Field-induced textures of polymer-dispersed chiral liquid crystal microdroplets

Abstract

We have microscopically observed the textures of very large droplets of cholesteric liquid crystal in a polymer matrix under the influence of an electric field E. When E = 0, the droplets exhibit rings and often a disclination line extends from the centre to the periphery of the droplet. As E increases, the droplet undergoes a progressive transition to a uniform-appearing texture. This uniform region first occurs near the centre of the droplet, then increases in radius as the field is increased. We propose that the field-off texture corresponds to the Frank-Pryce spherulite model while the uniform field-on texture is the planar texture.     

Field-induced textures of polymer-dispersed chiral liquid crystal microdroplets

We have microscopically observed the textures of very large droplets of cholesteric liquid crystal in a polymer matrix under the influence of an electric field E. When E = 0, the droplets exhibit rings and often a disclination line extends from the centre to the periphery of the droplet. As E increases, the droplet undergoes a progressive transition to a uniform-appearing texture. This uniform region first occurs near the centre of the droplet, then increases in radius as the field is increased. We propose that the field-off texture corresponds to the Frank-Pryce spherulite model while the uniform field-on texture is the planar texture.     

Microquantitative determination of hesperidin by pulse perturbation of the oscillatory reaction system

A simple and reliable kinetic method for the determination of hesperidin (Hesp) is developed. It is based on potentiometric monitoring of the concentration perturbations of the matrix reaction system which is in a stable non-equilibrium stationary state close to the bifurcation point. The Bray–Liebhafsky oscillatory reaction is used as the matrix system. The response of the matrix to perturbations by different concentrations of Hesp is followed by using a Pt electrode. A linear relationship between maximal potential shift and the logarithm of Hesp concentrations is obtained between 7.5 and 599.4 g mL^–1. The limit of detection is 0.65 g mL^–1. The described procedure has been successfully applied to the determination of Hesp from different sources (capsules, industrial and hand-squeezed orange juice, and white wine).     

Entropy production in a mesoscopic chemical reaction system with oscillatory and excitable dynamics

Stochastic thermodynamics of chemical reaction systems has recently gained much attention. In the present paper, we consider such an issue for a system with both oscillatory and excitable dynamics, using catalytic oxidation of carbon monoxide on the surface of platinum crystal as an example. Starting from the chemical Langevin equations, we are able to calculate the stochastic entropy production P along a random trajectory in the concentration state space. Particular attention is paid to the dependence of the time-averaged entropy production P on the system size N in a parameter region close to the deterministic Hopf bifurcation (HB). In the large system size (weak noise) limit, we find that P ～ N(β) with β = 0 or 1, when the system is below or above the HB, respectively. In the small system size (strong noise) limit, P always increases linearly with N regardless of the bifurcation parameter. More interestingly, P could even reach a maximum for some intermediate system size in a parameter region where the corresponding deterministic system shows steady state or small amplitude oscillation. The maximum value of P decreases as the system parameter approaches the so-called CANARD point where the maximum disappears. This phenomenon could be qualitatively understood by partitioning the total entropy production into the contributions of spikes and of small amplitude oscillations.     

Pulse perturbation technique for determination of piroxicam in pharmaceuticals using an oscillatory reaction system

A simple and reliable novel kinetic method for the determination of piroxicam (PX) was proposed and validated. For quantitative determination of PX, the Bray-Liebhafsky (BL) oscillatory reaction was used in a stable non-equilibrium stationary state close to the bifurcation point. Under the optimized reaction conditions (T = 55.0°C, [H₂SO₄]₀ = 7.60×10^?2 mol L^?1, [KIO₃]₀ = 5.90×10^?2 mol L^?1, [H₂O₂]₀ = 1.50×10^?1 mol L^?1 and j ₀ = 2.95×10^?2 min^?1), the linear relationship between maximal potential shift ΔE _m , and PX concentration was obtained in the concentration range 11.2–480.5 μg mL^?1 with a detection limit of 9.9 μg mL^?1. The method had a rather good sample throughput of 25 samples h^?1 with a precision RSD = 4.7% as well as recoveries RCV ≤ 104.4%. Applicability of the proposed method to the direct determination of piroxicam in different pharmaceutical formulations (tablets, ampoules and gel) was demonstrated.      

