Network solids classification

The elements that form network solids lie on the right side of the periodic table, bordering the elements that form molecular crystals on one side and those that form metals on the other. Thus they are intermediate between the metals and the nonmetals. In this borderline region classifications are sometimes difficult. Whereas one property may suggest one classification, another property may lead to a different conclusion. Figure 17-3 shows some elements that form solids that are neither wholly metallic nor wholly molecular crystals. 

The forces between O2 molecules, the lattice particles of solid oxygen, do not fit into any of the classifications we have discussed to this point. The O2 molecule is not polar and contains no ionic or metallic bonds, and solid oxygen melts at much too low a temperature to fit into the category of a network solid. The forces between O2 molecules are called dispersion forces and result from momentary nonsymmetric electron distributions in the molecules. 

Part of the process to make cheese involves the flocculation of an electrostatically stabilized colloidal O/W emulsion of oil droplets coated with milk casein. The flocculation is caused by the addition of a salt, leading to the formation of networks which eventually gel. The other part of the process involves reaction with an enzyme (such as rennet), an acid (such as lactic acid), and possibly heat, pressure and microorganisms, to help with the ripening [811]. The final aggregates (curd) trap much of the fat and some of the water and lactose. The remaining liquid is the whey, much of which readily separates out from the curd. Adding heat to the curd (-38 °C) helps to further separate out the whey and convert the curd from a suspension to an elastic solid. There are about 20 different basic kinds of cheese, with nearly 1000 types and regional names. Potter provides some classification [811]. 

Recourse to characterization of tertiary and finer sub-classification is generally unnecessary. Indeed, despite this complex frame, in the majority of the situations, drug release field deals with polymeric membranes made up of a continuous phase (usually a liquid phase) trapped in a swollen solid phase (polymeric netwoik) [4]. This stmctme can be seen as a coherent system, having mechanical characteristics in between those of solids and liquids [5]. The presence of cross-links between polymeric chains hinders polymer dissolution in the liquid phase that can only swell the network. This stmctme is roughly similm to that of sponge filled by a liquid phase. Nevertheless, this is a particulm sponge as, in... 

The geometrical structure of pores is of great concern, but the three-dimensional description of pores is not established in less-crystalline porous solids. Only intrinsic crystalline intra-particle pores offer a good description of the structure. The hysteresis analysis of molecular adsorption isotherms and electron microscopic observation estimate the pore geometry such as cylinder (cylinder closed at one end or cylinder open at both ends), cone shape, slit shape, interstice between closed-packing spheres and inkbottle. However, these models concern with only the unit structures. The higher order structure of these unit pores such as the network structure should be taken into accoimt. The simplest classification of the higher order structures is one-, two- and three-dimensional pores. Some zeolites and aluminophosphates have one-dimensional pores and activated carbons have basically two-dimensional slit-shaped pores with complicated network structures [95]. 

The present article will aim to give a literature survey of the studies pertaining to the production of porous carbons (in particular activated carbon) from plastic polymers. As far as the authors are aware, this is the first such review article. Activated carbons (ACs) are porous solids with desirable properties that include high thermal stability, high surface area, and high chemical resistance. The high surface area is produced by the numerous pore networks inside the material. It should be noted that the current lUPAC classification of pores is as follows [17] ... 

Conner and coworkers (refs. 7,8) have recently utilized a pore/throat network model to obtain information about the morphology of materials from mercury penetration data. The void/solid structure is viewed as an interconnected network so that adsorption/desorption and retraction/intrusion can be associated with the openings and constrictions within the void network. These latter investigators analyzed the data as if the materials consisted of agglomerated microspheres. The measured ratio of the most probable radii of intrusion to those of retraction seemed to be characteristic of the void structure and pore shape. Conner et al. (ref. 8) developed a heuristic diagram for the classification of void/solid morphologies from a... 

---