Double layer in colloids

Hunter, R. J., The double layer in colloidal systems, CTEy 1, 397 (1980). 

The surfaces being considered are not planar, and therefore instead of Helmholtz-Perrin parallel-plate condensers, one has concentric-sphere capacitors Gouy-Chapman regions show radial instead of planar symmetry. All such points complicate the mathematics, but lead to few new truths. Hence, such details will be ignored in this very simple account of the dominating role of double layers in colloid chemistry. 

Source J. Lyklema, Fundamentals of Electrical Double Layers in Colloidal Systems. In Colloidal Dispersions (J. Goodwin, Ed.), Royal Society of Chemistry, London, 1982, pp. 47-70. aThe signs in the table indicate the sign of the acquired charges. 

Overbeek, J. Th. G., Electrochemistry of Double Layer. In Colloid Science, Vol. 1 (H. R. Kruyt, Ed.), Elsevier, Amsterdam, Netherlands, 1952a, pp. 115-193. (See the annotation under Kruyt 1952.)... 

Electrical transients (12) can also be used to evaluate particle mobilities in special circumstances. Charged particles, their counterions and other excess ions present in the suspending fluid contribute to the electrical current. When the concentration of excess ions is very low compared to the concentration of counterions, it is sometimes possible to determine the current contributed by particles versus that contributed by ions. Ionic concentrations define the extent of double layers in colloids. Transient and AC conductivities can be related most directly to the ionic concentrations and mobilities. But, again, the measurements in low conductivity fluids have to be performed in planar cells with narrow electrode spacings in order to ensure well defined electric fields. 

The role of double layers in colloid science is outlined in ... 

J. Lyklema, Fundamentals of Electrical Double Layers in Colloidal Systems , in Colloidal Dispersions , Ch. 3. 

Attard, P. 2001. Recent advances in the electric double layer in colloid science. Current Opinion in Colloid Interface Science 6, no. 4 (August) 366-371. doi 10.1016/ 81359-0294(01)00102-9. 

P. Attard, Curr. Opin. Colloid Interface Sci., 6, 366 (2001). Recent Advances in the Electric Double Layer in Colloid Science. 

Splelman L A and Friedlander S K 1974 Role of the electrical double layer In particle deposition by convective diffusion J. Colloid. Interfaoe. Sol. 46 22-31... 

Nikbin N, Watts P (2004) Solid-supported continuous flow synthesis in micro-reactors using electroosmotic flow. Org Process Res Dev 8 942 Overbeek JTG (1952) Electro chemistry of double layer. In Kruyt HR (ed) Colloid science, Vol. 1. Elsevier, Amsterdam... 

Spielman, L. A. and FRIEDLANDER, S. K. J. Colloid and Interface. Sci. 46 (1974) 22. Role of the electrical double layer in particle deposition by convective diffusion. 

Silver iodide particles in aqueous suspension are in equilibrium with a saturated solution of which the solubility product, aAg+ai, is about 10 16 at room temperature. With excess 1 ions, the silver iodide particles are negatively charged and with sufficient excess Ag+ ions, they are positively charged. The zero point of charge is not at pAg 8 but is displaced to pAg 5.5 (pi 10.5), because the smaller and more mobile Ag+ ions are held less strongly than-the 1 ions in the silver iodide crystal lattice. The silver and iodide ions are referred to as potential-determining ions, since their concentrations determine the electric potential at the particle surface. Silver iodide sols have been used extensively for testing electric double layer and colloid stability theories. 

During the 1930 s a clearer idea of the role of the electrical double layer in stabilising colloidal particles began to emerge, particularly in the work of Verwey (5), Kruyt (6) and Derjaguin (7). In 1938 in a classic paper Langmuir (8) showed that when an overlap of double layers occurred, with two flat plates whose surfaces were at the same electrostatic potential, then a repulsion pressure was developed between them. 

Gouy1 and Chapman,2 who were the first to predict the distribution of electrolyte ions in water around a charged flat surface, demonstrated that the ions form a diffuse layer (the electric double layer) in the liquid near the interface. The interaction between two charged surfaces, due to the overlapping of the double layers, was calculated much later by Deryaguin and Landau3 and Verwey and Overbeek.4 The stability of the colloids was successfully explained by them in terms of a balance between the double layer and van der Waals interactions (the DLVO theory).3 4... 

The relative motion of the two sides of the mobile double layer in electrokinesis transports electricity consequently the presence of any surface in a fluid ought to increase the conductivity of that fluid this effect is called surface conductivity . When the area of surface is considerable, as with a suspension or colloidal solution, the increase should be appreciable. Smoluchowski predicted this effect, which has been shown to exist by a good many workers,3 but the theory and its verification by experiment appear at the present time to be in a state of considerable confusion. Some workers, indeed, claim that there is no correlation4 between the potential and the surface conductivity, which seems improbable. The reader must draw his own conclusions from the litera-... 

