Nematic-isotropic phase transition of binary liquid crystal mixtures

We experimentally studied the nematic-isotropic phase transition of (a) binary mixtures consisting of nematic and racemic liquid crystals and (b) binary mixtures consisting of positive and negative dielectric liquid crystals. We observed that the phase transition temperature is very sensitive to the chemical structures of the constituent components. We also used Maier-Saupe theory to calculate the transition temperature of binary mixtures. By fitting the experimental data, we obtained the interaction coupling constant between the constituent components.     

Polydisperse polymer liquid crystals near the anisotropic-isotropic transition

Polydisperse systems of polymer liquid crystal (PLC) macromolecules consisting of LC and flexible sequences are considered, particularly at the phase transition LC-i from an anisotropic to the isotropic phase. The Flory and Matheson lattice model is used for formulation of the partition function; the orientation distribution function for LC orienting potential interactions introduced by Flory and Ronca is included. Using a Landau approach to calculating the Helmholtz function of the system as a function of the orientation parameter s, we obtain the formulae for parameters of the model in the vicinity of the LC-i transition. The critical values at the transition of s, of the strength of orienting interactions, the system temperature, and the anisotropy of LC sequences are calculated and discussed as functions of the polydispersity parameter d. Regions of d are found where polydispersity has a weak influence on the LC-i equilibrium parameters. There is also a region of d values in which the influence of polydispersity is quite pronounced.     

Anomalous viscoelasticity near the isotropic-nematic phase transition in liquid crystals

Recent optical Kerr effect experiments have shown that orientational relaxation of nematogens shows a pronounced slow down of the response function at intermediate times and also a power law decay near the isotropic-nematic (I-N) transition. In many aspects, this behavior appears to be rather similar to the ones observed in the supercooled liquid near-glass transition. We have performed molecular dynamics simulations of model nematogens (Gay-Berne with aspect ratio 3) to explore the viscoelasticity near the I-N transition and also investigated the correlation of viscoelasticity (if any) with orientational relaxation. It is found that although the viscosity indeed undergoes a somewhat sharper than normal change near the I-N transition, it is not characterized by any divergence-like behavior (like the ones observed in the supercooled liquid). The rotational friction, on the other hand, shows a much sharper rise as the I-N transition is approached. Interestingly, the probability distribution of the amplitude of the three components of the stress tensor shows anisotropy near the I-N transition-similar anisotropy has also been seen in the deeply supercooled liquid. Frequency dependence of viscosity shows several unusual behaviors: (a) There is a weak, power law dependence on frequency [eta(')(omega) approximately omega(-alpha)] at low frequencies and (b) there is a rapid increase in the sharp peak observed in eta(')(omega) in the intermediate frequency on approach to the I-N transition density. These features can be explained from the stress-stress time correlation function. The angular velocity correlation function also exhibits a power law decay in time. The reason for this is discussed.     

The study of phase transition in some irradiated cholesteric liquid crystalline mixtures

We present the study of binary and multicomponent cholesteric mixtures undertaken with the aim of forming a system with the temperature of the phase transition close to the room temperature, which could be suitable for the detection of ionizing radiation. The phase diagrams were established on the basis of data from the optical microscopy and differential scanning calorimetry (DSC). The mixtures were exposed to the continual spectrum of X-Ray radiation in the period of 30/60 min. The mixtures react by changing the color of the mesophase, and a shift of the mesophase transition towards lower temperatures. The duration of the effects exceeds six months.     

Ionic liquid near a charged wall: structure and capacitance of electrical double layer

We study the effects of ion size asymmetry and short-range correlations on the electrical double layer in ionic liquids: we perform molecular dynamics simulations of a model ionic liquid between two "electrodes" and calculate the differential capacitance of each as a function of the electrode potential. The capacitance curve has an asymmetric "bell-shape" character, in qualitative agreement with recent experiments and the mean- field theory (MFT) which takes into account the limitation on the maximal local density of ions. The short-range ionic correlations, not included in the MFT, lead to an overscreening effect which changes radically the structure of the double layer at small and moderate charging. With the radius of cations taken to be twice as large as anions, the position of the main capacitance maximum is shifted positively from the potential of zero charge (PZC), as predicted by MFT. An extension of the theory (EMFT), however, reproduces the simulated capacitance curve almost quantitatively. Capacitance curves for real ionic liquids will be affected by nonspherical shape of ions and sophisticated pair potentials, varying from liquid to liquid. But understanding the capacitance behavior of such model system is a basis for rationalizing those more specific features.     

