Changes In Ionic Conductivity Research Articles

EFFECT OF ADDITIVES ON THE CRYSTALLIZATION OF BARIUM SULPHATE

Abstract The kinetics of crystallization of barium sulphate for both seeded and unseeded systems in presence of additives have been studied by following the changes in ionic conductivity in supersaturation solutions containing stoichiometric concentrations of the lattice ions at 298 K. The additives investigated include: 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), hexa methylenediaminetetramethylenephosphonic acid (HMNTMP), ethylenediaminetetramethylenephosphonic acid (ENTMP) and nitrilotrimethylenephosphonic acid (NTMP). The effect of these additives on the growth kinetics has been studied at several inhibitor concentrations. The retarding effect of these inhibitors is interpreted in terms of adsorption of inhibitor ions at the active crystal growth sites following the Langmuir isotherm. The order, in terms of decreasing the inhibitory effect on the rate of growth of barium sulphate of the studied additives is HEDP > HMNTMP > ENTMP > NTMP. This order holds for both seeded and unseeded systems.

Presynaptic facilitation revisited: state and time dependence

The mechanisms underlying short-term presynaptic facilitation, the enhancement of transmitter release from sensory neurons in Aplysia, induced by serotonin (5-HT), can be divided into two categories: (1) changes in ionic conductances leading to spike broadening and enhancement of Ca2+ influx; and (2) actions on the machinery for transmitter release that are independent of spike broadening and the resulting increases in Ca2+ influx. Spike broadening and the associated enhancement of excitability are induced by the modulation of K+ conductances in the sensory neuron. The cellular mechanisms that contribute to the enhancement of release that is independent of spike broadening are not known and may involve vesicle mobilization or other steps in exocytotic release. These two facilitatory actions of 5-HT are mediated by at least two second-messenger-activated protein kinase systems, protein kinase A (PKA) and protein kinase C (PKC). These two second-messenger cascades overlap in their contributions to synaptic facilitation. However, their relative contributions to enhancement of transmitter release are not simply synergistic but are state- and time-dependent. The state dependence is a reflection of the synapse's previous history of activity. When the synapse is rested (and not depressed), a brief pulse of 5-HT (lasting from 10 sec to 5 min) produces its actions primarily through PKA via both spike broadening-dependent and -independent mechanisms. The broadening primarily involves the modulation of a voltage-dependent K+ current, IKV, with a small contribution by a voltage-independent K+ current, IKS. By contrast, the enhancement of excitability is mediated primarily by the modulation of IKS. As the synapse becomes depressed with repeated activity, the contribution of PKC becomes progressively more important. As is the case with PKA, PKC produces its action both by broadening the spike via modulation of IKV and by a spike broadening-independent mechanism. In addition to being state-dependent, the mechanisms of facilitation are time-dependent. There are differences in the response to 5-HT when it is given briefly to produce short-term facilitation or when the exposure is prolonged. When exposure is brief (< or = 5 min), PKA dominates. When exposure is prolonged (10-20 min), PKC becomes dominant as it is with depressed synapses. Thus, synaptic plasticity appears to be expressed in several overlapping time domains, and the transition between very short-term facilitation and various intermediate duration phases seems to involve interactive processes between the kinases.

Cellular responses to endogenous electrochemical gradients in morphological development

Endogenous electric fields give vectorial direction to morphological development in Zea mays (sweet corn) in response to gravity. Endogenous electrical fields are important because of their ability to influence: 1) intercellular organization and development through their effects on the membrane potential, 2) direct effects such as electrophoresis of membrane components, and 3) both intracellular and extracellular transport of charged compounds. Their primary influence is in providing a vectorial dimension to the progression of one physiological state to another. Gravity perception and transduction in the mesocotyl of vascular plants is a complex interplay of electrical and chemical gradients which ultimately provide the driving force for the resulting growth curvature called gravitropism. Among the earliest events in gravitropism are changes in impedance, voltage, and conductance between the vascular stele and the growth tissues, the cortex, in the mesocotyl of corn shoots. In response to gravistimulation: 1) a potential develops which is vectorial and of sufficient magnitude to be a driving force for transport between the vascular stele and cortex, 2) the ionic conductance changes within seconds showing altered transport between the tissues, and 3) the impedance shows a transient biphasic response which indicates that the mobility of charges is altered following gravistimulation and is possibly the triggering event for the cascade of actions which leads to growth curvature.