Osmotic Pressure of the Double Layer in a Colloidal Suspension... 

In the theory developed by Derjaguin and Landau (24) and Verwey and Overbeek (25.) the stability of colloidal dispersions is treated in terms of the energy changes which take place when particles approach one another. The theory involves estimations of the energy of attraction (London-van der Walls forces) and the energy of repulsion (overlapping of electric double layers) in terms of inter-oarticle distance. But in addition to electrostatic interaction, steric repulsion has also to be considered. 

Geoiogy. An example of electrochemistry in geology concerns certain types of soil movements. The movement of earth under stress depends on its viscosity as a siurry that is, a viscous mixture of suspended solids in water with a consistency of very thick cream. Such mixtures of material exhibit thixotropy, which depends on the interactions of the double layers between colloidal particles. These in turn depend on the concentration of ions, which affects the field across the double layer and causes the colloidal structures upon which the soil s consistency depends to repel each other and remain stable. Thus, in certain conditions the addition of ionic solutions to soils may cause a radical increase in their tendency to flow. 

Electrical Double Layer. In order to model the structure of the electrical double layer (EDL) of oxide colloids, it is necessary to formulate 1) the reactions which result in the formation of surface charge (cTq), and 2) the potential and charge relationships in the interfacial region. It has been generally assumed that surface charge (O ), defined experimentally by the net uptake of protons by the surface, results from simple ionization of oxide surface sites (5, 11 12, 13), i.e.. 

Concave double layers, l.e. double layers at the hollow side of curved interfaces, are of more general interest, "nius double layers inside spherical cavities are relevant for micro-emulsions and vesicles, and those inside cylinders for electrokinetics and membrane transport. Certain contact points exist with double layers in overlap, as occurring in colloid stability. 

More Indirectly, Information on the charge- or potential distribution is also obtainable from studies involving two double layers in interaction, as in colloid stability, soap films and wetting films. Overlap Is essentially determined by the diffuse parts, the Stem layers defining the boundary conditions. Most of this will be deferred to Volume IV. 

Double layers in non-polar media recur in colloid stability (Volume IV). The slow decay dv /dr (or dy//dx) means that the field strength is low. and so is the interparticle force. On the other hand, the range of the Interaction is very high, so that even in dilute sols the particles feel each other s presence. Absence of screening means that the pair interaction between particles is completely described by Coulomb s law. In emulsions and at oil-water Interfaces a "double diffuse double layer may be formed, which is more extended in the oil phase K... 

R. J. Hunter, Zeta Potential in Colloid Science. Principles and Applications, Academic Press (1981). (Although electrokinetics are emphasized there is also much information on the composition of double layers in disperse systems.)... 

B.V. Deijaguln, (= Deryagin), S.S. Dukhln and V.N. Shilov, Kinetic Aspects of Electrochemistry of Disperse Systems, Adv. Colloid Interface Set 13 (1980) 141. (Emphasis on double layers in an external field.)... 

Adamson (51) proposed a model for W/0 microemulsion formation in terms of a balance between Laplace pressure associated with the interfacial tension at the oil/water interface and the Donnan Osmotic pressure due to the total higher ionic concentration in the interior of aqueous droplets in oil phase. The microemulsion phase can exist in equilibrium with an essentially non-colloidal aqueous second phase provided there is an added electrolyte distributed between droplet s aqueous interior and the external aqueous medium. Both aqueous media contain some alcohol and the total ionic concentration inside the aqueous droplet exceeds that in the external aqueous phase. This model was further modified (52) for W/0 microemulsions to allow for the diffuse double layer in the interior of aqueous droplets. Levine and Robinson (52) proposed a relation governing the equilibrium of the droplet for 1-1 electrolyte, which was based on a balance between the surface tension of the film at the boundary in its charged state and the Maxwell electrostatic stress associated with the electric field in the internal diffuse double layer. 

Kosmulski, M., Oxide/electrolyte interface Electric double layer in mixed solvent systems. Colloids Surf. A, 95, 81,1995. 

Janusz, W. et al.. Investigation of the electrical double layer in ametal oxide/monova-lent electrolyte solution system, 7. Colloid Interf. Sci., 187, 381,1997. 

Siffert. B.. Jada, A., and Letsango, J.E., Location of the shear plane in the electric double layer in an organic medium, J. Colloid Intetf. Sci., 163, 327, 1994. 

FIGURE 7.1 Schematic representation of the electrical double layer in the vicinity of the solid-liquid interface. (From Williams, R.A., in Colloid and Surface Engineering Applications in the Process Industries, Butter-worth-Heinemann, Oxford, 1992. With permission.)... 

Fig. V-29. Schematic representation of dynamic polarization of the electrical double layer in the field of acoustic wave, leading to the appearance of colloid vibration potential (CVP) [35]...

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