what is double layer theory

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What is electrical double layer in pharmacy?

Definition. Electrical Double Layer: A double layer around a charged particle formed by the particle surface charge and its counter ions, forming an ionic cloud surrounding the particle.

What is double diffusion layer?

Diffuse double layer (DDL) is an ionic structure that describes the variation of electric potential near a charged surface, such as clay, and behaves as a capacitor. Formation: Clays are aluminosilicates in which some of the aluminum and silicon ions are replaced by elements with different charge.

What is electrical double layer potential?

Electric Double Layer. Electric Double Layer. The double layer model is used to visualize the ionic environment in the vicinity of a charged surface. It can be either a metal under potential or due to ionic groups on the surface of a dielectric.

What is Stern double layer?

The Stern Layer is the first (internal) layer of the electric double layer, which forms at a charged surface in an ionic solution. The Stern Layer is immobile relative to the surface and is comprised of a layer of ions charged oppositely to the surface which attach to the surface.

Why are double layers formed?

During the process of adsorption of surfactant ions at a liquid-fluid interface, the surface electric potential and charge density increase with time. This leads to the formation of an electric double layer inside the solution.

What causes the formation of the double layer?

Electric double layers form whenever two conducting phases meet at an interface. Generally, one of the phases acquires a positive excess charge on its surface, which is balanced by a countercharge of the same magnitude and opposite sign on the other phase.

What is electrical double layer and zeta potential?

The zeta potential gives an indication of the potential stability of the colloidal system. Electrical double layer: There is a region around each particle where the particle charge attracts the free ions to form an electrical cloud called the electrical double layer.

What is the meaning of zeta potential?

Zeta potential is a physical property which is exhibited by any particle in suspension, macromolecule or material surface. It can be used to optimize the formulations of suspensions, emulsions and protein solutions, predict interactions with surfaces, and optimise the formation of films and coatings.

What is the thickness of double layer?

The Electric Double Layer In a metallic electrode the thickness is around 0.1 nm and is a function of the electron density due to the stationary nature of atoms in solids. In the electrolyte the thickness varies more, in a range of .

What is Helmholtz layer?

Helmholtz double layer (HDL) refers to the structural representation of the accumulation of electrical charges present at the boundary of an electrode and electrolyte when they are in contact with each other. HDL is most readily identifiable in fluid-based mixture systems, such as paints used for corrosion prevention.

What is the difference between single and double Immunodiffusion?

In double immunodiffusion, both antigen and antiserum together diffuses in the gel while in Single immunodiffusion only antibody is involved in the gel and only antigen diffuses in the gel.

Why is ouchterlony called double diffusion?

In the Ouchterlony double diffusion, both the antigen and the antibody diffuse toward each other in a semisolid medium to a point till their optimum concentration is reached. A band of precipitation occurs at this point.

What is a diffusion boundary layer?

The boundary layer diffusion is the rate-determining step for growth under typical experimental conditions (20–40 Torr, ∼300 °C), and high-quality nanosheets can only grow in the diffusion-limited regime.

What is diffusion layer in cyclic voltammetry?

In electrochemistry, the diffusion layer, according to IUPAC, is defined as the "region in the vicinity of an electrode where the concentrations are different from their value in the bulk solution.

What is the DLVO theory?

In the classical Derjaguin, Landau, Verwey and Overbeek (DLVO) theory electrical double layer repulsions represented one half of the interactions studied and these repulsions closely resembled those occurring in aqueous systems. However, when using water as the liquid medium it is indispensable to extend the classical DLVO theory by including the polar interaction energies which always occur in water, using the Lewis acid–base approach. (Using the classical DLVO approach for an aqueous system limits one to content oneself with analyzing only about 10% of the forces that are actually operating in aqueous systems.) Each one of the three different non-covalent forces involved in the extended DLVO (XDLVO) theory, i.e.: (1) Lifshitz–van der Waals (LW), or electrodynamic interactions; (2) Lewis acid-base (AB) or electron-acceptor/electron-donor interactions and: (3) electrical double layer (EL) interactions, decays according to fundamentally different regimens. This requires that for each one of the three types of interaction the free energies as a function of distance be determined and treated separately. It is only after each energy versus distance curve thus obtained for LW, AB and EL interactions has been obtained separately, that all three (expressed in the same energy units) may be added together, to obtain the final XDLVO plot.

