Gel structure

In biological systems molecular assemblies connected by non-covalent interactions are as common as biopolymers. Examples arc protein and DNA helices, enzyme-substrate and multienzyme complexes, bilayer lipid membranes (BLMs), and aggregates of biopolymers forming various aqueous gels, e.g, the eye lens. About 50% of the organic substances in humans are accounted for by the membrane structures of cells, which constitute the medium for the vast majority of biochemical reactions. Evidently organic synthesis should also develop tools to mimic the Structure and propertiesof biopolymer, biomembrane, and gel structures in aqueous media. 

Microreticular Resins. Microreticular resins, by contrast, are elastic gels that, in the dry state, avidly absorb water and other polar solvents in which they are immersed. While taking up solvent, the gel structure expands until the retractile stresses of the distended polymer network balance the osmotic effect. In nonpolar solvents, little or no swelling occurs and diffusion is impaired. 

Carbopol is widely used in cosmetic and pharmaceutical practice as a gel-former. Carbopol resins are hydrophilic polymers which swell in water solutions and transform into the gel form at neutralization. At the elaboration of special cosmetic preparations in which carbopol is used, the problem of raw materials compatibility appears. For example, some extracts of aromatic pectin containing materials destroy the gel structure of carbopol. High contents of bivalent metal ions, in particular calcium ions, destructively influence onto the gel-making ability of the system too. 

Beside the crystalline material, a certain portion of amorphous lead dioxide is always observed. In the working electrode such amorphous material is apparently hydrated and forms a gel structure at the phase boundary between the solid material and the electrolyte (cf. Ref. [6]). 

An advanced solution to the problem of decreasing the free mobility of the electrolyte in sealed batteries is its gel formation. By adding some 5-8 wt.% of pyrogenic silica to the electrolyte, a gel structure is formed due to the immense surface area (-200-300 m2 g ) of such silicas, which fixes the sulfuric acid solution molecules by van der Waals bonds within a lattice. These gels have thixotropic properties i.e., by mechanical stirring they can be liquefied and used to Filled into the... 

Elharfaoui N., Djabourov M., Babel W.. Molecular weight influence on gelatin gels structure, enthalpy and rheology. Macromolecular Symposia 256 (2007) 149-157. 

Johansson and coworkers [182-184] have analyzed polyacrylamide gel structure via several different approaches. They developed an analytical model of the gel structure using a single cylindrical unit cell coupled with a distribution of unit cells. They considered the distribution of unit cells to be of several types, including (1) Ogston distribution, (2) Gaussian distribution of chains, and (3) a fractal network of pores [182-184]. They [183] used the equilibrium partition coefficient... 

Maaloum, M Pernodet, N Tinland, B, Agarose Gel Structure Using Atomic Force Microscopy Gel Concentration and Ionic Strength Effects, Electrophoresis 19, 1606, 1998. Mackie, IS Meares, P, The Diffusion of Electrolytes in a Cation-Exchange Resin Membrane I. Theortical, Proceedings of the Royal Society of London Series A 232, 498, 1955. 

The gelling temperature is an important factor for the characterization and application of pectins. The pectin consumer wants a pectin fulfilling his special requirements, this can mean either working with or without pregelation. Pregelation, the weakening of gel structure, occurs when pectin preparations are stressed below their gelation temperature so that the mechanical treatment leads to an irreversible destruction of the three-dimensional network. 

Qualitatively, the results observed with ferric chloride and lead acetate were highly variable. It was first noted with ferric chloride that gels were forming within the reaction vessel We observed the formation of translucent gels in the reaction vessel with calcium chloride, spermidine and ferric chloride. In the case of lead acetate, a feathery type precipitate formed in the reaction vessel. Macdonald et al. (14) observed formation of a clear gel and flocculated precipitate in high ester pectin treated with lemon endocarp and peel PE isozymes, respectively. They hypothesized that the different gel structures were due to unique mechanism of deesterification by the PE isozymes. Our results with different cations and formation of different gel structure or precipitate seems to be similar to that reported by Macdonald et al. (14). If there is a different mechanism of de-esterification for plant PEs,... 

Reference to the infinite structures as networks would seem inconsistent with our assumption, introduced as an approximation, that no intramolecular reactions occur. A randomly branched structure devoid of intramolecular linkages could hardly be called a network the latter term conveys the notion of circuitous interconnections within the structure. Actually, as will appear later, the assumption referred to need only be applied to the finite molecular species its extension to the infinite structure is superfluous. Certainly it will contain an abundance of intramolecular connections, which, in fact, is an essential feature of the gel structure... 

The fact that the initial setting process for magnesium oxychloride cements takes place without observable formation of either the 5 1 8 or the 3 1 8 phase is important. It indicates that formation of an amorphous gel structure occurs as the first step, and that crystallization is a secondary event which takes place from what is effectively a supersaturated solution (Urwongse Sorrell, 1980a). This implies that crystallization is likely to be extremely dependent upon the precise conditions of cementition, including temperature, MgO reactivity, heat build-up during reaction and purity of the components in the original cement mixture. 

The senior author first became interested in acid-base cements in 1964 when he undertook to examine the deficiencies of the dental silicate cement with a view to improving performance. At that time there was much concern by both dental surgeon and patient at the failure of this aesthetic material which was used to restore front teeth. Indeed, at the time, one correspondent commenting on this problem to a newspaper remarked that although mankind had solved the problem of nuclear energy the same could not be said of the restoration of front teeth. At the time it was supposed that the dental silicate cement was, as its name implied, a silicate cement which set by the formation of silica gel. Structural studies at the Laboratory of the Government Chemist (LGC) soon proved that this view was incorrect and that the cement set by formation of an amorphous aluminium phosphate salt. Thus we became aware of and intrigued by a class of materials that set by an acid-base reaction. It appeared that there was endless scope for the formulation of novel materials based on this concept. And so it proved. 

Although there seem to be definite categories, there is an amazing degree of interrelation between these functions. Viscosity alters a gel structure. A... 

H. Muller, W. Breuer, C. P. Herold, P. Kuhm, and S. von Tapavicza. Mineral additives for setting and/or controlling the rheological properties and gel structure of aqueous liquid phases and the use of such additives. Patent US 5663122,1997. 

Thixotropy is a phenomenon that occurs frequently in dispersed systems. It is defined as a reversible, time-dependent decrease in viscosity at a constant shear rate. Generally, a dispersion that shows an isothermal gel-sol-gel transformation is a thixotropic material. The mechanism of thixotropy is the breakdown and reforming of the gel structure. 

Thixotropic formulations such systems exhibit shear thinning when agitated and thus are pumpable. Yet, when agitation stops, the system rapidly establishes a gel structure, thereby avoiding leakage. 

Phospholipids, which are one of the main structural components of the membrane, are present primarily as bilayers, as shown by molecular spectroscopy, electron microscopy and membrane transport studies (see Section 6.4.4). Phospholipid mobility in the membrane is limited. Rotational and vibrational motion is very rapid (the amplitude of the vibration of the alkyl chains increases with increasing distance from the polar head). Lateral diffusion is also fast (in the direction parallel to the membrane surface). In contrast, transport of the phospholipid from one side of the membrane to the other (flip-flop) is very slow. These properties are typical for the liquid-crystal type of membranes, characterized chiefly by ordering along a single coordinate. When decreasing the temperature (passing the transition or Kraft point, characteristic for various phospholipids), the liquid-crystalline bilayer is converted into the crystalline (gel) structure, where movement in the plane is impossible. 

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