Ionic double layer, interface

An electric potential drop across the boundary between two dissimiliar phases as well as at their surfaces exposed to a neutral gas phase is the most characteristic feature of every interface and surface electrified due to ion separation and dipole orientation. This charge separation is usually described as an ionic double layer. 

The surface potential of a liquid solvent s, %, is defined as the difference in electrical potentials across the interface between this solvent and the gas phase, with the assumption that the outer potential of the solvent is zero. The potential arises from a preferred orientation of the solvent dipoles in the free surface zone. At the surface of the solution, the electric field responsible for the surface potential may arise from a preferred orientation of the solvent and solute dipoles, and from the ionic double layer. The potential as the difference in electrical potential across the interface between the phase and gas, is not measurable. However, the relative changes caused by the change in the solution s composition can be determined using the proper voltaic cells (see Sections XII-XV). 

Equations (25) to (29) concern the case of neutral adsorbates, where there is no ionic double layer to contribute to the surface potential. In the case of charged (i.e., ionic) adsorbates, the measured potential consists of two terms. The first term is due to dipoles oriented at the interface, which may be described by the above formulas, and the second term presents the potential of the ionic double layer at the interface from the aqueous... 

The structure of the double layer can be altered if there is interaction of concentration gradients, due to chemical reactions or diffusion processes, and the diffuse ionic double layer. These effects may be important in very fast reactions where relaxation techniques are used and high current densities flow through the interface. From the work of Levich, only in very dilute solutions and at electrode potentials far from the pzc are superposition of concentration gradients due to diffuse double layer and diffusion expected [25]. It has been found that, even at high current densities, no difficulties arise in the use of the equilibrium double layer conditions in the analysis of electrode kinetics, as will be discussed in Sect. 3.5. 

It is well known that solid-state LECs exhibit a significant response time since electroluminescence can only occur after the ionic double-layers have been built up at the electrode interfaces [79,82]. Since in this case only the PFg anion is mobile, the double-layers are formed by accumulation and depletion of PFg at the anode and cathode, respectively. The LEC device with 45 started to emit blue-green light at a bias of 5 V after several minutes. The electroluminescence spectrum, as shown in Fig. 36 (trace a), is very similar to the photoluminescence spectrum recorded for a spin-coated film on glass and of a solution of the complex. For comparison, the electroluminescence... 

Matijevic, E. and Pethica, B.A. (1958) The properties of ionized monolayers. Part 1. Sodium dodecyl sulphate at the air/water interface. Part 2. The thermodynamics of the ionic double layer of sodium dodecyl sulphate. Trans. Faraday Soc., 54, 1382-99. 

Blok, L. and de Bruyn, P. L., The ionic double layer at the ZnO/solution interface. I. The experimental point of zero charge. J. Coll. Interface Sci. (3), 518-526 (1970). 

At the pristine water-water vapour Interface spontaneous polarization of the water molecules takes place, leading to the fpotentIal. Prlstlnlty implies that there are no other ions or dissolved molecules apart from minute amounts of H and OH Ions, created by spontaneous dissociation of water molecules and which may give rise to a weak superimposed Ionic double layer. There Is no operational procedure to establish this f-potentlal but present-day consensus has It that the alr-slde Is negative, see sec. 3.9. At Issue Is now the formation of ionic double layers in addition to this when the solution contains simple electrolytes. The more dramatic changes caused by adsorbed or spread surfactants will not be addressed here. 

Blok, L. and de Bruyn, P.L., The ionic double layer at the ZnO/solution interface,... 

One can identify three physical phenomena which lead to the observed values of Ax- First, an ionic double layer can be established if the distance of closest approach for cations and anions to the interface is not the same. Second, if one of the components of the solution has a dipole moment, it may assume a preferred orientation at the interface, thereby giving rise to dipolar potential drop. Finally, the presence of the solute can change the orientation of water molecules at the interface from that present in the pure solvent. The fact that Ax is usually positive is evidence that the anion approaches the surface more closely than the cation. This is not difficult to understand given that anions are more weakly solvated than... 

The monolayers chosen included cephalins and lecithins as examples of molecules expected to have a high polarizability normal to the interface (9, 26). Long chain sulfate and quaternary ammonium ions were studied as examples of monolayers with diffuse ionic double layers. Other experiments were made with protein films, with a long chain /5-alanine, and with monolayers of equimolar mixtures of long chain sulfates and quaternary ammonium ions. The various results can be explained by the effects illustrated below for long chain sulfates and lecithin. 

A substantial amount of data already exists on reactions at room temperature in various solvent systems. Temperature-dependent data, however, are quite sparse, and there are virtually no data at sufficiently low temperatures to test certain quantum statistical mechanical aspects such as tunneling in reaction coordinate space. More reliable and extensive ionic double-layer data for various electrochemical interfaces are needed to facilitate the comparison of theoretical and experimental rate constants. 

First models of liquid-liquid interfaces in electrochemistry treated them as flat and sharp [289, 290]. The nonlinear capacitance due to two back-to-back ionic double layers at such an interface is given simply by [289]... 

The experimental methods available for determination of the surface concentration of adsorbed intermediates formed in charge-transfer processes have been reviewed recently by Gileadi and Conway. They have been divided into two groups (a) methods in which the charge needed to form or desorb the surface species is directly measured and b) methods in which the capacity at the interface (i.e., the sum of the ionic double-layer capacity and the adsorption pseudocapacity) is measured as a function of potential and integrated to obtain the charge. These methods will not be discussed further here. The experimental methods described below pertain to the determination of adsorption of uncharged species on solid electrodes. 

SFG intensity in the spectral region that is associated with an increase in the ordering of hydrogen-bonded water molecules.The increase in the SFG intensity was attributed to an increase in the ordering of water molecules caused by the formation of an ionic double layer at the interface. 

Fig. 8. Resonant SHG of NaJ and KJ. The high concentration increase in the resonant SHG intensity is due to iodide anions at the interface forming a dense ionic double layer with the cations. Due to the relatively small hyperpolarisability of iodide, it is not possible to determine whether the iodide anions are enhanced at the interface or whether the surfece mole fraction increases linearly with the bulk. (Reprinted with permission from Ref 76. CopyTight 2006 American Physical Society.)...

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