Steric repulsion substances

This pathway is electronically favorable, but the steric interference between the groups attached to the double bonds is likely to be severe. Such repulsions can be relieved if there are no groups sticking out sidewise at one end of the double bond, as with the central carbon of 1,2-propadiene, CH2=C=CH2, and ketene, CH2=C=0. These substances often undergo. [2 + 2] cycloadditions rather readily (Section 13-3D), and it is likely that these are concerted additions occurring by the Mobius route. 

Emulsions made by agitation of pure immiscible liquids are usually very unstable and break within a short time. Therefore, a surfactant, mostly termed emulsifier, is necessary for stabilisation. Emulsifiers reduce the interfacial tension and, hence, the total free energy of the interface between two immiscible phases. Furthermore, they initiate a steric or an electrostatic repulsion between the droplets and, thus, prevent coalescence. So-called macroemulsions are in general opaque and have a drop size > 400 nm. In specific cases, two immiscible liquids form transparent systems with submicroscopic droplets, and these are termed microemulsions. Generally speaking a microemulsion is formed when a micellar solution is in contact with hydrocarbon or another oil which is spontaneously solubilised. Then the micelles transform into microemulsion droplets which are thermodynamically stable and their typical size lies in the range of 5-50 nm. Furthermore bicontinuous microemulsions are also known and, sometimes, blue-white emulsions with an intermediate drop size are named miniemulsions. In certain cases they can have a quite uniform drop size distribution and only a small content of surfactant. An interesting application of this emulsion type is the encapsulation of active substances after a polymerisation step [25, 26]. 

In many dispersions of ice crystals, however, the latter are clearly separate from each other see for example Figure 9.1, depicting ice cream. This may be due to colloidal repulsion between crystals. Several substances can presumably adsorb onto ice, like proteins and ions, providing steric and electrostatic repulsion. However, this has not been systematically studied to the author s knowledge. 

Deflocculants are additives that prevent the close approach of particles. They adsorb on the particles and increase the repulsive forces. Coagulants do the opposite function. They reduce the repulsive forces or steric hindrance and promote particle agglomeration. When the agglomeration is produced by bridging action, the substances used are called flocculants, known... 

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