Rietveld X-ray Powder Profile Analysis and Electrical Conductivity of Fastion Conducting Gd2(Til-ySny)2O7 Solid Solutions

AbstractThe A3+2B4+2O7 pyrochlores have fast-ion conduction properties that make them attractive candidates for applications in fuel cells. Rietveld powder-profile analysis of x-ray diffraction data has been used to determine structural parameters for Gd2(Ti1-ySny)2O7 solid solutions (y = 0.20, 0.40, 0.60, and 0.80) for correlation with composition-dependence of the specimens' electrical conductivity. In accord with Vegard's law, the lattice constants increase linearly with y as the larger Sn ion replaces Ti4+. The structures in the system remain remarkably ordered in view of the fact that it had been suggested that increasing the average radius of the ions that occupy the B4+ site relative to A3+ drives the system toward a disordered defect-fluorite state. A small but significant increase in mixing of the occupancy of the cation sites was found with increasing y to a maximum of [GdB] = 0.05. We could detect no significant disorder in the anion array. The activation energy and pre-exponential term for oxygen ion conduction were found to increase linearly and exponentially with y, respectively, such that at a given temperature, the ionic conductivity changes by less than an order of magnitude across the system. The behavior is in marked contrast to other systems where the substitution of a larger cation in the B site increases conductivity by up to three orders of magnitude as a consequence of substantial disorder that is introduced in both the cation and anion arrays.

Fast-Ion Conducting Y2(ZryTi1-y)2O7 Pyrochlores: Neutron Rietveld Analysis of Disorder Induced by Zr Substitution

The structure of Y2(ZryTi1-y)2O7 solid solutions progressively changes, with increasing y, from an ordered pyrochlore structure A2B2O7□, space group Fd3m, to a deflect-fluorite structure (A, B)4 (O0.875□0.125)8, space group Fm3m, at y = 0.90. The anion array consists of three independent sites O(1), O(2), and O(3), occupying positions 48f, 8a and 8b, respectively, of which 8b is unoccupied in a fully ordered pyrochlore. Rietveld powder-profile analysis of data collected with 1.5453-Å thermal neutrons was used to determine the structural state of four samples with increasing Zr content (y = 0.30, 0.45, 0.60, and 0.90). Refinements that employed only the pyrochlore superstructure intensity data provided weighted profile residuals that ranged 8.06 to 8.67% compared with expected values of 7.13 to 7.87% derived from counting statistics. The onset of disorder at y = 0.30 is marked by filling of the vacant 8b site with oxygen ions displaced from the nearest-neighbor anion shell—i.e., O(1) in 48f. Only for y > 0.45 does O(2) participate in the disorder. Mixing of the occupancy of cation sites A and B begins only with under occupancy of the O(2) site. The eight-coordinated A site, position 16c, is occupied solely by Y for y ≤ 0.45 and is predominantly Y at y = 0.60. The substituted Zr4+ thus replaces Ti4+ in the six-coordinated B site for most of the solid-solution series, Complete mixing of all three cation species occurs abruptly over the narrow compositional range 0.60 < y ≤ 0.90. The disordered structural states cannot be described by a single-order parameter. The O(1) ion is displaced 0.46 Å toward the vacant O(3) site in the ordered pyrochlore at y = 0. The displacement relaxes to the ideal position of the fluorite structure with a quadratic dependence on y. Increase of the anisotropic temperature-factor coefficients with y indicates general softening of the structure with increased disorder. The array of O(1) ions forms a continuous path for migration of anions through the structure. The magnitude and composition dependence of the reported change of ionic conductivity with y may be satisfactorily explained by the variation of the product of charge-carrier concentration and vacancy concentration N(N - 1), where N is taken as the site occupancy of O(1).

