Bridging mechanism, coagulation

Alternatively, several workers have shown that not only is the soluble, zero-charged hydrolysis product considerably more surface active than the free (aquo) ion but also a polymeric charged or uncharged hydrolysis product may be formed at the solid-liquid interface at conditions well below saturation or precipitation in solution. Hall (5) has considered the coagulation of kaolinite by aluminum (III) and concluded that surface precipitates related to hydrated aluminum hydroxide control the adsorption-coagulation behavior. Similarly Healy and Jellett (6) have postulated that the polymeric, soluble, uncharged Zn(OH)2 polymer can be nucleated catalytically at ZnO-H20 interfaces and will flocculate the colloidal ZnO via a bridging mechanism. 

It is also important to note that the coagulation mechanism in vivo does not occur in solution, but is localized to activated cell surfaces expressing anionic phospholipids such as phosphatidylserine, and is mediated by Ca2+ bridging between the anionic phospholipids and 7-carboxyglutamic acid residues of the clotting factors. This is the basis for using calcium chelators such as ethylenediamine tetraacetic acid (EDTA) or citrate to prevent blood from clotting in a test tube. 

AHAs do not coagulate proteins. Applying them in a peel should not therefore produce a whitening elfect . According to Forestier, the mechanism of action of AHAs is as follows. Even at low concentrations, AHAs can insert themselves between two protein chains. Here, they build a sort of bridge that reduces corneocyte cohesion. As a result of the lytic action of AHAs on corneodesmosomes, corneo-cytes are shed more easily from the skin, and the thickness of the stratum corneum is reduced. The skin appears more hydrated as the stratum corneum is thinned or disappears temporarily. 

Colloidal aggregates may be formed by different mechanisms. For instance, particles surrounded by an electrical double layer coagulate either in the primary or secondary minimum by the addition of salt. Polymer-coated particles may flocculate by polymer bridging or by changing the composition of the medium such that the polymer coats become insoluble. 

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