Mobility of water near charged interfaces

The location of the hydrodynamic shear surface is discussed for micelles of Na dodecyl sulfate and for clay particles (platelets of montmorillonite and of vermiculite). Micelles are characterized by a combination of experiments: light scattering, micellar self-diffusion, intrinsic viscosity, electrophoresis and electric conductance. The concerted interpretation of these experiments shows that the shear surface of micelles coincides within 0.1 nm with the surface enveloping the heads of the micellized ions. Claims of structured water in clays and an abnormally high viscosity of clay-held water have been based on the low self-diffusion of water in swollen clays, and on the temperature dependence of the hydraulic resistance of clay plugs (anomalous activation energy). It is shown that the self-diffusion of water between the platelets requires corrections for a wall effect and for the hydration of the exchangeable, slow moving cations. After application of such corrections, the viscosity of water in clay is found to be about the same as of bulk water, with the shear surface located at 0.1 ± 0.1 nm from the clay/water interface. The small anomaly in the activation energy of water in clay plugs is reasonably explained by a slight change with temperature of the pore size distribution in the plugs. Approximate calculations of the dielectric constant of water in electric double layers suggest some restriction in the orientation of water molecules in the first layer next to highly charged interfaces such as vermiculite/water. The various results all indicate that changes in the water mobility induced by a charged interface are small and do not reach beyond the first layer of water molecules.     

Behavior of mixtures of symmetric and asymmetric electrolytes near discretely charged planar surfaces: a Monte Carlo study

Canonical Monte Carlo (CMC) simulations are employed in this work in order to study the structure of the electrical double layer (EDL) near discretely charged planar surfaces in the presence of symmetric and asymmetric indifferent electrolytes within the framework of a primitive model. The effects of discreteness and strength of surface charge, charge asymmetry, and size asymmetry are specific focuses of this work. The CMC simulation protocol is initially tested against the classical theory, the modified Gouy-Chapman (GC) theory, in order to assess the reliability of the simulation results. The CMC simulation results and the predictions of the classical theory show good agreement for 1:1 electrolytes and low surface charge, at which conditions the GC theory is valid. Simulations with symmetric and asymmetric electrolytes and mixtures of the two demonstrate that size plays an important role in determining the species present in the EDL and how the surface charge is screened. A size-exclusion effect could be consistently detected. Although it is energetically favorable that higher-valence ions screen the surface charge, their larger size prevents them from getting close to the surface. Smaller ions with lower valences perform the screening of the charge, resulting in higher local concentrations of small ions close to the surface. The simulations also showed that the strength of the surface charge enhances the size-exclusion effect. This effect will definitely affect the magnitude of the forces between interacting charged surfaces.     

Relaxation processes in mixtures of liquid crystals and polymers near phase boundaries and during phase separation

We present experimental studies of the relaxation of concentration fluctuations in a semidilute solution of polystyrene (PS) (30% by weight) in 4-cyano-4'-n-octyl-biphenyl (8CB) (70% by weight) using the photon correlation spectroscopy (PCS). In the homogeneous phase there are two modes of relaxation. The slow one (typical time scale is taus = 0.001 s) is due to the diffusion of polymer chains (of molecular mass 65,000) in the LC matrix (of molecular mass 290), while the fast one has the time scale of the order of tauf approximately 0.00001 s. The amplitude of the fast mode is much weaker than the one for the slow mode. Moreover it does not depend on the scattering wave vector, q. The value of the diffusion coefficient, Dc = 1/(tausq2) for the slow mode decreases with temperature according to the Arhenius law until we reach the coexistence curve. Its value close to the coexistence is Dc = 4 x 10(5) nm2/s and the activation energy in the homogeneous mixture is Ec=127 kJ/mol. If we gradually undercool the mixture below the coexistence into the metastable two-phase region without inducing the phase separation we find unexpectedly that Dc does not change with temperature even 4 degrees below the coexistence curve. The characteristic time of the fast mode does not depend on the scattering wave vector indicating that it is related to the transient gel structure. We have shown that it is possible to measure the short time relaxation of concentration fluctuations during the phase separation in the mixture. At low temperature close to the isotropic-nematic phase transition we have observed that the relaxation is well separated in time from the typical time of the domain growth. This relaxation mode is characterized by the large diffusion coefficient D = 2 x 10(8) nm2/s. The mode probably comes from the coupling between the orientational dynamics of liquid crystals and the transient gel structure of polymers.     