Characteristics of a complex of ethylene oxide-phosphate copolymer with LiClO 4 as a polymer electrolyte

Methoxy phosphoryl chloride was reacted with polyethylene glycol of molecular weight of 300 to produce a linear ethylene oxide-phosphate copolymer. The polymer is characterized and the ionic conductivity of the complex with LiClO 4, measured by means of frequency response analyzer, is in the range of 4 × 10 −5 S/cm at room temperature when the mole ratio of the ethoxy group to the salt is 60:1. The polymer shows glass transition temperature of −61°C which jumps to −48°C when mixed with the salt by the mole ration of 60:1. A further increase in the weight of the salt shows little change in glass transition temperature. It is noted that the ionic conductivity changes little with an increase of temperature when the mole ratio of the polymer and the salts is 60:1.

Perturbation of Ion Channel Conductance Alters the Hypnotic Response to the α2-Adrenergic Agonist Dexmedetomidine in the Locus Coeruleus of the Rat

The alpha 2-adrenergic agonists are members of a novel class of hypnotic-anesthetic agents that selectively bind to alpha 2 adrenoceptors in the locus coeruleus (LC) to initiate their pharmacologic action. The postreceptor molecular mechanism of the hypnotic action of alpha 2-adrenergic agonists remains unknown. In this study we addressed the role of conductance through a variety of calcium and potassium ion channels in the hypnotic action of dexmedetomidine in the LC of the rat. Cannulas were inserted stereotactically into the LC of halothane-anesthetized rats (n = 318). After at least 48 h, rats were tested for loss of righting reflex in response to administration of the alpha 2-adrenergic agonist dexmedetomidine at a hypnotic (7.0 micrograms LC) or subhypnotic (3.5 micrograms LC) dose. To establish the mediating role of various species of calcium and potassium ion channels in the hypnotic response, rats were pretreated with the following drugs before the administration of dexmedetomidine LC:S(+)202791 (L-type calcium-channel activator), nifedipine and R(-)202791 (L-type calcium-channel blocker), SNX 111 (N-type calcium-channel blocker), SNX 230 (P-type calcium-channel blocker), quinine (calcium-activated and voltage-gated potassium-channel blocker), charybdotoxin (calcium-activated potassium-channel blocker), dendrotoxin (voltage-gated potassium-channel blocker), or glybenclamide (adenosine triphosphate-sensitive potassium-channel blocker). The drugs were used in doses not causing behavioral effects that could have confounded the interpretation of loss of righting reflex. SNX 230 and the dihydropyridines nifedipine and R(-)202791 produced loss of righting reflex in the presence of a subhypnotic dose of dexmedetomidine. The hypnotic-enhancing effects of the dihydropyridines could be blocked with S(+)202791, which also diminished loss of righting reflex in response to dexmedetomidine 7.0 micrograms LC. Quinine, dendrotoxin, and charybdotoxin each attenuated the hypnotic response to dexmedetomidine 7.0 micrograms LC. The hypnotic response to dexmedetomidine was not significantly altered by SNX 111 or glybenclamide. Inhibition of ion conductance through L- or P-type calcium channels and facilitation of conductance through voltage-gated or calcium-activated potassium channels may be involved in the mechanism of hypnotic action of alpha 2-adrenergic agonists. These changes in ion conductance were capable of producing membrane hyperpolarization and decreasing neuronal excitability. There was no evidence for the involvement of adenosine triphosphate-sensitive potassium channels or N-type calcium channels in the hypnotic response to dexmedetomidine.

