Double Layer Impedance in Mixtures of Acetonitrile and Water

Double layer (DL) impedances were evaluated in mixed solutions of water and acetonitrile at various ratios in the polarized potential domain in order to find competitive orientation of the two solvent molecules on the platinum electrode. The DL capacitance at any ratio of the mixture shows the common power law of the ac‐frequency. The capacitance at molar fractions of water less than 0.2 increases linearly with the fraction, and reaches the value for aqueous solution. This variation indicates accumulation of water molecules on the electrode excluding acetonitrile molecules. It is modelled on the concept of the Langmuir‐type isotherm in which water and acetonitrile molecules have competitive interaction with the electrode. The experimental variations by the use of the isotherm yield the difference in the interaction energy, 6 kJ mol⁻¹. The accumulation of water in the DL is supported by the formation of the adsorbed layer by insoluble ferrocene.     

Electrophoresis of solid particles at large Peclet numbers

A theory of concentration polarization of a thin electrical double layer (DL) on a spherical particle is developed for the regime of large Peclet numbers which is realized in strong electric fields. In this regime, the concentration field arising outside DL is estimated under influence of diffusion and convection. According to the theory developed, polarization of DL at large Peclet numbers causes a change in the Stern potential, the formation of a dipole moment and the long-range potential. The diffuse layer deviates strongly from spherical symmetry and electroneutrality, and the screen of the surface charge is provided not only by the diffuse atmosphere but also by the charge induced in the convective-diffusion layer. The effect of electric field on the induced charge gives rise to the additional electroosmotic slip, that was called "secondary electroosmosis". Thus, a nonlinear additional term for the Smoluchowski formula of electrophoretic velocity is based on the changes of zeta-potential and on the secondary electroosmotic slip. The comparison of theory with experimental results revealed considerable fitting.     

Effect of an electric field applied during the injection procedure on the pretilt angle of a nematic liquid crystal

We examined the effect of an electric field applied during the injection procedure on the polar pretilt angle of a nematic liquid crystal (LC). The pretilt angle of the sample injected at 25°C gradually increased as the electric field was increased. On the other hand, the pretilt angle of a sample injected at 90°C (which is above the nematic-isotropic phase transition temperature of LC) showed a sudden increase in the presence of the electric field and also increased with a greater electric field. We think the alignment layer might be swollen with LC molecules, and the rotation of the immersed LC molecules by the electric field induces a deformation of the alignment layer. These results imply LC and the alignment layer were coupled, and their cooperation had an influence on determining the bulk pretilt angle.     

Electrokinetics at high ionic strength and hypothesis of the double layer with zero surface charge

A growing number of publications in the last two decades have suggested that the structure and other properties of the interfacial water layer can significantly affect the double layer (DL) because of changes in ion solvatation energy. Most interesting is the possibility that a double layer might in fact exist, even when there is no electric surface charge at all, solely because of the difference in cation and anion concentrations within this interfacial water layer. Dukhin, Derjaguin, and Yaroschuk suggested this possibility 20 years ago and developed a phenomenological theory. Recently, Mancui and Ruckenstein created more sophisticated microscopic model. In this article, we present our first experimental result regarding the verification of this "zero surface charge" DL model. The electroacoustic technique allows testing at high ionic strength (up to 2 M). As a first step, we confirm the surprising result of Johnson, Scales, and Healy regarding large zeta potential of alumina (8 +/- 1 mV) in 1 M KCl. As a second step, we suggest using nonionic surfactant Tween 80 for probing and modifying the structure of the interfacial layer at high ionic strength. The application of surfactant at moderate ionic strength (i.e., <0.1 mol/dm3), as might be expected, reduces the zeta potential simply by shifting the slipping plane. However, there is no influence of surfactant on the zeta potential observed at high ionic strength. It turns out that a high concentration of KCl simply eliminates surfactant adsorption. We develop a new technique for characterizing the adsorption of nonionic surfactant using an acoustic attenuation measurement. We hope that these methods in combination with a proper surfactant and electrolyte selection would allow us to gain more detailed information on the interface structure at high ionic strength.     

Molecular-dynamics simulation of the surface layer of a nonionic micelle

A spherical micelle structure has been studied for cationic (n-dodecyltrimethylammonium chloride) and nonionic (hexaethylene glycol mono-n-hexyl ether) surfactants in pure water and a sodium chloride solution. The molecular-dynamics has been used to simulate the self-assembly of aggregates from an initially homogeneous mixture of water and surfactant molecules and to gain insight into the structure of micelles and their surface layers. The radial distribution functions obtained for charged components have been employed to calculate the local electric potentials of the micelles and the contributions from the charges of water atoms, ions, and a surfactant to it. It has been shown that, similarly to previously studied ionic micelles, in nonionic surfactant micelles, the contributions from water molecules and polar groups (and ions in the case of the salt solution) to the electric potential are mutually compensated in the region of the electrical double layer. Therefore, the resultant electric potential of the surface layer rapidly tends to zero.     

