivity of the planar homogeneous cell with and without the DC bias. Then the dielectric tensor components were calculated based on the molecular orientational models. We present the theory and experimental procedure of the MOM method. Measurements have been performed on Merck FLC compound SCE-8. The following novel dielectric behaviour was observed, as the DC bias voltage was increased the dielectric permittivity of the planar homogeneous cell decreased at the low frequencies (∼ 1 kHz) while increased at the high frequencies (10kHz ∼). The sign of the dielectric biaxiality ∂εε (= ε₂ - ε₁) inverted around 1 kHz, being negative at low frequencies and positive at high frequencies. The roles of the biaxiality on the dielectric behaviour of SSFLC cells are discussed.     

Effect of pH on the electrophoretic mobility of a particle with a charge-regulated membrane in a general electrolyte solution

The electrophoretic motion of an entity comprised of a rigid, uncharged core covered by a charge-regulated membrane which simulates a biological cell, in a general a:b electrolyte solution is analyzed. The membrane carries a fixed charge which arises from the dissociation of the acidic functional group HA. We show that the higher the concentration of cations in the bulk liquid phase, the lower the absolute Donnan potential, _D, and the lower the concentration of functional group, N₀, the lower the _D. Also, the higher the pH, the higher the absolute electrical potential, and the greater the N₀, the lower the pH. The absolute mobility of a cell, μ, increases with pH, but decreases with the increase in the friction coefficient of the membrane phase, γ. For a fixed total number of HA, if γ is large, μ/μ_s is less than unity, μ_s being the mobility of the corresponding rigid particle, and it decreases with the thickness of membrane d, and the inverse is true if γ is small. For a medium γ, the variation of μ/μ_s as a function of d has a local maximum, and depending upon d, it can be either greater or less than unity.     

A monolayer study on three binary mixed systems of dipalmitoyl phosphatidyl choline with cholesterol, cholestanol and stigmasterol

The monolayer behavior of three mixed systems of dipalmitoyl phosphatidyl choline (DPPC) with sterols; cholesterol (Ch), stigmasterol (Stig), and cholestanol (Chsta) formed at the interface of air/water (phosphate buffer solution at 7.4 with addition of NaCl) was investigated in terms of surface pressure (π) and molecular occupation surface area (A) relation. A series of π–A curves at every 0.1 mol fraction of each sterol for the three combinations of mixed systems were obtained at 25.0 °C.

On the basis of the π–A curves, the additivity rule in regard to A versus sterol mole fraction (X_st) was examined at discrete surface pressures such as 5, 10, 15, 20, 25, 30 mN m⁻¹, and then from the obtained A–X_st curves the partial molecular areas (PMA) were determined. The A–X_st relation exhibited a marked negative deviation from ideal mixing in the pressure range below 10 mN m⁻¹, i.e. in the expanded liquid film region (below the transition pressure of DPPC).

The PMA of Ch at π=5 mN m⁻¹, for example, was found to be conspicuously negative in the range of X_Ch=0–0.2 (about −0.4 nm² per molecule) and slightly positive (ca. 0.1 nm² per molecule) in the range X_Ch=0.2 to 0.4. Above X_Ch=0.5, Ch’s PMA was almost the same as the surface area of pure Ch, while DPPC’s PMA was reduced to 60% of that of the pure system.

Excess Gibbs energy (ΔG_(ex)) as a function of X_st was estimated at different pressures. Applying the regular solution theory to thermodynamic analysis of ΔG_(ex), the activity coefficients (f₁ and f₂) of DPPC and the respective sterols as well as the interaction parameter (I_p) in the mixed film phase were evaluated; the results showed a marked dependence on X_st.

Compressibility C_s and elasticity C_s⁻¹ were also examined. These physical parameters directly reflected the mechanical strength of formed monolayer film.

Phase diagrams plotting the collapse pressure (π_c) against X_st were constructed, and the π_c versus X_st curves were examined for the respective mixed systems in comparison with the simulated curves of ideal mixing based on the Joos equation.

Comparing the monolayer behavior of the three mixed systems, little remarkable difference was found in regard to various aspects. In common among the three combinations, the mole fraction dependence in monolayer properties was classified into three ranges: 0<X_st<0.2, 0.2<X_st<0.4 and 0.5<X_st<1. How the difference in the chemical structure of the sterols influenced the properties was examined in detail.     

Antiferroelectric liquid crystals with amide linking group positioned at chiral tail

A homologous series of chiral liquid crystal compounds, N-methyl-N-pentyl-(S)-2-(6-(4-(4-alkyloxyphenyl)benzoyloxy)-2-naphthyl)propionamide, with an amide linkage in a chiral tail was synthesized and their mesomorphic properties studied. All the materials possessed an antiferroelectric smectic C (SmC_A*) phase, which was confirmed by observations of microscopic texture, switching current behaviour and electro-optical responses. The spontaneous polarization, P _s, and apparent tilt angle, θ, were also measured. The maximum P _s values are in the range of 173-222 nC cm^-2, and the maximum θ values are in the range of 26-30°.     

Application of the spectroscopic modifications of Pippard relations to NaNO2 in the ferroelectric phase

This study examines a linear variation of the specific heat C_P with the frequency shifts 1/ν(∂ν/∂T) for the Brillouin frequencies of the L-mode [010], [001] and [100] in the ferroelectric phase of NaNO₂ according to our spectroscopically modified Pippard relation. We obtain this linear relationship for those modes studied and calculate dT_C/dP in the ferroelectric phase of NaNO₂. Our calculated values of dT_C/dP for the [001] and [100] modes are in good agreement with the values given in the literature.     