Expression of phosphatidylserine-dependent antigens on the surface of differentiating human trophoblast

The syncytiotrophoblast of the human placenta arises from fusion of stem cells called cytotrophoblasts. The molecular mechanisms associated with cell fusion and syncytiation of cytotrophoblastic cells remain largely unknown. In the present study, we investigated the morphological and electrical properties of BeWo cells, a human choriocarcinoma-derived trophoblast cell model, with several features of the human cytotrophoblast. Cultured cells tended to cluster, but only fused into small, multinucleated syncytia in the presence of cAMP (72 h). The morphological features of both the actin and microtubular cytoskeletons indicated that within 72 h of constant exposure to cAMP, intracellular cortical actin cytoskeleton disappeared, which was the most prominent inducing factor of multi-nucleation. The presence of the cation channel protein, polycystin-2 (PC2), a TRP-type cation channel, associated with placental ion transport in term human syncytiotrophoblast, co-localised with acetylated tubulin in midbodies, but was found non-functional under any conditions. Different electrical phenotypes were observed among control BeWo cells, where only 26% (8 of 31 cells) displayed a voltage-dependent outwardly rectifying conductance. Most quiescent BeWo cells had, however, a low, slightly outwardly rectifying basal whole cell conductance. Acute exposure to intracellular cAMP (<15 min) increased the whole cell conductance by 122%, from 0.72 nS/cell to 1.60 nS/cell, and eliminated the voltage-regulated conductance. The encompassed evidence indicates that the early events in BeWo cell fusion and syncytiation occur by cAMP-associated changes in ionic conductance but not morphological changes associated to chronic exposure to the second messenger. This suggests a tight regulation, and important contribution of cation conductances in cytotrophoblastic cells prior to syncytiation.

Voltage-dependent ionic conductances in Chinese hamster ovary cells.

Chinese hamster ovary (CHO) cells are becoming a widely used biological material. A number of studies report membrane ion conductance changes after transfection of channels and receptors, but there are few data available on the properties of membrane ion conductances of CHO cells before transfection. In this work we studied voltage-dependent ionic conductances in cultures of CHO native (CHO-K1) cells. Three types of voltage-dependent ionic conductances were identified: 1) a K+ conductance showing sensitivity to Ca2+ and a unit conductance of approximately 210 pS in symmetrical 150 mM K+ outside-out patches (this conductance, which did not inactivate during a 160-ms pulse, was inhibited by 30 nM charybdotoxin but not by 30 mM extracellular tetraethylammonium); 2) a rapidly activating and inactivating tetrodotoxin (TTX)-sensitive inward current, peaking at about -10 to 0 mV (this current showed characteristics similar in many respects to Na+ current recorded in neurons); and 3) another voltage-dependent inward current, which had slow inactivation, was TTX insensitive but was blocked by Co2+ (current was also carried by Ba2+, peaked at approximately 0 to +10 mV, was identified as a Ca2+ conductance, and was inhibited by dihydropyridines but not by 10 microM omega-conotoxin). Cell-attached patch recordings of single Ca2+ channel currents demonstrated a unitary conductance of approximately 20 pS.

Phase Relations And Conductivity In ZrO2-Sc2O3-La2O3 System

ABSTRACTThe oxygen-ion conductor 0.88ZrO2-0.12Sc2O3 has a discontinuous change in ion conductivity at about 660°C. This change accompanies the structural transition from rhombohedral to cubic phase. Since the high temperature cubic phase shows large ion conductivity, it is of interest to examine whether or not the cubic phase stabilizes in the low temperature region by another dopant . By adding only 0.005 mole % La2O3, the cubic phase is stabilized below about 500 °C without any loss of conductivity compared with 0.88ZrO2-0.12Sc2O3. The ion conductivity of cubic stabilized ZrO2-Sc2O3-La2O3 system is around 1×10-1 S/cm at 800°C. Cubic phase stabilization using second dopant in a ZrO2-Sc2O3 system led to the finding of a fast oxygen-ion conductor in the ZrO2-Sc2O3-La2O3 system.

Uniaxial Stress Dependence of the Ionic Conductivity of Some Alkali Halides

AbstractA sensitive method to measure ionic conductivity changes of uniaxially stressed crystals is described. The stress effect in the extrinsic regime of the conductivity of NaCl and LiF single crystals is used to determine the elastic dipole tensor of a migrating cation vacancy. The elastic dipole tensor is transformed to the double force tensor which gives insight into the jump path the ion takes and into the elastic displacements in the immediate neighbourhood of the jumping ion.

