Water Dipole Orientation Research Articles

Multicomponent space‐charge transport model for ion‐exchange membranes

AbstractA multicomponent space‐charge transport model for ion‐exchange membranes was developed, where the membrane structure was modeled as an array of cylindrical pores with a uniform distribution of fixed‐charge sites on the pore walls. Ion/fixed‐charge site electrostatic interactions, electric‐field‐induced water dipole orientation, ion‐hydration free‐energy changes during ion partitioning, and concentration‐dependent transport parameters were considered in the analysis. The model predicted experimental concentration vs. time data accurately for Donnan dialysis separations with a DuPont Nafion 117 cation‐exchange membrane, where the membrane separated a dilute H2SO4 solution from an aqueous mixture of either Cs2SO4 + Li2SO4 or Cs2SO4 + Na2SO4. Both computer predictions and experimental measurements showed that the alkali metal cation with the larger hard‐sphere radius (lower surface charge density) was selectively absorbed in and transported across the membrane during a multicomponent separation. The cation/cation transport permselectivity was less than the selectivity for equilibrium uptake due to slow ion transport near the pore wall, where discrimination between like‐charge cations was greatest.

Molecular dynamics simulation of the water|nitrobenzene interface

The structure and dynamics of the neat water|nitrobenzene liquid|liquid interface are studied at 300 K using molecular dynamics computer simulations. The water is modeled using the flexible SPC potential, and the nitrobenzene is modeled using an empirically determined nitrobenzene potential energy function. Although nitrobenzene is a polar liquid with a large dielectric constant, the structure of the interface is similar to other water|non-polar organic liquid interfaces. Among the main structural features we describe are an enhancement of interfacial water hydrogen bonds, the specific orientation of water dipoles and nitrobenzene molecules, and a rough surface that is locally sharp. Surface roughness is also characterized dynamically. The dynamics of molecular reorientation are shown to be only mildly modified at the interface. The effect due to the polarizable many-body potential energy functions of both liquids is investigated and is found to affect only mildly the above results.

Molecular dynamics simulations of water within models of ion channels

The transbilayer pores formed by ion channel proteins contain extended columns of water molecules. The dynamic properties of such waters have been suggested to differ from those of water in its bulk state. Molecular dynamics simulations of ion channel models solvated within and at the mouths of their pores are used to investigate the dynamics and structure of intra-pore water. Three classes of channel model are investigated: a) parallel bundles of hydrophobic (Ala20) alpha-helices; b) eight-stranded hydrophobic (Ala10) antiparallel beta-barrels; and c) parallel bundles of amphipathic alpha-helices (namely, delta-toxin, alamethicin, and nicotinic acetylcholine receptor M2 helix). The self-diffusion coefficients of water molecules within the pores are reduced significantly relative to bulk water in all of the models. Water rotational reorientation rates are also reduced within the pores, particularly in those pores formed by alpha-helix bundles. In the narrowest pore (that of the Ala20 pentameric helix bundle) self-diffusion coefficients and reorientation rates of intra-pore waters are reduced by approximately an order of magnitude relative to bulk solvent. In Ala20 helix bundles the water dipoles orient antiparallel to the helix dipoles. Such dipole/dipole interaction between water and pore may explain how water-filled ion channels may be formed by hydrophobic helices. In the bundles of amphipathic helices the orientation of water dipoles is modulated by the presence of charged side chains. No preferential orientation of water dipoles relative to the pore axis is observed in the hydrophobic beta-barrel models.

Molecular Dynamics Simulation of Water between Two Charged Layers of Dipalmitoylphosphatidylserine

A molecular dynamics simulation of water between two charged layers of dipalmitoylphosphatidylserine in its liquid-crystalline state with atomic detail was carried out. From an analysis of a trajectory of 184 ps of length, we obtained information about the dynamics and structure of water between such charged layers. The most remarkable conclusions of this work indicate that the presence of counterions and charge interactions between adjacent phospholipids produce a screening of the electric field which reduces the strength of the lipid−water interactions. In this way, only minor differences of the behavior of the water compared to uncharged phospholipids were found. This conclusion was based on the calculation of the following properties: electric potential and water dipole orientation across the water layer, radial distribution function of water about lipid atoms, and diffusion and orientational relaxation time of water.