Rotational viscosity γ1 of nematic liquid crystals as a function of temperature, pressure and density

Measurements of the rotational viscosity γ₁ and the density are presented for a mixture of 4'-methoxybenzylidenebutylaniline (MBBA) and its ethoxy homologue EBBA and a mixture of cyclohexylphenylnitriles (ZLI 2413 from Merck AG) as a function of temperature and pressure. A new set-up for the measurement of densities under pressures of up to 3kbar is described. It is shown that the pressure dependence of the kinematic rotational viscosity γ₁/ρ and the temperature dependence of γ₁ under isobaric and isochoric conditions have common features with that of the shear viscosity of isotropic liquids. Furthermore, it is found that the curves γ₁ = f(1/T) for constant p and γ₁ = g(ρ) for constant T can be shifted one onto another by an appropriate shift of the scale of the independent variable.     

The influence of double bonds in the terminal chain of 2-ring compounds on the physical properties of S*c mixtures

We present the properties of S*_c mixtures containing new 2-ring 5-n-alkyl-2-(4-n-alkenyloxyphenyl)pyridines and pyrimidines with systematically varying positions and configurations of the double bond Trans configurations at odd positions (counting the number of atoms from the core including the oxygen and the first carbon atom of the double bond) suppress the S_A phase, increase the S*_c tilt angle, Θ, and the spontaneous polarization, P_s, and lead to long switching times τ. Cis configurations at even positions suppress the nematic phase in favour of smectic phases, decrease Θ and P_s, and shorten τ. Other positional configurational combinations strongly reduce the clearing point. Furthermore, our results indicate that the preferred conformation of the alkenyloxy chain consists of alternating cis and trans units.     

The system CePO4–K3PO4

The phase diagram of the system CePO₄–K₃PO₄ has been determined based on investigations by differential thermal analysis, X-ray powder diffraction, IR spectroscopy and optical microscopy. The system contains only one intermediate compound K₃Ce(PO₄)₂, which melts incongruently at (1500±20)°C. This compound is stable down to room temperature and exhibits a polymorphic transition at 1180°C. It was confirmed that the low-temperature form β-K₃Ce(PO₄)₂ crystallizes in a monoclinic system, space group P2₁/m with unit cell parameters a=9.579 (5), b=5.634 (6), c=7.468 (5) Å; =γ=90°, β=90.81 (3)°; V=403.083 Å³.     

Rotational viscosity of nematic liquid crystals and their shear viscosity under flow alignment

The influence of molecular properties on the rotational viscosity, γ₁, of nematic liquid crystals is studied. The shear viscosity under flow alignment, η_s, is determined for the same liquid crystals. A significant correlation between both quantities is found. An equation is presented which allows the calculation of γ₁ from η_s with an error of about 20 per cent for the liquid crystals studied.     

Dielectric studies of the hexatic B-smectic A and crystal B-smectic A transitions

Static dielectric constants, ε_∥ and ε_⊥ as well as the low frequency dielectric dispersion of ε_∥ have been studied in the vicinity of the hexatic B-smectic A transition in n-hexyl-4'-pentyloxybiphenyl-4-carboxylate (65OBC) and n-butyl-4'-hexyloxybiphenyl-4-carboxylate (46OBC) and also near the crystal B-smectic A transition in n-(4-n-butyloxybenzylidene)-4-n-octylaniline (4O. 8). While the behaviour of the dielectric anisotropy is essentially similar for both transitions, the dispersion results show significant differences.     

Helix twist inversion in ferroelectric liquid crystals with one chiral centre

The helix twist inversion in the chiral smectic C phase was studied for two new homologous series of lactic acid derivatives. The inversion phenomenon was attributed to a competition between spatial conformers leading to opposite helix handedness. The temperature of inversion was found to increase with increasing length of the chiral molecular tail. With all substances studied, high values of the spontaneous polarization P_s were found which did not reach saturation on cooling, in spite of a saturated tilt angle θ_s.     

Collective dynamic modes in ferroelectric liquid crystal-aerosil dispersions

The influence of the concentration of hydrophilic aerosil particles on the collective dynamic modes of the ferroelectric liquid crystal S-(-)-2-methylbutyl 4-n-nonanoyloxybiphenyl-4'-carboxylate near the SmA-SmC* phase transition is investigated by means of dielectric spectroscopy in the frequency range 10^-2-10⁷ Hz. For aerosil densities ρ_s = 0.025, 0.05, 0.08 and 0.15 g cm^-3 considerable changes in the dielectric intensities of Goldstone and soft modes are observed. The characteristic frequency of the Goldstone mode slightly increases with increasing concentration of aerosil. The frequency degeneracy occurring in the SmA-SmC* phase transition is lifted in the presence of aerosil and an increase in the frequency gap is observed. Complete disappearance of the Goldstone mode at ρ_s = 0.20 g cm^-3 occurs, along with significant broadening of the soft mode. The results are interpreted as an effect of structure, surface interactions and length scale of the helix in disordered confinement.     

Induced smectic C* phases

Induced ferroelectric S*_C phases are formed by non-chiral S_C host phases doped with chiral dipolar guest molecules. In those mixtures the spontaneous polarization P_s and the tilt angle Θ has been investigated as a function of the mole fraction x_G of the chiral dopant. In most cases the reduced polarization P₀ = P_S/ sin Θ has been found to depend linearly on x_G. The polarization power which is defined by δ_P=(∂P₀/∂x_G)ΔT is discussed in terms of the molecular structure of the chiral dopants. There are systems in which P₀(x_G) deviates positively from linearity. This behaviour can be understood by considering a local field correction to P₀. By assuming a local field of Lorentz type a theoretical relation for P₀(x_G) has been derived which explains the experimental results. The effect of a local field is considerable if the transverse dipole moment and the polarizability of the chiral dopant are large.