Photoresponse in ionic conductivity for poly[(ω‐hydroxy)oligo(oxyethylene) methacrylate‐co‐butyl methacrylate] doped with spiropyran derivative and LiClO4

AbstractPhotoresponse in ionic conductivity has been realized in the solid polymer electrolytes, consisting of poly[(ω‐hydroxy) oligo(oxyethylene) methacrylate] [P(MEO)OH] or poly[(MEO)OH‐co‐butyl methacrylate] (PBMA), spiropyran derivative (SP) and LiClO4. SP showed reversible isomerization upon photoirradiation in these films. The ionic conductivity in these films decreased upon ultraviolet light (UV) irradiation and subsequently increased upon visible light (Vis) irradiation. The interaction between photoisomerized merocyanine form (PMC form) and the hydroxyl group on the polymer side chain was confirmed by an IR shift assignable to OH stretching vibration. From DSC thermograms, the change of ionic conductivity was revealed to be considerably affected by this interaction and attributed to reversible photoisomerization of SP. Reversibility of ionic conductivity was improved in copolymer P[(MEO)OH‐co‐BMA] in comparison with that in P(MEO)OH.

Effect of free radical scavengers on changes in ion conductance during exposure to therapeutic ultrasound.

Ultrasound has been used in physical therapy for > 4 decades. Recent studies indicate that non-thermal mechanisms such as cavitation are involved in the observed effects. Free radicals and other highly reactive compounds are known to form during sonochemical reactions associated with acoustic cavitation. Using frog skin as a biological model, the possibility that the increase in ionic conductance (Gt) upon exposure to therapeutic ultrasound is due to the effect of free radicals generated by sonochemical reactions, was investigated. It was found that the presence of cystamine, cysteamine and sodium ascorbate significantly reduced the increase in conductance caused by the exposure to 300 mW/cm2 (1 MHz CW) therapeutic ultrasound. The attenuation in the effects was dependent on the concentration of the radical scavengers/antioxidants used, the incubation time, and the intensity of ultrasound. The effects were also dependent on the lipid solubility of free radical scavengers/antioxidants. The time constant for the recovery process of Gt in the presence of free radical scavengers and antioxidants after exposure to ultrasound was found to be not significantly different from control. These results suggest that the increase in Gt due to ultrasound is induced by free radicals and other reactive species generated from acoustic cavitation. This study provides an indirect evidence to the contingent that free radicals are generated and act inside the cells. Furthermore, the radical scavengers and antioxidants used provide protection from oxidative damage without being involved in the recovery of Gt towards steady state values after sonication.

Reversible change in ionic conductivity for solid polymer electrolyte with spiropyran group in the polymer side chain upon photoirradiation

Reversible change in ionic conductivity for solid polymer electrolyte with spiropyran group in the polymer side chain upon photoirradiation

Structural phase transition and ion conductivity in 0.88ZrO 2−0.12Sc 2O 3

It is found that oxygen ion conductor 0.88ZrO 2−0.12Sc 2O 3 shows a discontinuous change in ion conductivity and a structural phase transition at around 650°C. The temperature dependence of ion conductivity shows a clear hysteresis at transition temperature. The crystal structure changes reversibly from rhombohedral to cubic at the transition temperature. The compound 0.88ZrO 2− 0.12Sc 2O 3 exhibits a first-order phase transition to a fast oxygen-ion conductor. The high temperature phase with fast ion conductivity is stabilized in the low temperature region by the addition of a dopant such as Gd 2O 3.

Entropy effects on conductivity of the blend-based and composite polymer solid electrolytes

The Meyer-Neldel rule is applied to describe the relation between the pre-exponential factors σ o and activation energies of various blend-based and composite polymeric electrolytes for which the temperature dependence of the conductivity satisfies the Arrhenius law. It is shown that for these electrolytes changes of ionic conductivity come from the variation of the migration entropy term. The correlations between the entropy of ion migration, phase structure and phase transitions are discussed. The physical meaning of the temperature T D calculated from the Meyer-Neldel equation is proposed.