Modeling the induction of lipid membrane electropermeabilization

Experiments show significant effects of an electric field on lipid membrane, leading to a pore formation when a high intensity field is applied. The phenomenon of electroporation is preceded by the induction and expansion of defects, responsible for the pre-pore excitation. We examine the mechanism of the induction of the field-driven defects by Monte Carlo simulations. The study is based on the improved Pink's model, which includes explicit interactions between the polar heads and energy of interactions between the heads and the field. No anomalous deformation of the molecules is considered. The study, provided for bilayer dipalmitoyl-phosphatidylcholine (DPPC) membrane in the gel (300 K) and fluid (330 K) phases, shows dependence of the membrane conformational and energetical state on the value of the electric field. We observe that the electric field affects the number of molecules in the gel and in the fluid states. In the layer at the negative potential, when the transmembrane voltage is above U(c) approximately 280 mV, lipid heads abruptly reorient and the number of local spots with fluid conformation increases. The other layer slightly tends to tighten its structure, producing additional mechanical stress between layers. Lipids showed complete insensitivity to the electric field within physiological limits, U<70 mV.     

Separation of Nucleic Acid Constituents by Column Liquid Chromatography

As a general rule the electrophoretic mobility of molecules and particles is solely governed by the potential at their surface of shear (ζ-potential) and does not depend on their size or shape. There are hawever two sets of exceptional conditions under which the size or shape of molecules and particles may influence their electrophoretic mobility. These are:

1) Conditions under which the ratio between the dimensions of the molecules or particles and the thickness of their diffuse ionic double layer is such that the distortion of the electric field by the particles is neither at its minimum nor at its maximum.

2) Conditions under which the molecules or particles are migrating under the influence of an electric field inside a porous medium, the size of the pores of which is such that they impede the larger molecules or particles more than the smaller ones.     

Molecular simulation of an aerosol OT reverse micelle: 2. Energy and kinetic characteristics

The effect of the structure of a reverse micelle on the energy and kinetic characteristics of system components is studied via the molecular dynamics method with the use of the coarse-grain model of a surfactant ion. Partial energies are calculated and the behavior of the local electric potential is determined. It is shown that, despite a high ion concentration, the electric potential in the surface layer of a micelle behaves itself as in the surface layer of pure water. Coefficients of translational diffusion of the components are calculated on the basis of the Einstein relation for the dependence of rms displacement on time. Autocorrelation functions of the angular velocity of eigenvectors of water molecules are obtained.     

Electro-disclinic effect in tilted smectic phases of banana-shaped liquid crystal materials

We give the first evidence that the director tilt angle can be reduced by electric fields in the tilted smectic phase of banana-shaped molecules. In these phases the value of polarization is determined by the molecular packing and no electro-clinic effect is expected. Our studies show that high electric fields eventually induce a meta-stable phase with zero director tilt. The tilted phase slowly recovers at low fields. We propose that the field-induced quenching of the layer fluctuations is responsible for the observed effects.     

Electro-disclinic effect in tilted smectic phases of banana-shaped liquid crystal materials

We give the first evidence that the director tilt angle can be reduced by electric fields in the tilted smectic phase of banana-shaped molecules. In these phases the value of polarization is determined by the molecular packing and no electro-clinic effect is expected. Our studies show that high electric fields eventually induce a meta-stable phase with zero director tilt. The tilted phase slowly recovers at low fields. We propose that the field-induced quenching of the layer fluctuations is responsible for the observed effects.     

Electrophoretic mobility of a charged spherical colloidal particle covered with an uncharged polymer layer

A general expression is derived for the electrophoretic mobility of a spherical charged colloidal particle covered with an uncharged polymer layer in an electrolyte solution in an applied electric field for the case where the particle zeta potential is low. It is assumed that electrolyte ions as well as water molecules can penetrate the polymer layer. Approximate analytic expressions for the electrophoretic mobility of particles carrying low zeta potentials are derived for the two extreme cases in which the particle radius is very large or very small.     