Crystal-face specificity of electrical double-layer parameters at metal/solution interfaces

The structure of the boundary region between a metal and a solution (the so-called electrical double layer) depends on the chemical nature of the metal and of the solvent, the type and amount of solute, and the structure of the metal surface. This article is focussed on the effect of varying the crystallographic orientation of the metal surface of an electrode on ‘classical’ double-layer parameters such as (i) potential of zero charge, (ii) its temperature coefficient, (iii) the interfacial permittivity (capacitance), (iv) and interfacial intermolecular interactions (adsorption). Similarities and differences between UHV experiments and related electrochemical situations are first scrutinized. The components of the electrode potential are analysed and their relevance to gas-phase experiments is discussed. Discrepancies between UHV and electrochemical data related to the same situation are explained in terms of temperature of experiment, lack of potential control and deviation of the structure of the surface solvent layer from bulk properties. A few experimental problems related to the preparation of single crystals, their use in an electrochemical cell, and the instrumental technique are then described, pointing to some ambiguous aspects. An interfacial parameter, ΔX, measuring the extent of the modifications occurring in the surface potential at the surface regions of the two phases as they are brought into contact, is derived from potential of zero charge–work function plots. For metal electrodes in aqueous solutions, ΔX is proportional to the metal–water interaction strength, although it is not only related to the surface water dipole orientation. ΔX is shown to correlate quite well with other double-layer parameters for different metals. The crystal-face specificity of the double-layer parameters is also correlated to ΔX, which thus turns out to be a unifying parameter for predictive analysis. The crystal-face specificity of capacitance and adsorption is discussed on the basis of several examples.

Studies on the electrical double layer structure at the water/air interface

Effective dipole moments (calculated from experimental data of surface tension and electric surface potential) of some homologous normal alcohols and carboxylic acid were found to vary linearly with the number of carbon atoms in the hydrocarbon chain. Values of effective dipole moments were used for the determination of the effective dipole moments of water molecules\((\bar \mu _{{\text{H}}_{\text{2}} {\text{O}}} )\), and the dielectric permittivity of the water subphase (e1), as well as in the vicinity of the hydrophobic part of adsorbed molecule (e2). The latter was found to decrease with the increase of the hydrocarbon chain length. Knowing the effective dipole moment of surface water dipoles, the average orientation angle (α) of water molecules at the inteface was estimated. The calculated potential drop of water varies within the range −0.038 to −2.38 V for two extreme orientations of water dipoles at the surface.

The study of specific adsorption of a polar solvent at solid electrodes by means of step elastic stretching under potentiostatic conditions

An approach is developed for the study of specific adsorption of polar solvents at solid electrodes based on the analysis of the dependence of stimulation by elastic deformation of the electrode specific increment of surface charge density Δ q A on potential E. Based on the analysis of (Δ q, E) curves, an assumption was made that, contrary to copper and cadmium, polycrystalline silver does not manifest a notable affinity to water, while cadmium displays a higher adsorptive activity as compared to copper. It was assumed that minimal values of specific increment of integral capacity Δ K stimulated by elastic deformation, in the region of small negative surface charges (σ) on the (Δ K, σ) curves of copper and cadmium are due to minimal orientation of water dipoles at the interface.

Formation of Blocking Contacts at p-Type Porphyrin/ITO or Electrolyte Junctions

Abstract Energetics of ITO/p-type porphyrin/electrolyte interfaces are discussed on the basis of photoelectrochemical and photovoltaic measurements. When sensitized by a layer of 5,10,15,20-tetraphenylporphyrinatomagnesium(II)(MgTPP), the indium-tin oxide (ITO) electrode acts as only a photocathode in an aqueous electrolyte solution. However, the ITO electrode modified with a film of ZnTPP, CuTPP or H2TPP having oxidation potential more positive than that of MgTPP acts as not only a photocathode but also a photoanode, depending on the electrode potential and the direction of irradiation on the ITO/porphyrin electrode. The cathodic component in the photocurrents is attributed to the downward band-bending in a space charge layer at the porphyrin/electrolyte (blocking) contact and the anodic component to that at the ITO/porphyrin (blocking) contact. The upward band-bending, on the contrary, is found inefficient in producing charge carriers, because the accumulation layer is conducting (ohmic) for majority carriers (holes). The porphyrin/electrolyte junction shows certain similarities to an ITO or metal/porphyrin (semiconductor) junction. However, it should be noted that a Helmholtz double layer exists at the porphyrin/electrolyte interface. The results are rationalized from the energy levels of the porphyrin films, ITO and the redox couple solutions with due regard to the orientation of water dipoles at the porphyrin surface.

Application of the immersion/emersion technique to the junction n-TiO 2/H 2O

The energy situation of the junction n-TiO 2/H 2O is reported, using information taken from the voltage pulses appearing upon contact of n-TiO 2 electrodes to electrolytic solutions. These pulses were recorded versus references potentials supplied from RHE and SCE. Furthermore, it was shown that upon emersion of the semiconducting electrode from the electrolyte, the double layer formed during the immersion procedure retains its structure. The results also reflect the importance of microscopic orientations of interfacial water dipoles on the energetics of the semiconductor/electrolyte junction.

