Photoluminescence of liquid-crystal azo derivatives in nanopores

Optics and Spectroscopy - The fluorescence of the nematic liquid crystal n-butyl-n′-methoxyazoxybenzene (BMAOB) in the form of a layer and in porous glasses with pores of different diameter...     

Dielectric exclusion of ions from membranes

Dielectric exclusion is caused by the interactions of ions with the bound electric charges induced by ions at interfaces between media of different dielectric constants. It is considered as one of mechanisms of nanofiltration. The transport properties of capillary model are expressed through ion distribution and diffusion coefficients. Due to local equilibrium the distribution coefficient is directly related to the excess solvation energy of ion. First, this energy is considered for single ions in single neutral pores in terms of pore size, ion charge, dielectric constants of solvent and membrane matrix and pore geometry. The dielectric exclusion from pores with closed geometry like circular cylinders is shown to be essentially stronger than that from pores with relatively open geometry like slits. Furthermore, the role of finite membrane porosity is analysed for the model of infinite slabs with alternating dielectric constants. The presence of other ions is accounted for within the scope of a mean-field approach, and the screening of dielectric exclusion is thus introduced and considered in some detail. A fixed electric charge is shown to cause additional screening. At the same time the dielectric exclusion makes the Donnan exclusion of ions stronger. Therefore the interaction between those two rejection mechanisms turns out to be non-trivial. Finally, the effect of solvent molecular structure is considered within the scope of non-local electrostatics. It is shown that the solvent non-locality typically results in somewhat stronger dielectric exclusion, however, its most important effect is slowing down the decline of dielectric exclusion with increasing bulk electrolyte concentration.     

Comparative Dielectric Investigations on Nematic Liquid Crystals with Aerosil

Dielectric measurements on a nematic room temperature mixture and the mixture with 20% aerosil were carried out. Addition of aerosil results in a small increase of the clearing temperature and in increasing relaxation frequency for the reorientation around the short molecular axis. The results are interpreted as orientation effect of the nematic sample at the fractal aerosil.     

Effect of electrooptical memory in suspensions of carbon nanotubes in liquid crystals

Electrooptical response and microstructure of dispersions of multiwall carbon nanotubes in N-(4-ethoxybenzilidene)-4-n-butyl aniline nematic liquid crystal (LC) are studied. Irreversible response on the applied electric field (electrooptical memory) was revealed in oriented layers of such suspensions. The essence of this effect consists in the fact that, after the switch-on and subsequent switch-off of the field, the optical transmittance of suspension layer placed between two crossed polarizers substantially increases compared to the initial value which is typical for homeotropic orientation of LC. The efficiency of electrooptical memory nonmonotonically depends on the concentration of nanotubes in suspension, c _CNT, reaching its maximum at c _CNT = 0.02–0.05 wt %. It is shown that the memory of suspensions is caused by the incomplete relaxation of LC molecules from planar to initial homeotropic state after the electric field switch-off. The model is proposed and substantiated, according to which the planar state of LC is stabilized by the network of nanotubes formed upon the disintegration of aggregates under the action of electrohydrodynamic flows. The disclosed memory effect is rather common; it is brought about in the suspensions of carbon nanotubes based on other LCs in which electrohydrodynamic instabilities are developed.     

Photoinduced orientational ordering in the series of methacrylic azopolymers

Polymethacrylates with side azo fragments containing various end substituents and spacers are synthesized and characterized. Spatial ordering of azo fragments in the series of the synthesized azo polymers under the action of polarized excitation light is studied by the methods of null ellipsometry and polarization spectroscopy. Two types of anisotropy development during photoirradiation are found. In homologs with strong acceptor substituents of azo fragments, biaxial orientational structure can be observed; as the radiation dose is increased, this structure is transformed into a uniaxial structure with a negative order parameter and with an axis parallel to the polarization vector of the excitation light. In this structure, the maximum degree of orientational order is achieved in polymers with high concentrations of azo fragments. In homologs with donor substituents of azo fragments, the initial stages of irradiation likewise lead to the development of biaxial orientation; later, this orientation is transformed into an isotropic distribution. The mode of anisotropy development is controlled by the lifetime of photoisomers of azo fragments.     

Fluorescence of the nematic liquid crystal 5CB in nanoporous glasses

The fluorescence spectra of the nematic liquid crystal n-pentyl-n′-cyanobiphenyl (5CB) in porous glasses with pores from 1 to 44 nm in diameter are investigated. A decrease in the pore diameter leads to suppression of some long-wavelength spectral components corresponding to H-type predimer and dimer pairs (the molecular sieve effect). The spectrum of 5CB in small pores (smaller than 4 nm in diameter) can be explained by the superposition of the monomer fluorescence and the fluorescence of J-type dimer pairs of 5CB molecules, as well as associates of 5CB molecules and surface groups on pore walls. Exposure of samples to UV light enhances the molecular interaction in associates, possibly, due to the formation of strong chemical bonds.     

Electrophoresis and stability of nano-colloids: History,theory and experimental examples

The paper contains an extended historical overview of research activities focused on determining interfacial potential and charge of dispersed particles from electrophoretic and coagulation dynamic measurements. Particular attention is paid to nano-suspensions for which application of Standard Electrokinetic Model (SEM) to analysis of experimental data encounters difficulties, especially, when the solutions contain more than two ions, the particle charge depends on the solution composition and zeta-potentials are high. Detailed statements of Standard Electrokinetic and DLVO Models are given in the forms that are capable of addressing electrophoresis and interaction of particles for arbitrary ratios of the particle to Debye radius, interfacial potentials and electrolyte compositions. The experimental part of the study consists of two groups of measurements conducted for Pt/C nano-suspensions, namely, the electrophoretic and coagulation dynamic studies, with various electrolyte compositions. The obtained experimental data are processed by using numerical algorithms based on the formulated models for obtaining interfacial potential and charge. While analyzing the dependencies of interfacial potential and charge on the electrolyte compositions, conclusions are made regarding the mechanisms of charge formation. It is established that the behavior of system stability is in a qualitative agreement with the results computed from the electrophoretic data. The verification of quantitative applicability of the employed models is conducted by calculating the Hamaker constant from experimental data. It is proposed how to explain the observed variations of predicted Hamaker constant and its unusually high value.     

Numerical modeling of one-dimensional problem of self-action of a wave in a stochastic nonlinear medium

Pacific Oceanology Institute, Far East Scientific Center, Academy of Sciences of the USSR. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 31, No. 1, pp. 53–60, January, 1988.     

Enhancement of fluorescence of porous silicon upon saturation by liquid crystal

The fluorescence of samples of porous silicon of various morphologies that are filled with a liquid crystal (LC), n-pentyl-n′-cyanobiphenyl (5CB), is studied. The fluorescence spectra of the sample, along with the long-wavelength band of porous silicon with a maximum in the range 627–667 nm, exhibit a short-wavelength band of 5CB with a maximum in the range 385–410 nm. The radiative relaxation times of porous silicon and 5CB lie in the micro- and nanosecond ranges, respectively. It is found that the filling of pores with 5CB enhances the fluorescence of porous silicon by two to three times. This enhancement is caused by non-radiative energy transfer from 5CB to the porous matrix as a result of efficient interactions between LC molecules and pore walls. Using IR spectroscopy, it is shown that the formation of hydrogen bonds between cyano groups of 5CB molecules and silanol groups of pore surface is the predominant type of these interactions. A transfer mechanism is suggested according to which excited associates of 5CB molecules transfer their energy via surface channels to excitons of porous silicon, enhancing its fluorescence.