Active microeletronic chip devices which utilize controlled electrophoretic fields for multiplex DNA hybridization and other genomic applications

Microelectronic DNA chip devices that contain planar arrays of microelectrodes have been developed for multiplex DNA hybridization and a variety of genomic research and DNA diagnostic applications. These devices are able to produce almost any desired electric field configuration on their surface. This ability to produce well-defined electric fields allows charged molecules (DNA, RNA, proteins, enzymes, antibodies, nanobeads, and even micron scale semiconductor devices) to be electrophoretically transported to or from any microlocation on the planar surface of the device. Of key importance to the device function is the permeation layer which overcoats the microelectrodes. The permeation layer is generally a porous hydrogel material that allows water molecules and small ions (Na+, CI-, etc.) to freely contact the microelectrode surface, but impedes the transport of the larger analytes (oligonucleotides, DNA, RNA, proteins, etc.). The permeation layer prevents the destruction of DNA at the active microelectrode surface, ameliorates the adverse effects of electrolysis products on the sensitive hybridization reactions, and serves as a porous support structure for attaching DNA probes and other molecules to the array. In order to maintain rapid transport of DNA molecules, facilitate hybridization, and work within constrained current and voltage ranges, low conductance buffers and various electronic pulsing scenarios have also been developed. These active microelectronic array devices allow electrophoretic fields to be used to carry out accelerated DNA hybridization reactions and to improve selectivity for single nucleotide polymorphism (SNP), short tandem repeat (STR), and point mutation analysis.     

Improving breakdown strength and energy storage efficiency of poly(vinylidene fluoride-co-chlorotrifluoroethylene) and polyurea blend films by double layer structure design

All-organic composites are widely used in energy storage application due to the high breakdown strength performance, but the improvement of energy storage was limited by the relatively low dielectric constant. Therefore, to satisfy the high demands of dielectric materials, energy storage properties of polymer composites should be further enhanced. In this article, poly(vinylidene fluoride-co-chlorotrifluoroethylene) (P(VDF-CTFE)) and polyurea (PUA), which are known as high dielectric ferroelectric material and linearly high energy storage efficiency material respectively, are composited through double layer (DL) casting method for the first time. The properties of DL structured composite film is contrasted with solution blending structure especially in energy storage efficiency, and the results demonstrate that DL structure design can make great use of advantages of two materials and also can avoid the influence of phase separation between P(VDF-CTFE) and PUA efficiently. Moreover, high breakdown strength (6180 kV/cm) and high energy storage efficiency (77%) of DL composites can be realized simultaneously by incorporating PUA as an insulating layer, and the mechanism is discussed in detail. This work provides an effective route to improve the energy storage properties of polymer dielectric materials and shows great application potential.     

Molecular-dynamics simulation of the effect of ions on a liquid-liquid interface for a partially miscible mixture

Molecular-dynamics simulations were performed to model the effect of added salt ions on the liquid-liquid interface in a partially miscible system. Simulations of the interface between saturated phases of a model 1-hexanol+water system show a bilayer structure of 1-hexanol molecules at the interface with -OH heads of the first layer directed into the water phase and the opposite orientation for the second layer. The alignment of the polar -OH groups at the interface stabilizes a charge separation of sodium and chloride ions when salt is introduced into the aqueous phase, producing an electrical double layer. Chloride ions aggregate nearer the interface and sodium ions move toward the bulk water phase, consistent with the explanation that the -OH alignment presents a region of partial positive charges to which the hydrated chloride atoms are attracted. Ions near the interface were found to be less solvated than those in the bulk phase. An electric field was also applied to drive ions through the interface. Ions crossing the interface tended to shed water molecules as they entered the hexanol bilayer, leaving a trail of water molecules. Stabilization and facilitated transport of the ion by interactions with the second layer of hexanol molecules appeared to be an important step in the mechanism of sodium ion transport.     

Polarization interaction of disperse particles with not thin Debye atmosphere

The electric field of the equilibrium double layer acts only at the distance of the Debye length. Therefore, the distances at which the electrostatic interaction forces act between particles in the absence of external fields are limited by the same length. As shown previously, the external electric field results in the appearance of volume charges outside DL. As a result, the long-range acting electric forces appear. In addition, the forces become anisotropic.

The polarization forces are proportional to at least the square of the external field. Previously we had shown that quadratic forces occurred not only as a result of the interaction of the charges being linear to an external field, but as a result of the interaction between the charges being quadratic to an external field as well as the equilibrium charges. We have described these quadratic charges in the theory of the double layer nonlinear polarization without any restrictions on the double layer thickness. This theory was developed by the method of successive approximations on the extemal field and the electrokinetic ς-potential. These results are obtained in analytical form and agree with the Fixman and Jagannathan's 1983 numerical solutions.

Our calculations of the polarization interaction energy showed: (a) in suspension of polar nonconductive particles in the less polar medium the orientation of the duplet perpendicular to field is more advantageous than along the field; (b) the attraction of the charged polar particles is substituted by the repulsion with the particle ς-potential increase both for longitude duplet (ς=2.5 mV) and for transverse duplet (ς=1.5 mV); (c) with the polar medium conductivity growth the repulsion forces between particles of the longitude duplet are changed by the attraction forces.     

