Protective double layer

Double layer paint provides additional protection since such coatings would be less porous than single layer paint. It Is also noted that In all specimens that are not rinsed there Is a tendency to show Inductive loops In the impedance plot. It is not clear If this Is due to the adsorption of Inhibitor on steel surface or due to the formation of oxides or due to Increased porosity (28). 

Decrease In charge transfer resistance and Increase In double layer capacitance Is observed with Increasing time of Immersion or with Increasing test temperature and gives Information on the degree of protection efficiency of a coating. 

Impregnation of cobalt and molybdenum (without sodium) increases largely the isomerizing activity of the catalyst the /3-pinene is then completely converted. The catalysts prepared with sodium molybdate and sodium hydroxide (Co-Mo-Na and Na-Co-Mo-Na) have lower isomerizing activities while their HDS activities are significantly increased. As in the case of alumina supported catalysts the sulfided CoMo phase protected by a double layer of alkaline ions on the carbon support gives the best results in HDS of /3-pinene. The behaviour of this catalyst was examined in desulfurization of the turpentine oil (40% a-pinene, 25% /3-pinene, 25% A -carene and 10% camphene + dipentene + myrcene, 1500 ppm S). The results are recorded in Table 6. 

When the lipids that form cell membranes are mixed with water, they spontaneously form sheets in which the molecules are aligned in rows, forming a double layer, with their polar heads facing outward on each side of the sheet. These sheets form the protective membranes of the cells that make up living tissues. 

The stability of many protected colloidal dispersions cannot be explained solely on the basis of electric double layer repulsion and van der Waals attraction other stabilising mechanisms must be investigated. Steric stabilisation is a name which is used (somewhat loosely) to describe several different possible stabilising mechanisms involving adsorbed macromolecules. These include the following ... 

The combination of a double-layer deposit of nickel and either micro-cracked or micro-porous chromium provides the very best system of protection available by these means. Plastics components intended for particularly demanding applications (including exterior trim on the most expensive cars), are finished to such specifications. 

Addition of soluble macromolecules (polymers) in the colloidal dispersion can stabilize the colloidal particles due to the adsorption of the polymers to the particle surfaces. The soluble polymers are often called protective agents or colloids. If the protective agents are ionic and have the same charge as the particles, the electrical double-layer repulsive forces will be increased and thus the stability of the colloidal particles will be enhanced. In addition, the adsorbed polymers may help weaken the van der Waals attraction forces among particles. However, the double-layer repulsion and the van der Waals attraction cannot account for the entire stabilization of the particle dispersions. 

Mixed type of inhibitors are generally represented by organic compounds. Irrespective of the type of inhibitor, the inhibition process involves transport of inhibitor to the metal site followed by interaction of the inhibitor with the surface of the metal, resulting in protection. We now recall the electrical double layer consisting of inner and outer Helmholtz planes and the distribution of anions (A ), cations and water dipoles. This is schematically shown in Figure 1.59. When an inhibitor is added the structure of the double layer is affected, with the inhibitor displacing the adsorbed water molecules on the metal surface and taking their place on the metal surface. 

Broad Influence on Stability. In general, when electrical surface charge is an important determinant of stability, it is easier to formulate a very stable OAV emulsion than a W/O emulsion because the electric double-layer thickness is much greater in water than in oil. (This condition is sometimes incorrectly stated in terms of greater charge being present on droplets in an OAV emulsion). This is not to say that W/O emulsions cannot be stabilized, however. Many reasonably stable oil-field W/O emulsions are stabilized by another mechanism the protective action of viscoelastic, possibly rigid, films formed on the droplets by macromolecules or solid particles. 

Protective Agents and Sensitization. The stability of a dispersion can be enhanced (protection) or reduced (sensitization) by the addition of material that adsorbs onto particle surfaces. Protective agents can act in several ways. They can increase double-layer repulsion if they have ionizable groups. The adsorbed layers can lower the effective Hamaker constant. An adsorbed film may necessitate desorption before particles can approach closely enough for van der Waals forces to cause attraction. If the adsorbed material extends out significantly from the particle surface, then an entropy decrease can accompany particle approach (steric stabilization). Finally, the adsorbed material may form such a rigid film that it poses a mechanical barrier to droplet coalescence. 

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