Polyacids hydrated

According to this mechanism, the reaction rate is proportional to the concentration of hydronium ion and is independent of the associated anion, ie, rate = / [CH3Hg][H3 0 ]. However, the acid anion may play a marked role in hydration rate, eg, phosphomolybdate and phosphotungstate anions exhibit hydration rates two or three times that of sulfate or phosphate (78). Association of the polyacid anion with the propyl carbonium ion is suggested. Protonation of propylene occurs more readily than that of ethylene as a result of the formation of a more stable secondary carbonium ion. Thus higher conversions are achieved in propylene hydration. 

Large bound monovalent cations, e.g. tetrabutylammonium ions, are too large to penetrate any of the hydration regions. However, the smaller lithium, sodium and potassium ions are able to penetrate the outermost hydration region of the neutralized polyacid and this is accompanied by volume increases (Figure 4.9). These cations are probably not site-bound but are mobile in the outer cylindrical region of hydration (Figure 4.10). 

It is well known that lyophilic sols are coagulated by the removal of a stabilizing hydration region. In this case, conversion of a sol to a gel occurs when bound cations destroy the hydration regions about the polyanion, and solvated ion-pairs are converted into contact ion-pairs. Desolvation depends on the degree of ionization, a, of the polyacid, and the nature of the cation. Ba ions form contact ion-pairs and precipitate PAA when a is low (0-25), whereas the strongly hydrated Mg + ion disrupts the hydration region only when a > 0-60. 

The hydrates of the polyacids ( 10.24) resemble those of the zeolites in that the water molecules are again situated in the interstices of a very open structure, and it is noteworthy that in these structures, too, the water may be readily removed. 

Mixed vanadium-chromium oxide compounds present a wide range of interesting properties for instance, they have excellent catalytic properties, and recently they were shown to be potential candidates for anodes in lithium-ion batteries. DTA, TG and powder XRD analyses were used [101] to monitor the dehydration/crystallization and phase transitions upon heat treatment of the hydrated vanadates obtained through the reaction of peroxo-polyacids of vanadium and chromium, and to determine the ranges of coexistence of the phases in equilibrium. 

Glass-ionomers set rapidly by an acid-base (neutralization) reaction to form an insoluble salt. The first step involves attack by hydrated protons from the polyacid solution at susceptible sites on the surface of the glass particles. This Uberates ions such as Na" and (or Sr ), to be followed quite quickly by Al. ... 

The acid strength of the cross-linked polyacids falls with the size of the hydrated gegenions. The hydrated lithium ion is larger than the hydrated rubidium ion, so that the apparent acid strength of cross-linked polyacids is greater in the presence of rubidium ions than in the presence of lithium ions. This is exactly the reverse of the situation for non-cross-linked polyacids, where acid strength depends on the size of the unhydrated ions. 

There are also polyacids and polybases. Polyacids give several hydrated protons per molecule, while polybases give several hydroxide anions. 

In the study of polyelectrolytes the IR absorption technique has been used to obtain information on proton dissociation from polyacids, ionbinding, the structure of the water of hydration and conformational changes. 

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