Influence of Embedded Protons on the Structure of Ice Microcrystals. Computer Simulation

Ice microcrystals of 40 molecules with embedded proton and placed in the external electric field were simulated by the Monte Carlo method in connection with the problem of the destruction of the ozone layer in the stratosphere. The proton field does not disturb the cluster crystalline state, whereas the electric field of crystal defects on the ice surface can affect essentially the microcrystal melting and destroy the structural transition. The melting of microcrystals is mainly reduced to the distortion of orientational molecular order.     

The influence of layer curvature on switching in ferroelectric B2 phases of liquid crystals having bent-shape molecules

In the B₂ phase of liquid crystalline compounds with bent-shape molecules ferroelectric switching can occur either by molecular rotation on the cone or by rotation of the molecule about its long axis (so-called chirality flipping), or by both mechanisms simultaneously. When the smectic layers of the B₂ phase are non-deformed and parallel the rotation of molecules under an external electric field occurs readily on the surface of the cone, while rotation around the long molecular axis is hindered by an energy barrier. Imposed deformation of smectic layers leads to interaction between local layer curvatures and molecular orientation, which results in the energy barrier hindering the molecular rotation by a cone. For appropriate constants describing this interaction the energy barrier can be so high that chirality flipping becomes the principal switching mode. An increase in the electric field can eliminate layer curvature, and therefore the energy barrier, so that switching with molecular rotation on the cone becomes possible. In the present contribution these mechanisms of switching are discussed and the influence of layer curvature on the switching mode is demonstrated.     

Interface between ionic and nonionic liquids: theoretical study

We use a Flory-Huggins type approach to calculate the structure and the surface tension coefficient of the boundary between ionic and nonionic liquids. The mixture of ionic and nonionic liquids is treated as a "three-component" system including anions, cations, and neutral molecules. We show that if the affinities of the cations and the anions to the neutral molecules are different, the interface comprises an electric double layer. The presence of this layer (uncompensated electric field) stabilizes the interface: the field inhibits the ions segregation at the interface and increases the surface tension. On the other hand, the short-range volume interactions promote the segregation and decrease the surface tension. Furthermore, the surface tension coefficient can be negative, if the difference of the affinities is high enough. It implies a possibility of microphase separation of the system.     

Synthesis of hexagonal ZnO clubs with opposite faces of unequal dimensions for the photoanode of dye-sensitized solar cells

Novel sub-micro sized hexagonal clubs of ZnO (HC-ZnO), which are coated as a scattering layer (SL) for the photoanode of a DSSC, are synthesized. X-ray diffraction (XRD) patterns of the ZnO clubs show clear peaks corresponding to wurtzite crystal phase of ZnO. Scanning electron microscopic (SEM) images show that each club has two opposite hexagonal faces (parts) of unequal dimensions. High resolution transmission electron microscopic (HR-TEM) image of a single ZnO club reveals that the ZnO is single crystalline and has wurtzite crystal structure; the image indicates a lattice spacing (d) of 0.26 nm; this is ascribed to the (002) planar spacing of the hexagonal ZnO. A solar-to-electricity conversion efficiency (η) of 3.36% is achieved for the cell with the double layer (DL) film, which is 16% higher than that of the cell with only transparent layer (TL) of commercial ZnO (2.89%) and far higher than that of the cell with SL (0.05%). The η of the cell with the DL (3.36%) could further be improved to 4.28% through the modification of the DL surface with TiO(x). Incident photo-to-current conversion efficiency (IPCE) curves, UV-vis absorption spectra, energy dispersive X-ray (EDX) spectra, and electrochemical impedance spectra (EIS) are also used to substantiate the results.     

Unusual electro-optical response of an oblique columnar phase formed by a bent-core mesogen

Mesophases formed by bent-core mesogens have attracted special attention because they can organise into fluid phases with polar order and supramolecular chirality. In this paper, a new five-ring bent-core mesogen is presented which forms a columnar mesophase. The structure is built up by layer fragments and possesses an oblique two-dimensional lattice with a layer group of the type p112/a. The columnar phase can be transformed into a ferroelectric SmCP phase (SmCP_F) by application of a sufficiently high electric field (25 V µm^-1). This field-induced transition was found to be reversible. The mechanism of the polar switching depends on the frequency of the applied electric field. The switching takes place in the usual way by a collective rotation of the molecules around the tilt cone. At very low frequencies (0.1 Hz and lower), the polar switching is based on a collective rotation of the molecules around their long axes. In the latter case, the switching is accompanied by an inversion of the layer chirality.