Study of the isotropic to smectic-A phase transition in liquid crystal and acetone binary mixtures

The first-order transition from the isotropic (I) to smectic-A (Sm?A) phase in the liquid crystal 4-cyano-4(')-decylbiphenyl (10CB) doped with the polar solvent acetone (ace) has been studied as a function of solvent concentration by high-resolution ac-calorimetry. Heating and cooling scans were performed for miscible 10CB+ace samples having acetone mole fractions from x(ace)=0.05 (1 wt?%) to 0.36 (10%) over a wide temperature range from 310 to 327 K. Two distinct first-order phase transition features are observed in the mixture whereas there is only one transition (I-Sm?A) in the pure 10CB for that particular temperature range. Both calorimetric features reproduce on repeated heating and cooling scans and evolve with increasing x(ace) with the high-temperature feature relatively stable in temperature but reduced in size while the low-temperature feature shifts dramatically to lower temperature and exhibits increased dispersion. The coexistence region increases for the low-temperature feature but remains fairly constant for the high-temperature feature as a function of x(ace). Polarizing optical microscopy supports the identification of a smectic phase below the high-temperature heat capacity signature indicating that the low-temperature feature represents an injected smectic-smectic phase transition. These effects may be the consequence of screening the intermolecular potential of the liquid crystals by the solvent that stabilizes a weak smectic phase intermediate of the isotropic and pure smectic-A.     

Dynamics near a liquid-liquid phase transition in a non-tetrahedral liquid: the case of gallium

We use molecular simulation to analyze liquid dynamics in the vicinity of the liquid-liquid phase transition (LLPT) recently discovered in the modified embedded-atom model for elemental gallium. For this purpose we analyze the diffusive behavior in terms of the mean-squared displacement and self-intermediate scattering functions for two systems obtained by cooling the stable liquid through the LLPT at different cooling rates. The results show a pronounced heterogeneity of the dynamics upon the onset of the LLPT. Furthermore, it is found that this heterogeneity is closely correlated to the structural properties of the 9-fold coordinated high-density and 8-fold coordinated low-density liquid forms involved in the transition, showing a mixture of domains with very different diffusion time scales. The dynamics of the low-density liquid is found to be much more sluggish than that of the high-density form. Analysis of the energetics suggests that the origin of this difference is rooted in the fact that the cohesion in the former is significantly stronger than that in the latter.     

Heteroaggregation in binary mixtures of oppositely charged colloidal particles

Heteroaggregation (or heterocoagulation) rate constants have been measured in mixtures of well-characterized colloidal particles of opposite charge with multiangle static and dynamic light scattering. This technique permits routine measurements of absolute heteroaggregation rate constants, also in the presence of homoaggregation. Particularly with multiangle dynamic light scattering, one is able to estimate absolute heteroaggregation rate constants accurately in the fast aggregation regime for the first time. Heteroaggregation rate constants have also been measured over a wide range of parameters, for example, ionic strength and different surface charge densities. Amidine latex particles, sulfate latex particles, and silica particles have been used for these experiments, and they were well characterized with respect to their charging and homoaggregation behavior. It was shown that heteroaggregation rate constants of oppositely charged particles increase slowly with decreasing ionic strength, and provided the surface charge is sufficiently large, the rate constant is largely independent of the surface charge. These trends can be well described with DLVO theory without adjustable parameters.     

Influence of dye molecules on the birefringence of liquid crystal mixtures at near infrared frequencies

The optical properties of nematic liquid crystals have been extensively exploited in the production of devices working in the visible range of the spectrum. These same properties can be employed to make devices that function in the near infrared as required for telecommunications applications. However, it is generally observed that the birefringence of liquid crystal mixtures decreases with increasing wavelength, making it important to identify new materials, optimized for use in the near infrared region. One route to high birefringence is to operate close to an absorption band edge, which in the present context implies choosing highly conjugated materials which are potentially colored and, thus, not suited to traditional display applications. In this paper we explore the usefulness of dye molecules as birefringence enhancers in mixtures with conventional nematic liquid crystals. The optical properties, in particular, the absorption edge, polarizability, and birefringence, of families of known dyes are calculated at optical (589 nm) and infrared (1550 nm) wavelengths, using electronic density functional theory. We demonstrate the expected correlation between the proximity of the absorption edge and the magnitude of the birefringence, and estimate the birefringence enhancement occurring when each dye is incorporated in a guest-host system.     

Dynamics of the Goldstone mode near the point of polarization sign reversal in ferroelectric liquid crystal-aerosil mixtures

Dielectric spectroscopy was used to study the influence of random disorder in aerosil-ferroelectric liquid crystal dispersions on the dynamics of the Goldstone mode near the point of polarization sign reversal and the relaxation rate and dielectric strength of the collective modes. In general, the dielectric strength and the relaxation frequency of the Goldstone mode decrease, in comparison with the bulk, with increasing aerosil density near a point of polarization sign reversal. However, the characteristic frequency of the Goldstone mode varies with silica density in an opposite manner on each side of this point. This can be explained as a different sensitivity on the spatial confinement of different molecular conformers above and below the point of polarization sign reversal. The experimental results of temperature and frequency dependence of the complex dielectric constant are compared with predictions of the generalized Landau model for ferroelectric liquid crystals.     

