Contact Adsorption and Electric Double Layer

In comparing Eqn. 5-39 with Eqn. 5-9 (4 h = gia.+ M.d - s,dip), which is based on the classical double layer model, it appears that the sum of the first, second and third terms on the right hand side of Eqn. 5-39 corresponds to the sum of gbm due to the interfadal charge and gM,dii due to the interfacial dipole on the metal side and the fourth term corresponds to gs.dip due to the interfadal dipole of adsorbed water molecules on the solution side. These equivalences give Eqns. 6-40 and 5-41  

These equations amplify the meaning of the classical model in terms of the improved jellium-sphere model. 

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In some cases, e.g., the Hg/NaF q interface, Q is charge dependent but concentration independent. Then it is said that there is no specific ionic adsorption. In order to interpret the charge dependence of Q a standard explanation consists in assuming that Q is related to the existence of a solvent monolayer in contact with the wall [16]. From a theoretical point of view this monolayer is postulated as a subsystem coupled with the metal and the solution via electrostatic and non-electrostatic interactions. The specific shape of Q versus a results from the competition between these interactions and the interactions between solvent molecules in the mono-layer. This description of the electrical double layer has been revisited by... 

The electrical double layer has been studied at the interface of acidified (pH = 3) KCIO4 and K2SO4 solutions in contact with an Sn solid drop electrode with an additionally remelted surface (SnDER).616 The E, is independent of ctl as well as of the electrolyte. Weak specific adsorption of CIO4 at SnDER is probable around <7 = 0. This view is supported by the high value of/pz for SnDER/H20 + KCIO4 (fpz = 1 -27). A value of fpz = 0.99 for SnDER/H20 + K2S04 indicates that the surface of SnDER is geometrically smooth and free from components of pseudo-capacitance.616... 

At present it is impossible to formulate an exact theory of the structure of the electrical double layer, even in the simple case where no specific adsorption occurs. This is partly because of the lack of experimental data (e.g. on the permittivity in electric fields of up to 109 V m"1) and partly because even the largest computers are incapable of carrying out such a task. The analysis of a system where an electrically charged metal in which the positions of the ions in the lattice are known (the situation is more complicated with liquid metals) is in contact with an electrolyte solution should include the effect of the electrical field on the permittivity of the solvent, its structure and electrolyte ion concentrations in the vicinity of the interface, and, at the same time, the effect of varying ion concentrations on the structure and the permittivity of the solvent. Because of the unsolved difficulties in the solution of this problem, simplifying models must be employed the electrical double layer is divided into three regions that interact only electrostatically, i.e. the electrode itself, the compact layer and the diffuse layer. 

Fig. 6-53. Interfadal charges, electron levels and electrostatic potential profile across an electric double layer with contact adsorption of dehydrated ions on semiconductor electrodes ogc = space charge o = charge of surface states = ionic charge due to contact adsorption dsc = thickness of space charge layer da = thickness of compact la3rer.

When particles or large molecules make contact with water or an aqueous solution, the polarity of the solvent promotes the formation of an electrically charged interface. The accumulation of charge can result from at least three mechanisms (a) ionization of acid and/or base groups on the particle s surface (b) the adsorption of anions, cations, ampholytes, and/or protons and (c) dissolution of ion-pairs that are discrete subunits of the crystalline particle, such as calcium-oxalate and calcium-phosphate complexes that are building blocks of kidney stone and bone crystal, respectively. The electric charging of the surface also influences how other solutes, ions, and water molecules are attracted to that surface. These interactions and the random thermal motion of ionic and polar solvent molecules establishes a diffuse part of what is termed the electric double layer, with the surface being the other part of this double layer. 

