Aqueous gels definition

This study has shown that MCC forms a gel-like structure at low solids concentration in the presence of salt. It has also been known for a considerable time (20) that MCC forms aqueous gels and pastes but at much higher solids concentration. It would appear that further studies with the MCC gel in dilute form may elucidate the mechanism of gelation more definitively. 

Control of the particle size while retaining precise control over the release rate is enabled by compartmentalization of the sol-gel solution into droplets of definite size. This can be achieved by emulsification of the sol-gel solution by mixing it with a solution composed of a surfactant and a non-polar solvent (Figure 2.13). When an active molecule is located in the aqueous droplet of a W/O emulsion, encapsulation occurs as the silicon precursors polymerize to build an oxide cage around the active species. By changing the solvent-surfactant combination, the particle size can be varied from 10 nm to 100 pm as the size of the particles is controlled by the size of the emulsion droplet, which acts as a nano-reactor for the sol-gel reaction (Figure 2.13). 

Precipitation of ferric hydroxide gel was also observed in the preparation of spindlelike hematite (a-Fe203) particles in a dilute ferric chloride solution in the presence of phosphate (9). In this case, however, the positive role of the gel was not definite since similar uniform hematite paricles were obtained as well in homogeneous systems in the presence of the same anions (9). Also, Hamada and Matijevic (10) prepared uniform particles of pseudocubic hematite by hydrolysis of ferric chloride in aqueous solutions of alcohol (10-50%) at I00°C for several days. In this reaction, it was observed that acicular crystals of (3-FeOOH precipitated first, and then they dissolved with formation of the pseudocubic particles of hematite. The intermediate P-FeOOH appears to work as a reservoir of the solute to maintain an ideal supersaturation for the nucleation and growth of the hematite. Since the (3-FeOOH as an intermediate and the pseudocubic shape tire not peculiar to the alcohol/water medium... 

The general conclusion of the mentioned works was that the appearance of the jump on the dependences of network volume on the composition of the solvent or on temperature is reached only at some definite content of ionic groups in the network chains. For neutral networks with flexible chains, the collapse is usually not observed. Exceptions to this rule were reported for poly(isopropylacrylamide) (PIPAA) [16], poly(vinylcaprolactam) and poly-(2-vinylpyrrolidone) [17] gels. The specific feature of these systems is that the transition takes place in structured solvents water or concentrated aqueous solutions of aluminium sulfate. 

The definition of the limit for dilute solutions is obviously important for the proper use of available chemical data. Moreover, as this property is also of particular importance for the sol-gel process discussed later it deserves further analysis. In the Debye-Hiickel ionic-cloud model, the influence of dissolved ions on a particular central ion is evaluated by comparing the mean distance between ions to the dimension of the ion cloud surrounding it. The latter is evaluated by the so-called Debye (or Debye-Hiickel reciprocal) length (1/k), which may be approximatively written for aqueous solutions at 25°C as ... 

Any formal definition of a gel has been avoided, because its formulation would be difficult on the basis of external properties. By any criterion, a 1 % aqueous solution of agar does form a gel when it is cooled from 95 to 20°, whereas a 1 % solution of sucrose does not but properties may be so continuous between these extremes that any dividing line would be arbitrary. Most of this Chapter is concerned with fairly permanent network structures formed from polymer solutions. As a definition of gels, this description is incomplete. Some cellulose gels, and pastes of gelatinized, starch granules, would be excluded, because they are formed by limited dispersion of solids. In other words, an arbitrary choice will be made to focus on those gels that are dilute with respect to polymer, because the information available about them at the molecular level is more precise than for others. 

However, by making use of this definition, one has to conclude that many systems which look like a gel are in fact not covered by this definition aqueous poly(vinyl alcohol)/borate systems, which are known to show liquid-like behaviour at low frequencies [7], solutions of phase separated atactic polystyrene at temperatures above the glass transition temperature of the swollen polystyrene crosslinks, solutions of ABA block copolymers above the glass transition temperature of the swollen A-blocks, and even gelatin, which also shows creep behaviour, as shown by Ross-Murphy et al. [8,9] and by Kramer et al. (private communication), and a relaxation mechanism at extremely low frequencies, as is shown in Fig. 7 of the Section on gelatin, and possibly ako poly(vinyl chloride) in plasticizers [10-13]. The advantage of the approach of Kramer et al. [3] is that these systems certainly are covered by their practical definition. 

Before this chapter is concluded, the question raised at the outset is to be answered. That is, whether or not one can reasonably attribute the nonpolar affinity of water-swollen hydrophilic gels to the hydrophobic interaction. The answer depends on the definition of the hydrophobic Interaction if it is defined as "any favorable nonpolar interaction in aqueous media", then the answer is "yes". It is further concluded that this nonpolar affinity stems from the intrinsic nonpolar affinity of the monomeric residues that is markedly enhanced in the gels by a cooperative mechanism involving the water/solute/gel matrix interactions. 

The sol-gel process - either aqueous or nonaqueous - is one of the most important processes for the preparation of oxidic nanopartides. For silica particles, the sol-gel-based Stober process is definitely the most used wet chemical preparation route. Particularly, the mild reaction conditions combined with the excellent control over nanopartides properties malce it a universal method for the production of colloids for various applications. In recent years, the metal oxide routes have also become more and more sophisticated. Another reason for the attractiveness of the preparation route is the activity of the derived particles toward surface functionalization. Such modified particles can be easily incorporated into polymer matrices to obtain nanocomposites with extraordinary properties. 

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