Thermally irreversible gel

Figure 12.1 Total and irreversible ammonia adsorption on silica gel, thermally pretreated at 473, 673 and 973 K.

Low methoxyl pectins (< 50% esterified) form thermoreversible gels in the presence of calcium ions and at low pH (3 - 4.5) whereas high methoxyl pectins rapidly form thermally irreversible gels in the presence of sufficient (e.g., 65% by weight) sugars such as sucrose and at low pH (< 3.5) the lower the methoxyl content, the slower the set. 

Whey proteins denature at temperatures above 70°C and become insoluble and form thermally irreversible gels. 

The thermally-reversible gel returns to the solution because the droving forces are noncovalent bonds such as hydrogen bonding, hydrophobic interaction, n-n interaction and electrostatic interaction. These noncovalent bonds are broken down by heating, so the gel reverts to the sol. Since the thermally-irreversible gel forms a network structure by firm chemical covalent bonds, the gel formed never changes to the sol. This is why the thermally-irreversible gel is called physical gel and the thermally-irreversible gel is referred to as chemical gel. Moreover, the gelation ability is found in not only biopolymers but also low molecular weight compounds. 

In chemical gels the network connection (crosslink) is usually a covalent bond, which leads to a thermally irreversible gel. When the crosslinking is purely physical in nature, a physical gel is formed which is thermally reversible. There have been at least two excellent works published in the last few years. Reversible Polymeric Gels and Related Systems by Russo [2] and Thermoreversible Gelation of Polymers and... 

Forms thermally irreversible gels on heating in the presence of high soluble solids (>50%) due to intermolecular hydrophobic association. 

Form thermally irreversible gels on heating. The protein chains unfold and reassociate to form aggregates which in turn generate a three-dimensional network. 

Forms thermally irreversible gels at room temperature in the presence of divalent ions (notably calcium) LM pectin... 

Forms very strong thermally irreversible gels in the presence of alkali following deacetylation by association of the polymer chains through hydrogen bonding. 

Konjac mannan dissolves in water to form highly viscous solutions. It is acetylated ( 1 acetyl group for every 19 sugar residues) and in the presence of alkali, deacetylation occurs and thermally irreversible gels are produced. The rate of gelation is dependent on the polymer and alkali concentration and also on the temperature. This is illustrated in Figure 4.20... 

This paper describes a novel thermally reversible chitosan oxalate gel, a thermally irreversible chitosan gel, and a thermally irreversible chitin gel (N-acetylchitosan gel or regenerated chitin). An intramolecular conversion of the chitosan oxalate gel into the chitin gel via the chitosan gel is discussed. The chitin gel may play a role of the structural processing of crab and shrimp shells and insect cuticle. A sheet of transparent film has been prepared from the gel slice by air-drying, 2 and a sheet of non-transparent porous by freeze-drying. The gel may be usable as a digestible material for drug delivery systems and as media for the immobilization of enzymes and cells. 

The ammoniation of chlorosilylated silica will be exemplified for the case of trichlorosilylated silica.15,18 Figure 12.5 shows the ammonia uptake (irreversibly adsorbed and/or chemisorbed) on silica gel with and without trichlorosilane modification. The silica gel was thermally pretreated at 973 K and the trichlorosilylation was performed at standard conditions (623 K, lh). 

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