tric potential on the external boundary of the Stern layer versus the bulk electrolyte is referred to as Stern potential. Electric potential difference between the fluid bulk and the surface is called the electric surface potential.zeta potential is used for estimating the degree of double layer charge. A characteristic value of this electric potential in the DL is 25 mV with a maximum value around 100 mV (up to several volts on electrodes). The chemical composition of the sample at which the ж-potential is 0 is called the point of zero charge or the iso-electric point. It is usually determined by the solution pH value, since protons and hydroxyl ions are the charge-determining ions for most surfaces.
. Detailed illustration of interfacial DL
potential can be measured using electrophoresis, electroacoustic phenomena, streaming potential, and electroosmotic flowaracteristic thickness of the DL is the Debye length, до? 1. It is reciprocally proportional to the square root of the ion concentration C. In aqueous solutions it is typically on the scale of a few nanometers and the thickness decreases with increasing concentration of the electrolyte.electric field strength inside the DL can be anywhere from zero to over +10 9 V/m. These steep electric potential gradients are the reason for the importance of the double layers.theory for a flat surface and a symmetrical electrolyte [20] is usually referred to as the Gouy-Chapman theory. It yields a simple relationship between electric charge in the diffuse layer у d and the Stern potential Ш d:
There is no general analytical solution for mixed electrolytes, curved surfaces or even spherical particles. There is an asymptotic solution for spherical particles with low charged double layers. In the case when electric potential over DL is less than 25 mV, the so-called Debye-Huckel approximation holds. It yields the following expression for electric potential Ш in the spherical DL as a function of the distance r from the particle center:
There are several asymptotic models which play important roles in theoretical developments associated with the interfacial DL.first one is thin DL raquo ;. This model assumes that DL is much thinner than the colloidal particle or capillary radius. This restricts the value of the Debye length and particle radius as following:
This model offers tremendous simplifications for many subsequent applications. Theory of electrophoresis is just one example. The theory of electroacoustic phenomena is another example. 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.opposing thick DL model assumes that the Debye length is larger than particle radius:
This model can be useful for some nano-colloids and non-polar fluids, where the Debye length is much larger.last model introduces overlapped DLs raquo ;. This is important in concentrated dispersions and emulsions when distances between particles become comparable with the Debye length.double layers have an additional parameter defining their characterization: differential capacitance. Differential capacitance, denoted as C, is described by the equation below:
where у is the surface charge and ш is the electric surface potential [1].
3. Methods of study
For the study of double electric layer use mainly three groups of methods. Firstly, the adsorption methods, which are based on the fact that the formation of the electrical double layer is due to the adsorption of various components of the solution and causes a change in their concentration. Specifically, adsorption methods are widely used to study the electric double layer formed on the fine particles in colloidal systems., Methods based on electrocapillary phenomena. Their essence is, that the formation of the electric double layer reduces the work required for creating a new surface interface and thereby leads to dependence of interfacial tension from electrode potential. The use of electrocapillary methods is limited by interfaces between the liquid phases on which possible direct measurement of the interfacial tension; for solid electrodes, these methods provide only qualitative information on the structure of the electric double layer. Third, methods, recording the amount of electricity spent on creating a certain electrode charge (charging the electric double layer). These include various galvanostatic and potentiostatic impulse techniques, as well as a method for measuring the electrical capacitance of the electric double layer using sinusoi...