What happens when the ions in the Stern layer are in equilibrium with the ions in the diffuse layer?

Since the ions in the Stern layer are in equilibrium with the ions in the diffuse layer, the increase of concentration or charge number of electrolyte in solution will make more counterions enter the Stern layer, leading to a quicker potential drop in the Stern layer, thus resulting in the decrease in ζ potential and compression of thickness of double layers, as shown in Fig. 7.16 a.

What is surface complexation model?

A complete Surface Complexation Model (SCM) consists of both mass-law expressions that describe ion binding to surface functional groups, and electrostatic correction terms for the mass law expressions derived from Electrical Double Layer (EDL) theory [ 60 ] , since the ion adsorption process results in charge development at the interface . Stern-based EDL models, which include both the earlier Helmholtz and diffuse-layer EDL representations, are most commonly incorporated into SCM's, and a schematic of our Stern-based EDL model as used to model our rutile adsorption data is given in Fig. 6. This 3-plane Stern model was originally due to Van Riemsdijk and co-workers [ 61 ], and is distinct from the widely-used original triple-layer model in that the capacitance of the outermost Stern plane (C 2 in Fig. 6) is not fixed at 0.2 F/m 2. A Stern-based MUSIC model is our SCM of choice because it is capable of linking macroscopic ion adsorption data with the underlying molecular-level details in a broadly realistic way [ 55 ].

How does EDL affect the viscosity of liquid?

The charged solid surface affects the viscosity of the liquid in the diffuse layer, due to (1) changes in the mobility of the ions in the diffuse layer with their distance from the charged solid surface , and (2) changes in the viscosity of the liquid molecules with their distance from the charged solid surface.

When specific adsorption takes place, it is possible, especially with polyvalent or surface active counterions, for?

When specific adsorption takes place, it is possible, especially with polyvalent or surface active counterions, for reversal of charge to take place within the Stern layer, that is, for ψ0 and ψδ to have opposite signs, as shown in Fig. 7.16 b.

Can oocysts be compared to DLVO?

Not all oocysts exhibited force-separation curves that could be fit with DLVO and Pincus predictions in the same manor as for Fig. 8. The inability to obtain agreement with DLVO theory is probably a combined result of the significant surface roughness and extension of the oocyst surface proteins into the electrical double layer. Indeed, it is somewhat surprising that frequent agreement with DLVO theory can be obtained at all, given the complexity of the surface. In order to make comparisons between all oocysts, the range of the repulsive force has been analysed in terms of the decay length ( Fig. 9) and magnitude ( Fig. 10 ).

What is the double layer of charge?

A double layer ( DL, also called an electrical double layer, EDL) is a structure that appears on the surface of an object when it is exposed to a fluid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The DL refers to two parallel layers of charge surrounding the object. The first layer, the surface charge (either positive or negative), consists of ions adsorbed onto the object due to chemical interactions. The second layer is composed of ions attracted to the surface charge via the Coulomb force, electrically screening the first layer. This second layer is loosely associated with the object. It is made of free ions that move in the fluid under the influence of electric attraction and thermal motion rather than being firmly anchored. It is thus called the "diffuse layer".

Why is the thin DL model valid for most aqueous systems?

The thin DL model is valid for most aqueous systems because the Debye length is only a few nanometers in such cases. It breaks down only for nano-colloids in solution with ionic strengths close to water.

How is an EDL formed?

