Networks cationic

The theory of polymer gel phase transition provides a novel paradigm of remarkable predictive power to explain both condensation and decondensation in secretion. It assigns specific functional roles to the polyanionic-network/ cation pairs ubiquitously found in secretory granules, and provides a working framework to formulate new, testable questions to investigate further the mechanisms of storage and release in secretion. 

In addition, alkali-ion transport is governed physically by the size of the bottlenecks (i.e. smallest opening at the site interface) between interstitial alkali-ion positions and chemically by the bonding energy between the mobile ions and the network anions. The shortest diameter of the bottlenecks should be larger than twice the sum of the mobile-ion and anion radii. The covalent bonding between the mobile ion and the anion network should be as weak as possible, which may be achieved if the anion forms a strong covalent complex with the network cations and/or if the anion is bonded to more than two of these cations. ... 

Yoshioka H, Mikami M, Mori Y, Tsuchida E (1993) Preparation of thermoresponsive surfaces using polyvinylchloride-graft-poly(A-isopropylacrylamide). Polym Adv Technol 4 519-521 Yu H, Grainger DW (1993) Thermosensitive swelling behavior in cross.linked N-isopropylacry-lamide networks-cationic, anionic, and ampholytic hydrogels. J Appl Polym Sci 49 1553-1563... 

The oxygen coordination number of the network cation is small. 

The sample must contain a high percentage of network cations which are surrounded by oxygen tetrahedra or triangles. 

Some oxygens are linked only to two network cations and do not form further bonds with any other cations. 

Network connectivity, however, should not be considered solely in terms of NBO concentrations. The coordination of the network cations is also an important factor in determining the connectivity of the... 

The cation CN should be small, i.e., 3—4-. For silicate (Si02) and phosphate (P2O5) networks, tetrahedral coordination is observed, whereas for borate (B2O3) network, the coordination is 3. Additionally, sufficient network cations should be present to sustain a continuous network structure. 

A typical absorption curve for vitreous siUca containing metallic impurities after x-ray irradiation is shown in Eigure 12. As shown, the primary absorption centers are at 550, 300, and between 220 and 215 nm. The 550-nm band results from a center consisting of an interstitial alkah cation associated with a network substituent of lower valency than siUcon, eg, aluminum (205). Only alkaUes contribute to the coloration at 550 nm. Lithium is more effective than sodium, and sodium more effective than potassium. Pure siUca doped with aluminum alone shows virtually no coloration after irradiation. The intensity of the band is deterrnined by the component that is present in lower concentration. The presence of hydrogen does not appear to contribute to the 550-nm color-center production (209). 

In addition to the Zachariasen and radius ratio rules, for oxides the electronegativity of the predominant cation should be between 1.7 and 2.1 (7). If the cation electronegativity is too high, the compound tends to form molecules or discrete polyatomic ions rather than a connected network. For example, CrO satisfies the radius ratio rule, but the highly electronegative Cr ions promote the formation of discrete dichromate(VI) ions, Cr202 , in the presence of other oxides. 

Modifiers in glass are compounds that tend to donate anions to the network, whereas the cations occupy "holes" in the disordered stmcture. These conditions cause the formation of nonbridging anions, or anions that are connected to only one network-forming cation, as shown in Figure 2. Modifier compounds usually contain cations with low charge-to-radius ratios (Z/r), such as alkali or alkaline-earth ions. 

Tricyclohexaprenol, a possible forerunner of sterols in the evolution of biomembranes, was synthesized by construction of the cyclic network in one step using cation-olefin tricyclization and subsequent stereocontroUed attachment of the Cio appendage to ring C. 

The [S4N4] cation 5.18 is a planar eight-membered ring with equal S-N bond lengths ( >4 ) of ca. 1.55 A. It is a fully delocalized ten r-electron system with a strong r-network. ... 

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