Phase-separation of cellulose from ionic liquid upon cooling: preparation of microsized particles

Cellulose - Cellulose is an historical polymer, for which its processing possibilities have been limited by the absence of a melting point and insolubility in all non-derivatizing molecular...     

Wetting reversal transition in phase-separated polymer mixtures

We investigate a wetting reversal transition in thin films of two-phase mixtures of poly(ethylene-propylene) (PEP) and its deuterated analogue (dPEP) on a substrate covered by a self-assembled monolayer (SAM) whose surface energy, _γSAM, is tuned by varying the SAM composition. As _γSAM increases from 21 to 24 mJ/m², a transition from a three-layer (air/dPEP/PEP/dPEP/SAM) to a two-layer (air/dPEP/PEP/SAM) structure occurs at increasing T_c - T, where T_c and T are the critical and transition temperatures, respectively. As the system structure changes from three-layer to two-layer, the thicknesses of the dPEP-rich wetting layers at the air/mixture and mixture/SAM interfaces are found to smoothly increase and decrease, respectively, while the thickness of the PEP-rich layer (ca. one half of the total film thickness) does not change. The dependence of the transition temperature on _γSAM is predicted by a simple model using the experimental data on the surface energies of PEP/dPEP and estimates of the interfacial energy between PEP and dPEP.     

Laser light-scattering spectroscopy on thermally excited capillary waves at liquid-liquid interfaces in binary liquid mixtures near critical end-points

With the aim of testing Widom's recent predictions regarding the surface and interfacial tensions of binary fluid mixtures near critical end-points we have constructed a laser light-scattering spectrometer for studying thermally excited waves at the liquid-liquid interface. This instrument permits the determination of interfacial tensions at temperatures very much nearer the critical temperature T_C than is possible by conventional capillarity. We have measured the interfacial tension of nitroethane+3-methylpentane over a range of temperatures closer by one order of magnitude in ε = |T ? T_C|/T_C to T_C than previously reported; in the region of overlapping ε the two sets of results agree well. The critical index μ is found to be 1.26 ± 0.02 in good agreement with values for pure fluids but somewhat smaller than the value, 1.33 ± 0.03, previously reported for this mixture, possibly reflecting a temperature range dependence of μ.     

Binary liquid mixtures in porous solids

The technique of nuclear magnetic resonance cryoporometry has been used to study the behavior of binary liquid mixtures of water and decane in porous sol-gel silicas. It was observed that the water preferentially adsorbed onto the silica surface and so was able to displace the decane from the pores.     

Description of intra-diffusion in liquid mixtures

Employing a previously derived model to describe intra-diffusion coefficients in liquid mixtures based on molecular simulations of spherical Lennard–Jones particles [T. Merzliak, A. Pfennig, Mol. Simul. 30 (7) (2004) 459–468], an improved set of coefficients was obtained from optimized molecular dynamics simulations. In these simulations, the thermodynamic states were planned with the help of optimal experimental design, which allows to reduce the number of simulations necessary for significant determination of the coefficients by roughly a decade. The model was then applied to the real liquid mixtures toluene + cyclohexane, toluene + 1,4-dioxane, n-hexane + toluene, 1,4-dioxane + cyclohexane and cyclohexane + n-hexane, which have molecular properties that correspond to the model assumptions. Experimental intra-diffusion coefficients for the mixtures toluene + cyclohexane, toluene + 1,4-dioxane, n-hexane + toluene and 1,4-dioxane + cyclohexane were determined with nuclear magnetic resonance (NMR) techniques in this work. Even without additional parameters for the mixture the proposed model can describe the diffusion coefficients with an average accuracy of 5%. Allowing a deviation from Lorentz–Berthelot mixing rules leads generally only to slight improvement.     

Excess compressibility in binary liquid mixtures

Brillouin scattering experiments have been carried out on some mixtures of molecular liquids. From the measurement of the hypersonic velocities we have evaluated the adiabatic compressibility as a function of the volume fraction. We show how the quadratic form of the excess compressibility dependence on the solute volume fraction can be derived by simple statistical effects and does not imply any interaction among the components of the system other than excluded volume effects. This idea is supported by the comparison of the experimental results with a well-established prototype model, consisting of a binary mixture of hard spheres with a nonadditive interaction potential. This naive model turns out to be able to produce a very wide spectrum of structural and thermodynamic features depending on values of its parameters. An attempt has made to understand what kind of structural information can be gained through the analysis of the volume fraction dependence of the compressibility.