The effect of salt concentration on the local atomic structure and conductivity of PEO-based NiBr 2 electrolytes

High molecular weight poly (ethylene oxide) (PEO) ( MW = 5 × 10 6) based NiBr 2 electrolytes of compositions, NiBr 2(PEO) n , n = 2, 4, 8, 16, 32, 100, where n is the number of oxygen per Ni(II) ion, were prepared by solution casting. An unusually high oxygen/metal ratio of 2 could be achieved with NiBr 2 salt. X-ray powder diffraction study and pair distribution function (PDF) analysis indicate that no crystalline PEO-salt complex phase exists until n≤16. At low salt concentration ( n> 16), the electrolyte consists of two components: amorphous PEO containing dissolved salt and crystalline PEO in which salt ions may exist as imperfections. The interchain correlation of PEO is virtually unaffected by dilute concentrations of salt. With increasing salt concentration, a new PEO-salt crystalline complex starts to form and the interchain correlation of PEO is drastically increased. These results are consistent with those obtained from differential scanning calorimetry (DSC) analysis and show a possible correlation between changes in local atomic structure and ionic conductivity.

Intracellular transduction mechanisms for the slow synaptic events.

Electrical activities of the postganglionic neurons in the superior cervical ganglia of rabbits are modulated in various ways following activation of the subtypes of muscarinic acetylcholine receptors. (i) M1 receptors mediate a slow depolarization consisting of at least three types of ionic conductance changes, and one of these is possibly mediated by cyclic GMP. (ii) M2 receptors mediate a slow hyperpolarization that seems to be generated by inositol triphosphate derived from phosphatidylinositol breakdown. (iii) M2 receptors also cause, through an activation of C kinase, a suppression of Ca entry during action potentials that results in a characteristic change in the action potentials and thereby modulates excitability of superior cervical ganglion neurons. Each subtype of muscarinic receptors thus regulates different pathways of intracellular transduction and modulates the electrical signaling of sympathetic neurons.

Photoinduced ionic conductivity in poly[(?-methoxy)oligo(oxyethylene)methacrylate] doped with triphenylmethane derivatives

The solid film of poly[(ω-methoxy)oligo(oxyethylene) methacrylate] [P(MEO)] doped with malachite green leuco hydroxide (MGLOH) or triphenylmethylalcohol (TPMOH) has been prepared as a new type of solid polymer electrolyte with controllable ionic conductivity upon photoirradiation. The ionic conductivity for P (MEO) film doped with 0.1 mol% MGLOH or 0.05 mol% TPMOH increased with UV irradiation. This suggests that the photogenerated ions act as conductive carrier in the matrix. In particular, for the TPMOH doped film, the effect of TPMOH content on the ionic conductivity change has been analyzed with the change of glass transition temperature (Tg) upon UV irradiation.

Merocyanin 540 fluorescent probe-induced changes in mechanical and electrical characteristics of lipid bilayers

The optical probe Merocyanin 540 (M-540) has been employed _ _ to monitor action membrane potentials [l]. In using the probe it is important to know how adsorbed probe molecules modify the characteristics of the respective biological objects. The effects of M-540 on electrical characteristics of bilayer lipid membranes were studied by Krasne [2]. She reported that (1) in contrast to other voltage-sensitive dyes, M-540 induced only very small changes in BLM conductance, (2) the dye caused large changes of ionic conductivity and kinetics mediated by ionophores; these changes were considerably larger for uncharged than for charged lipids, and larger for positively charged substances than for negatively charged ones. Estimates of the transmembrane potential to explain the sudden increase of ionic conductance, based on calculations of the electrophoretic potential in compliance with the Gouy-Chapman theory, yielded too small values to explain satisfactorily the effects of M-540 on the ionic conductivity of BLM. Based on M-540 optical spectra measurements [3-51 an interaction model was developed and tested for various membrane-adsorbed dye forms. The questions arise, how these interactions affect the physical characteristics of membranes and whether they can be employed to monitor membrane characteristics. In analyzing this approach, the effects were studied of M-540 adsorption on electrical and mechanical parameters of BLMs prepared from a mixture of egg