Temperature compensation in the oscillatory bray reaction

The influence of temperature on the oscillatory frequency of the hydrogen peroxide-iodate ion reaction is found to be two-sided: (i) the period length decreases with increasing temperature in most of the instances studied, (ii) or in some cases an opposite change is observed. A temperature-independent period length (temperature compensation) is also discovered experimentally in a rather wide temperature interval at a narrow concentration range of reactants both in a batch configuration and under flow conditions. A simple model was considered to simulate this behavior. Opposing effects of the composite reactions of the model on the calculated period length with changing temperature are shown to be responsible for temperature compensation or overcompensation.     

Chemical oscillatory reaction in a hydrodynamic oscillator

The periodic mixing of the reactants (bromate-thiocyanate or bromate-hydroxylamine) by the hydrodynamic oscillator gives rise to both periodic and aperiodic chemical oscillations.

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Limit cycle behaviour in the Belousov-Zhabotinskii oscillatory reaction

Oscillations in the bromate-Mn(II)-organic substrate system are studied by following the concentrations of Mn(III) and Br₂ spectrophotometrically, apart from monitoring the concentration of Br^- by potentiometry. Limit cycles for the Mn(III)-Br₂ combination are constructed experimentally and theoretically. The experimental results agree with the theoretical expectations based on a suggested mechanism.     

Investigation of oscillatory Briggs-Rausher reaction in a distributed medium

A study has been made of an oscillatory reaction in a chemical system consisting of iodate, hydrogen peroxide, manganese(II) sulfate, malonic acid, and sulfuric acid (Briggs-Rausher reaction) in unstirred solutions in the bulk and in a thin layer (distributed media). Autowave structures formed in the system at 20–25 °C are described for the first time, as well as changes in color of the solution as a result of local action of light.L. V. Pisarzhevskii Institute of Physical Chemistry, Ukrainian Academy of Sciences, Kiev. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 6, pp. 737–741, November–December, 1991. Original article submitted July 6, 1990.     

Morphological studies of a (self-assembling oil gelator/liquid crystal) composite system

The aggregation structure of a novel (self-assembling oil gelator/liquid crystal) composite was investigated using light scattering studies and morphological observations. The oil gelator forms a self-assembled-networks aggregate in an organic solvent with a low molecular weight liquid crystal (LC). It became apparent from Hv light scattering patterns and polarizing optical microscopy that two types of LC molecular alignments exist in the composite: a random orientation and a spherulite type one in a nematic gel state. Also, optical and atomic force microscopic observations revealed that fibrils which formed bundles in the fibre-like and spherulite-like aggregates, were formed in the composite. The alignment of the liquid crystal molecules was related to the aggregation structure of the self-assembling oil gelator in a liquid crystal gel state.     

Morphological studies of a (self-assembling oil gelator/liquid crystal) composite system

The aggregation structure of a novel (self-assembling oil gelator/liquid crystal) composite was investigated using light scattering studies and morphological observations. The oil gelator forms a self-assembled-networks aggregate in an organic solvent with a low molecular weight liquid crystal (LC). It became apparent from Hv light scattering patterns and polarizing optical microscopy that two types of LC molecular alignments exist in the composite: a random orientation and a spherulite type one in a nematic gel state. Also, optical and atomic force microscopic observations revealed that fibrils which formed bundles in the fibre-like and spherulite-like aggregates, were formed in the composite. The alignment of the liquid crystal molecules was related to the aggregation structure of the self-assembling oil gelator in a liquid crystal gel state.     

Kinetic determination of morphine by means of Bray-Liebhafsky oscillatory reaction system using analyte pulse perturbation technique

A novel kinetic method for micro-quantitative determinations of morphine (MH) is proposed and validated. The method is based on the potentiometric monitoring of the concentration perturbations of the oscillatory reaction system being in a stable non-equilibrium stationary state close to the bifurcation point between stable and oscillatory state. The response of the Bray-Liebhafsky (BL) oscillatory reaction as a matrix system, to the perturbations by different concentrations of morphine, is followed by a Pt-electrode. The proposed method relies on the linear relationship between maximal potential shift, ΔE_m, and the logarithm of the added morphine amounts in the range of 0.004-0.18 μmol. Under optimum conditions, the sensitivity of the proposed method (as the limit of detection) is 0.001 μmol and the method is featured by good precision (R.S.D. = 1.6%) as well as the excellent sample throughput (45 samples h⁻¹). In addition to standard solution analysis, this approach was successfully applied for quantitative determination of morphine in a typical pharmaceutical dosage form. Some aspects of the possible mechanism of morphine action on the BL oscillating chemical system are discussed in detail.     