The formation of electrical double layer (EDL) has been traditionally assumed to be entirely dominated by ion adsorption and redistribution. With considering the fact that the contact electrification between solid-solid is dominated by electron transfer, it is suggested by Wang that the EDL is formed by a two-step process. In the first step, when the molecules in the solution first approach a virgin surface that has no pre-existing surface charges, it may be possible that the atoms/molecules in the solution directly interact with the atoms on the solid surface to form strong overlap of electron clouds. Electron transfer occurs first to make the “neutral” atoms on solid surface become charged, i.e., the formation of ions. In the second step, if there are ions existing in the liquid, such as H+ and OH-, the loosely distributed negative ions in the solution would be attracted to migrate toward the surface bonded ions due to electrostatic interactions, forming an EDL. Both electron transfer and ion transfer co-exist at liquid-solid interface.

What is the BDM model?

In 1963 J. O'M. Bockris, M. A. V. Devanathan and Klaus Müller proposed the BDM model of the double-layer that included the action of the solvent in the interface. They suggested that the attached molecules of the solvent, such as water, would have a fixed alignment to the electrode surface. This first layer of solvent molecules displays a strong orientation to the electric field depending on the charge. This orientation has great influence on the permittivity of the solvent that varies with field strength. The IHP passes through the centers of these molecules. Specifically adsorbed, partially solvated ions appear in this layer. The solvated ions of the electrolyte are outside the IHP. Through the centers of these ions pass the OHP. The diffuse layer is the region beyond the OHP.

What is EDL in physics?

The electrical double layer ( EDL) is the result of the variation of electric potential near a surface, and has a significant influence on the behaviour of colloids and other surfaces in contact with solutions or solid-state fast ion conductors .

What is the theory of a flat surface and a symmetrical electrolyte?

The theory for a flat surface and a symmetrical electrolyte is usually referred to as the Gouy-Chapman theory . It yields a simple relationship between electric charge in the diffuse layer σ d and the Stern potential Ψ d:

Which model fails for highly charged DLs?

The Gouy-Chapman model fails for highly charged DLs. In 1924 Otto Stern suggested combining the Helmholtz model with the Gouy-Chapman model: In Stern's model, some ions adhere to the electrode as suggested by Helmholtz, giving an internal Stern layer, while some form a Gouy-Chapman diffuse layer.

What is double layer theory?

Traditionally, electrochemical double-layer theory has been concerned with the so-called ideally polarizable interfaces, at which by definition no electrochemical reaction takes place and hence no direct current passes through the interface. However, this condition can be relaxed: for double-layer studies, it is sufficient that any electrochemical reaction does not disturb the distribution of the excess charges at the interface, and this is the assumption that we shall make throughout this article. In this case, a potential difference can be applied to the two sides of the interface, which entails a change in the excess charges. Hence, the interface resembles a capacitor, and, indeed, much research has been focused on the capacity of the double layer.

How do double layers form?

Electric double layers form whenever two conducting phases meet at an interface. Generally, one of the phases acquires a positive excess charge on its surface, which is balanced by a countercharge of the same magnitude and opposite sign on the other phase. Well-known examples are the interfaces between two metals with different work functions, of two semiconductors with n- and p-type conductivities, and of a metal electrode in contact with an electrolyte solution. Here, we consider mainly metal/solution interfaces, because they are of greatest relevance to fuel cells, supercapacitors, and batteries. In addition, carbon is often used in devices, and we will treat this as a special case. Because of its unique band structure, its capacity shows quite an unusual dependence on the electrode potential.

What is the Stern layer?

Stern layer: counterions (charged opposite to the surface charge), attracted to the particle surface and closely attached to it by the electrostatic force.

What is the role of the double layer in electrostatic stabilization?

An electric double layer is a phenomenon that plays a fundamental role in the mechanism of the electrostatic stabilization of colloids. Colloidal particles gain negative electric charge when negatively charged ions of the dispersion medium are adsorbed on the particle surface. A negatively charged particle attracts the positive counterions surrounding the particle. As shown in Fig. 1-17, an electric double layer is the layer surrounding a particle of dispersed phase, including the ions adsorbed on the particle surface and a film of the countercharged dispersion medium. The electric double layer is electrically neutral.

How is a double electric layer formed?

Double electric layer can be formed by directionally absorbing the polar group of a phase to another phase, which can be carried out by the electrons on the polymer functional groups going through the phase interface.

Which layer contains free ions with a higher concentration of the counterions?