Jote the greater complexity of defining adsorption here in studies of electric double layers than, e.g., for metal-gas systems. With electric double layers, one is concerned with the whole interphasial region. The total adsorption is the sum of the increases of concentration over a distance, which in dilute solutions may extend for tens of nanometers. Within this total adsorption, there are, as will be seen, various types of adsorptive situations, including one, contact adsorption, which counts only Arose ions in contact with the electronically conducting phase (and is Aren, like the adsorption referred to in metal-gas systems, the particles on Are surface). Metal-gas systems deal with interfaces, one might say, whereas metal-electrolyte systems deal primarily with interphases and only secondarily with interfaces. 

Electroosmosis (p. 167) plays a very significant role in hpce because the interior surface of a quartz capillary develops a negative charge when in contact with aqueous solutions due to the ionization of surface silanol groups (Si-OH) above pH 4 and the adsorption of anions. As a result, a layer of cations from the bulk solution builds up close to the wall to maintain a charge balance by forming an electrical double-layer . The high fields employed... 

When an electrode (electronic conductor) is contacted with an electrolyte (ionic conductor), it shows some potential and attracts ions with opposite sign, forming electrical double-layer at the electrode/electrolyte interface, as shown in Figure 17. lu. Increasing its electrode potential causes further adsorption of ions... 

The results of these calculations imply that none of the ions would be contact adsorbed when no specific interactions between ions and metal are taken into account in the model. The Li+ ion, believed to be nonspecifically adsorbing, would be able to approach the surface more closely than the anions, mostly because of its small size, which allows it to penetrate the surface layers without displacing water molecules. The simulation results thus indirectly demonstrate the importance of specific chemical interactions for realistic models of the electric double layer. Apparently, also some specific features of the hydration shell structure of the ions must be taken into consideration in order to fully understand the adsorption of ions. 

Besides equilibria in the liquid phase (proteolytic, complex forming, etc.) that influence directly the values of effective mobilities of compounds to be separated, it is necessary to also establish, in the electrophoretic system, equilibria between the liquid and solid phase. In electrophoretic techniques which use solid stabilizing media adsorption of solutes on the sorbent surface is the main consideration. In capillary methods, and with colloid particles, similar effects have also to be considered (the surface of the solid phase that is in contact with the liquid phase is, with respect to the volume of the liquid, rather large). In both these latter cases the interaction between the solid and liquid phases participates in the formation of the electric double layer that conditions the electro-osmotic flow, and attributes the electric charge to colloid particles. 

The repulsive forces arise from the electromagnetic interactions of the charged layer surrounding the particles, the so-called electrical double layer. On the surface of the particles, a charged layer may be formed due to selective adsorption of ions. This part of the double layer is immobile and consists of tightly adsorbed ions in direct contact with the particle surface. In the solution adjacent to the particle, a second layer, in which the ions are more diffusely distributed, penetrates into the liquid. This part of the double layer is termed the diffusion layer. The extent of this diffusion layer depends on the electrolyte concentration increasing electrolyte concentration causes this diffuse double layer to shrink closer in to the particle, so that the electrostatic potential falls off more quickly with distance. The process by which the particles are stabilized by the repulsive forces of the electrical double layers is known as electrostatic stabilization. 

At least two of the recognized mechanisms for the formation of electrical double layers (Hunter, elal. 1981 Russel etal., 1989) are relevant to LB film depositions (1) ionization of carboxylic acid group and amphoteric acid groups on solid surfaces, and (2) differences between the affinities of two phases for ions or ionizable species. The latter mechanism includes the uneven distribution of anions and cations between two immiscible phases, the differential adsorption of ions from an electrolyte solution to a solid surface, and the differential solution of one ion over the other from a crystal lattice. Since the solid-liquid and the film-liquid interfaces are flat, large surfaces and since both have a large, solid-like concentration, the analysis that follows applies to both interfaces. For an interface conformed by a thin film of an amphiphilic compound with the hydrophilic end of the molecule in contact with the water subphase, the equilibrium of charges is based on pH and subphase concentration. The effect of pH is highlighted by the definition of the of the carboxylic acid ... 

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