Temperature (over)compensation in an oscillatory surface reaction

Biological rhythms are regulated by homeostatic mechanisms that assure that physiological clocks function reliably independent of temperature changes in the environment. Temperature compensation, the independence of the oscillatory period on temperature, is known to play a central role in many biological rhythms, but it is rather rare in chemical oscillators. We study the influence of temperature on the oscillatory dynamics during the catalytic oxidation of formic acid on a polycrystalline platinum electrode. The experiments are performed at five temperatures from 5 to 25 °C, and the oscillations are studied under galvanostatic control. Under oscillatory conditions, only non-Arrhenius behavior is observed. Overcompensation with temperature coefficient (q(10), defined as the ratio between the rate constants at temperature T + 10 °C and at T) < 1 is found in most cases, except that temperature compensation with q(10) ≈ 1 predominates at high applied currents. The behavior of the period and the amplitude result from a complex interplay between temperature and applied current or, equivalently, the distance from thermodynamic equilibrium. High, positive apparent activation energies were obtained under voltammetric, nonoscillatory conditions, which implies that the non-Arrhenius behavior observed under oscillatory conditions results from the interplay among reaction steps rather than from a weak temperature dependence of the individual steps.     

Periodic CO and CO2 evolution in the oscillatory Briggs-Rauscher reaction

While various reactions in the inorganic subset of the oscillatory Briggs-Rauscher (BR) reaction were clarified in the recent years, the organic subset of the present mechanisms contains only one process: the iodination of malonic acid. Further organic reactions can play a role, however, if malonic (MA) and iodomalonic (IMA) acids can be oxidized in the BR reaction. As CO2 and CO should be products if such oxidations can take place, the main aim of this work was to learn whether these gases are produced in a significant amount in a BR system. In our BR experiments, a rather intense evolution of both gases was observed with an oscillatory and a nonoscillatory component. With the initial conditions applied here, one from every 6 carbon atoms was oxidized either to CO2 or to CO in the course of the BR reaction. The amount of CO2 was about 4 times higher than that of CO. Experiments are in progress to disclose the reactions which generate the measured gases and their role in the mechanism of the BR reaction.     

Potential response in the uncatalyzed bromate oscillatory system

The potential response of the platinum indicator electrode in the uncatalyzed bromate oscillator (UBO) with three different substrates, namely gallic acid (GA), pyrogallol (PG) and veratraldehyde (VA) has been analyzed. The large amplitude oscillations obtained with a Pt electrode are due to the sequential response of the indicator electrode to the varying concentration ratios of the redox couples present in the reaction mixture.     

Calorimetry studies of a chemical oscillatory system

The effect of putrescine (PUT) on KSCN-H₂O₂-CuSO₄-NaOH oscillating system was investigated by calorimetric method. The oscillating reaction was monitored in a closed reactor with stirring, and the result showed that the oscillating period was linearly related with putrescine concentration and the numbers of oscillation decreased with increase in putrescine concentration. When [PUT]=2.83·10⁻⁴ M, no oscillation was observed. A possible mechanism is proposed that putrescine is a scavenger of the active-oxygen species. The result of numerical simulation by a simplified mechanism consisting of 18 kinetic steps and 16 variables is consistent with the experimental findings.     

Bifurcation of irregular oscillatory regimes in the Belousov-Zhabotinskii reaction in a closed reactor

We describe the bifurcation diagram for irregular oscillatory regimes in the Belousov-Zhabotinskii reaction in a closed reactor upon variation of the initial concentrations of ferroin and potassium bromate. We formulate rules for transition from one regime to another.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol, 28, No. 5–6, pp. 474–478, September–December, 1993.     

The morphology of the spherulitic surface in polyethylene

Studies of lamellar shapes and profiles in linear polyethylene have often been implemented by microscopic examination of appropriately etched random slices through spherulitic specimens. However, since a spherulite crystallized from a bulk specimen is a three-dimensional assembly of radially oriented lamellae with a twisting orientation (except for very high T_cs), it may be difficult to draw appropriate conclusions from a random slice. A technique has been developed that allows one to prepare spherulitic surfaces such that their distance relative to the geometric center can be well characterized. A computer-based graphical representation is also presented which suggests that the projection of lamellae on such surfaces is adequately represented by a radially oriented assembly of helicoidally twisted lamellae. Based on the agreement between the experimental results and the computer model, it is suggested that the C and S shapes previously observed by other researchers are not necessarily intrinsic to the lamellar profile but may arise due to geometrical effects as the lameliae project onto a surface at various angles. From these results it is also evident that in ringed spherulites lamellae undergo continuous twisting rather than successive misalignment of essentially untwisted segments.