Water adsorption on bromine-covered copper single crystal surfaces

The coadsorption of water with bromine on Cu(110) and Cu(111) has been studied using UPS, LEED, TDS and δφ. The results show that water is more strongly bound to the surface in the vicinity of the bromine than on the clean substrate. This behaviour is interpreted as surface hydration of the partially ionic bromine.Information about the dipole orientation of the solvation water is derived from the observed δφ-values. The LEED studies indicate that island formation with long-range order of the mixed bromine-water layer occurs at fairly low bromine coverages. The experimental results are qualitatively similar on the close-packed (111) and the more open (110) surfac. The relevance of these findings to the electrochemical phenomenon of specific anion adsorption on metal electrodes is briefly discussed.

Patchwise interfacial dipole layers on solids. Application to the silver iodide – aqueous solution interface

The hydration of solid particles in aqueous solution is treated as a patchwise phenomenon, in which the orientation of water dipoles varies in a patchwise fashion along the surface. The idea is applied to the AgI system where besides the neutral surface Ag+ patches and I− patches are distinguished. A first-order elaboration is offered, including an analysis of the relation between overall macroscopic quantities and local properties. The model offers an interpretation of two characteristics of double layers on silver iodide: the asymmetry of its p.z.c. and the conspicuous capacitance rise on the positive side and may serve as a starting point for a better description of the typical differences between the double layers on AgI and Hg.

Electrical Characteristics and Electret Behavior of Melanin

Conventional electrical measurements and the thermally stimulated depolarization current (TSDC) technique were exploited to investigate the electrical conductivity and the electret behavior of synthetic melanins. Our results can be summarized as follows: (i) Melanin contains deep traps at ≃1 eV and shallow traps giving rise to space charge‐limited currents; (ii) all the samples we examined show strong TSDC peaks at 370°–400°K without any applied polarizing field. These are caused by the release of trapped water molecules, the activation energy being about 1.45 eV; (iii) the orientation of water dipoles is partially responsible for the low temperature TSDC peak induced by applied electric field (activation energy ≃0.32 eV); (iv) the very complex structure of the TSDC plots can be attributed to an irregular distribution of charges and dipoles trapped in the heterogeneous macromolecules containing various amounts of the indole‐5,6 quinone precursors.

Studies on secondary electrocapillary effects: II. The electrocapillary phenomena in thin liquid film

The electrocapillary curves of thin liquid films of 1 N KCl aq on a mercury surface were obtained by direct measurements on sessile drops of mercury, which showed that the film pressure was dependent on the applied potential. This dependence can be ascribed not only to the change in double layer capacity, but also to the change in film thickness by the applied potential. The curves showed marked depression in interfacial tension of thin film near the electrocapillary maximum; this was the characteristic feature of thin films as compared with that of ordinary electrocapillary curves. This can be explained by the gas adsorption in the absence of double layer in the thin film. The interfacial entropy per square centimeter could be estimated from the slope of the interfacial tension vs temperature relations, while the interfacial entropy per mole could be estimated from the slope of the electrocapillary curve for thin films. It was also possible from these two values to divide the interfacial entropy change into two components, the one due to the formation of ionic double layer and the other mainly due to the orientation of water dipoles.

EPR of Ag° site conversion in gamma-irradiated frozen AgNO 3 ices

Gamma radiolysis of AgNO 3 ice at 77°K forms Ag° in 4 magnetically distinct sites due to different orientations of water dipoles. Site interconversion which occurs by rotation of water dipoles due to removal of the electric field of Ag + is studied by EPR.

Effects of Electrostatic Fields on the Surface Tension of Salt Solutions

The surface tension of air/ solution interfaces with applied electrostatic fields of up to 6700 v/cm was measured using a surface energy balance. The concentration was 0–3.5%. It was found that fields of either polarity lower the surface tension of all investigated solutions. Magnitude of the surface tension changes and dependence on direction of the field lead to the conclusion that the potential drop at air/water interfaces is smaller than previously thought, that the water dipole orientation is only slight, and that the hydrogen in the oriented water molecule is facing the air. The experiments point out that the effect of the field is predominantly on the sodium ion. It is shown that the difference in the energy for adsorption at the interface between cations and anions cannot be neglected. Calculations on the basis of a modified Gibbs equation on the assumption of a diffuse double layer at the interface leads to unreasonably high surface deficiencies. Neglecting the diffuse double layer and reckoning on the basis of an inner double layer only leads to values of the same order of magnitude as those calculated from the Gibbs equation alone.