Hydration and Steric Effects

Additional influences on dispersion stability beyond those accounted for by the DLVO theory, like surface hydration and steric effects, have received considerable attention over the past several decades [194,278], 

The use of natural and synthetic polymers to stabilize aqueous colloidal dispersions is technologically important, with much research in this area being focused on adsorption and steric stabilization [286-291]. Steric stabilization is discussed further in the next section. 

In principle, steric stabilization can result from any of the following [194,280]  

The most important factor influencing the degree of steric stabilization is the thickness of the adsorbed layer in comparison with the size of the particles [292], The term protection has also been used because the steric stabilizing effect can cause significant salt tolerance on the part of a colloidal dispersion. Some suspensions have been prepared, using high concentrations of polyelectrolytes, that are quite stable in concentrated salt solutions [49]. 

In almost all cases there will be adsorption layers on each approaching droplet, bubble, or particle surface so there will also be an osmotic pressure contribution, again at close approach, due to overlap of the adsorption layers. The osmotic contri- 

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PEG is widely known for its resistance to protein adsorption, nonimmunogenicity, and antithrombogenicity. It is believed that PEG S antifouling ability is related to hydration and steric effects. Several studies have been focused on PEG grafting onto PUs. Particularly, PEG was either introduced in the polymer backbone, 2" by Michael addition onto main chain double bonds and click chemistry, or grafted in the polymer side chain by urethane or allophanate linkages. ... 

Electronic and steric effects operate m the same direction Both cause the equilib rium constants for hydration of aldehydes to be greater than those of ketones... 

Effects of Structure on Rate Electronic and steric effects influence the rate of hydration in the sfflne way that they affect equilibrium. Indeed, the rate and equilibrium data of Table 17.3 parallel each other almost exactly. 

For borderline metals and/or borderline ligands, the order is determined by other factors such as dehydration (removal of hydrate water), steric effects, etc. 

In conclusion, the percentage of hydrate present in solution at equilibrium depends on both electronic and steric effects. Electron donation and bulky substituents decrease the percentage of hydrate present at equilibrium, whereas electron withdrawal and small substituents increase it. 

Visser [1988] also draws attention to the effects of surface hydration and steric hindrance that also prevent close approach of particles and surfaces therdry reducing the opportunity for adhesion. In both examples interpenetration of the adsorbed layers, i.e. a physical barrier of where say, a polymer is adsorbed, prevents collision and hence adhesion. 

Steric and Hydrodynamic Effacts. Additional influences on dispersion stability beyond those accounted for by the DLVO theory, like steric, sinface hydration, and hydrodynamic effects have received considerable attention over the past several decades (54). More generally, 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 ionisable groups. The adsorbed layers can lower the effective Hamaker constant. An adsorbed film may necessitate desorption before particles can approach close enough for van der Waals forces to cause attraction. 

Table 17 3 compares the equilibrium constants for hydration of some simple aldehydes and ketones The position of equilibrium depends on what groups are attached to C=0 and how they affect its steric and electronic environment Both effects con tribute but the electronic effect controls A hydr more than the steric effect... 

The hydroboration step, being very sensitive to steric effects, yields only secondary alkylboranes from trisubstituted double bonds, whereas the less hindered alkylborane is formed predominantly from disubstituted steroidal double bonds. The diborane attack occurs usually towards the a-side and hence results in overall a-hydration of double bonds after alkaline hydrogen peroxide oxidation. ... 

The cation of 4,4 -biquinazolinyl and its 2,2 -dimethyl derivative readily add water across the 3,4- and 3, 4 -double bonds, but the cation of 2,2 -biquinazolinyl is not hydrated. Hydration in the 4,4 -isomers has been attributed to restricted rotation about the 4,4 -bond, a steric effect which is relieved by hydration. The ultraviolet spectrum of 2,2 -biquinazolinyl (neutral species and cation) shows that there is considerable conjugation between the quinazoline groups. Covalent hydration is absent from the latter compound because it would otherwise destroy the extended conjugation present. 

When simply hydrated, the number of water molecules bonded to the metal (central) ion correspond to a value N, the coordination number, which is also termed the hydration number. In complexation, ligands displace the hydrate waters, although not necessarily on a 1 1 basis. Charge, steric, and other effects may cause the maximum number of ligands to be less than N. For example, in... 

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