Structure breakdown

Induced instability of a dispersion is the most important form of instability encountered in practice, for here the dispersion is unable to maintain a given structure during changes in storage conditions. Sometimes the structural breakdown is permanent (in which case the initial dispersion at rest is metastable) at other times the structure is rebuilt on removal of the external influence. 

Furin et al. [90] have conducted the electrochemical fluorination of a series of amines, tripropylamine, tributylamine and triamylamine, and a series of enamines formed by the reaction of hexafluoropropene, its dimers and trimers with the secondary amines dipropylamine, dibutylamine and dial-lylamine. As well as the anticipated saturated perfluoroanalogues of the starting materials by-products resulting from structural breakdown were also produced. 

The release of cations is interpreted to have resulted chiefly from two processes an initial release caused by rapid exchange of surface cations for hydrogen followed by a slow release due to structural attack and disintegration of the aluminosilicate lattice. Other processes which could complicate the form of the dissolution curves are adsorption of cations released by structural breakdown, ion exchange on interlayer sites of cations released by structural breakdown and surface exchange (shale only), precipitation of amorphous or crystalline material, and dissolution rate differences among the various crystalline phases. 

Structural breakdown. Calculations of the depth of leaching/ dissolution were made using the following assumptions ... 

Now, it has been observed that calcium ions are needed (11, 12, IS, 14) for the release of acetylcholine. If we assume the same trigger mechanism as was described previously, we see that the penetration of calcium gives rise to a much stronger water lattice and membrane structure breakdown than does that of sodium. The bilayer membrane of the vesicles, which is tightly coupled to the presynaptic membrane via the cytoplasmatic water, is affected equally strongly and the probability of... 

Thixotropy is the time-dependent analogue of shear-thinning and plastic behaviour, and arises from somewhat similar causes. If a thixotropic system is allowed to stand and is then sheared at a constant rate, the apparent viscosity decreases with time until a balance between structural breakdown and structure re-formation is reached. If the sheared system is then allowed to stand, it eventually regains its original structure. A thixotropic hysteresis loop (Figure... 

Fig. 1.8 Conceptual structural breakdown of polymer processing product fabrication operations (23).

After adsorption one side of the protein molecule is oriented towards the sorbent surface, turned away from the aqueous solution. As a consequence, hydrophobic parts of the protein that are buried in the interior of the dissolved molecule may become exposed to the sorbent surface where they are still shielded from contact with water. Because hydrophobic interaction between apolar amino acid residues in the protein s interior support the formation of secondary structures as a-helices and P-sheets, a reduction of this interaction destabilizes such structures. Breakdown of the a-helices and/or P-sheets content is, indeed, expected to occur if peptide units released from these ordered structures can form hydrogen bonds with the sorbent surface. This is the case for polar surfaces such as oxides, e.g. silica and metal oxides, and with sorbent retaining residual water at their surfaces. Then the decrease in ordered secondary structures leads to an increased conformational entropy of the protein. This may favour the protein adsorption process considerably.13 It may be understood that proteins having an intrinsically low structural stability are more prone to undergo adsorption-induced structural changes. 

Seff et al. (27) have reported that 20% or greater exchange of Cu2+ in acidic media into A-zeolites results in the breakdown of the crystal structure. Our studies involve an exchange of 0.3% Cu2+ per unit cell for which low exchange level we did not observe significant structural breakdown. At this low exchange level Cu2+ populates only the most preferential sites. 2+... 

Tattersall (T43) found that pastes of w/c ratio 0.28-0.32 and age 4.5 min followed the Bingham model at low rates of shear, but that at higher rates the structure broke down irreversibly. Several other investigators have obtained similar results, but negative hysteresis has also been observed (e.g. Ref. R30), probably due to the use of hysteresis cycles of long duration, in which the structural breakdown due to shear is outweighed by the effects of... 

Flowahle rheology studies, i.e., structure breakdown and reconstitution. 

Here we will discuss not only structure formation but also structural breakdown. 

In a fused-oxide system, the metal oxide (e.g., NajO) is the structure breaker in aqueous solutions, the electrolyte (e.g., NaCl) is the structure breaker. Does the extent of structural breakdown of the continuous Si-0 network present in pure silica before the addition of MO depend on the concentration of MO The extent of breakdown must indeed depend on the concentration of the stmcture breaker then one would expect that properties that depend on the size and nature of the structures present would also be concentration dependent. 

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