Diffuse layer contains free ions with a higher concentration of the counterions. The ions of the diffuse layer are affected by the electrostatic force of the charged particle. The electrical potential within the electric double layer has a maximum value on the particle surface (Stern layer).

What is the boundary of a colloidal particle?

The boundary of this layer is called the slipping plane (shear plane).The value of the electric potential at the slipping plane is called the zeta potential, which is a very important parameter in the theory of interaction of colloidal particles.

Why is the double layer formed?

Thus, the double layer is formed in order to neutralize the charged surface and , in turn, causes an electrokinetic potential between the surface and any point in the mass of the suspending liquid. This voltage difference is on the order of millivolts and is referred to as the surface potential. The magnitude of the surface potential is related ...

What is the theory of a cloud of oppositely charged ions?

The Gouy-Chapman theory describes a rigid charged surface, with a cloud of oppositely charged ions in the solution, the concentration of the oppositely charged ions decreasing with distance from the surface. This is the so-called diffuse double layer.

What is the boundary between two phases?

One of the first principles which must be recognized is that matter at the boundary of two phases possesses properties which differentiates it from matter freely extended in either of the continuous phases separated by the interface. When talking about a solid-solution interface, it is perhaps easier to visualize a difference between the interface and the solid than it is to visualize a difference between the interface and the extended liquid phase. Where we have a charged surface, however, there must be a balancing counter charge, and this counter charge will occur in the liquid. The charges will not be uniformly distributed throughout the liquid phase, but will be concentrated near the charged surface. Thus, we have a small but finite volume of the liquid phase which is different from the extended liquid. This concept is central to electrochemistry, and reactions within this interfacial boundary that govern external observations of electrochemical reactions. It is also of great importance to soil chemistry, where colloidal particles with different surface charges play a crucial role.

Is it easier to visualize a difference between a solid and a solid?

When talking about a solid-solution interface , it is perhaps easier to visualize a difference between the interface and the solid than it is to visualize a difference between the interface and the extended liquid phase. Where we have a charged surface, however, there must be a balancing counter charge, and this counter charge will occur in ...

Is double layer thickness accurate?

This theory is still not entirely accurate. Experimentally, the double layer thickness is generally found to be somewhat greater than calculated. This may relate to the error incorporated in assuming activity equals molar concentration when using the desired form of the Boltzman distribution. Conceptually, it tends to be a function of the fact that both anions and cations exist in the solution, and with increasing distance away from the surface the probability that ions of the same sign as the surface charge will be found within the double layer increase as well.

What is double layer theory?

The double layer theory combines the effects of the electrostatic repulsion and van der Waals attraction owing to the so-called double layer of counter ions. LS brine using the mechanism of the expansion of the electric double layer to reduce clay–clay attraction. Indirect interactions between oil, brine, and rock highly affect the discharge of clay particles; this mechanism normally occurs in kaolinite plates and involve in their charge distribution [46]. LS water makes water film more stable owing to this expanded double layer effect, resulting in more water-wet on clay surfaces and more oil is detached; conversely, adsorption of divalent at water/sand and water/oil interfaces changes the wettability from water-wet state to oil-wet condition [45,70,71].

What is the theory of a diffuse double layer?

Diffuse-double-layer theory states that as colloidal particles assume a positive or negative charge due to the presence of charged groups within, or adsorption of a charged layer from , the surrounding medium, an electrical double layer of the opposite charge is formed at the interface between the solid phase and the aqueous phase to ensure electroneutrality of the overall colloidal system. A fixed covering of positive ions is formed over a group of negatively charged particles. This fixed layer of charge is called the Stern layer, which in turn is surrounded by a thin movable layer of positive charges called the diffuse layer, as shown in Figure 2.4.

What is EDL theory?

A general EDL theory has been reported that applies to a large diversity of types of electrolytes used in electrochemical power generators, such as batteries, fuel cells, and super capacitors ( Figure 1.18b) ( Quiroga et al., 2014 ).

Overview

Development of the (interfacial) double layer

Mathematical description

Electrical double layers

See also

Further reading

External links