Superficial network

Addition of an alkali metal oxide as a "network modifier to the "network former causes pH sensitivity, i.e., small amounts of alkali metal induce superficial gel layer formation as a merely local chemical attack and so with limited alkali error larger amounts will result in more pronounced dissolving properties of the glass up to complete dissolution, e.g., water-glass with large amounts of sodium oxide. Simultaneous addition of an alkaline earth metal oxide, however, diminishes the dissolution rate. Substitution of lithium for sodium in pH-sensitive glass markedly reduces the alkali error. 

On exposure to water, an anhydrous oxide can become hydrated by physical adsorption of water molecules without dissociation, dissociative chemisorption of water leading to new hydroxy groups, and finally to the formation of superficial oxyhydroxide or hydroxide, such as for MgO [14]. When silica groups are exposed to water for an extended time, their hydroxylation produces polymeric chains of -Si(0H)2-0-Si(0H)2 0H groups which can link up to form three-dimensional silica gel networks. Around 2 nm thick silica gel layers have been observed on silica surfaces prepared by evaporation of silica on mica which were exposed to humid air [70], Thus, it may be postulated that surface groups are present not only in a two-dimensional oxide-liquid interface, but also in a bulk phase of finite thickness extending from the surface into the interior of the solid [71]. 

The scalp is supplied by branches of the external carotid artery, particularly the superficial temporal, occipital and posterior auricular arteries. Above the orbit, there is a contribution from terminal branches of the ophthalmic artery. There is a rich anastomotic network between the various arteries of the scalp. 

Moyamoya syndrome (Ch. 6) causes gradual stenosis or occlusion of the terminal portions of the internal carotid arteries or middle cerebral arteries. This leads to formation of an abnormal collateral network of fragile vessels, which occasionally rupture. It has been proposed that constructing a bypass to relieve the pressure on the collaterals would be beneficial, for example between the superficial temporal artery and the middle cerebral... 

The airways receive their blood supply from the systemic circulation via laryngeal, tracheal and bronchial arteries. In the trachea and large bronchi of humans and other large mammals, branches of arterioles deep in the airway wall supply the airway mucosa. In some peripheral airways, mucosal vessels may be supplied by the pulmonary circulation through vascular anastomoses. The airway mucosa has a rich blood supply, with a p>articularly extensive vascular network located immediately beneath the epithelium. Mucosal capillaries are tributaries of an extensive system of venules. In humans, a superficial plexus of venules connects with another plexus of venules located deeper in the mucosa. 

The purpose of the present survey is to examine and visualize the potential that organotin(IV) complexes can have in metal-directed self-assembly of macrocycles and extended networks. Their application in the formation of cage- and cluster-type structures is only mentioned superficially, since this chemistry is described in other sections within this chapter. 

---