Peptizing Gels

As early as 1864, Thomas Graham reported that silica gel could be liquefied by a trace of alkali, which he described as peptization of the jelly. In 1922, Praetorius and Wolf (102) produced silica sol from a gel by heating it in water at elevated temperature and pressure. Ncundlinger (103) prepared sols containing about 10% silica by treating the gel first with ammonia and heating without evaporating water until a sol was produced. Improved similar processes were invented by White (5) and Trail (104). 

A modification of the preparation of sol from gel is described by Ahlberg and Simpson (lOS), who formed the gel under alkaline conditions by incompletely neutralizing the alkali of a silicate such as sodium silicate with less than the equivalent acid, then washing out the salts and heating the wet gel to peptize it. Much higher conversion of gel to sol is claimed than when an acidic gel.is first made. Characteristics of sols made by this process are not available, but probably sols 15-45 nm in diameter were produced. A similar-process had been described by Legal (106). Conversion of gel to sol in an autoclave to obtain a 30% ammonia-stabilized sol was patented by Mertz (107). The effect of ultrasonic dispersion of silfca gel was examined by Bubyreva and Bindas (108). 

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Monomer conversion (79) is followed by measuring the specific gravity of the emulsion. The polymerization is stopped at 91% conversion (sp gr 1.069) by adding a xylene solution of tetraethylthiuram disulfide. The emulsion is cooled to 20°C and aged at this temperature for about 8 hours to peptize the polymer. During this process, the disulfide reacts with and cleaves polysulfide chain segments. Thiuram disulfide also serves to retard formation of gel polymer in the finished dry product. After aging, the alkaline latex is acidified to pH 5.5—5.8 with 10% acetic acid. This effectively stops the peptization reaction and neutralizes the rosin soap (80). 

Guizard et al. (1986), Cot, Guizard and Larbot (1988) and Larbot et al. (1989) used a sol-gel method to prepare zirconia membrane top layers on an alumina support. The water necessary for the hydrolysis of the Zr-alkoxide was obtained from an esterification reaction. The complete hydrolysis was done at room temperature and resulted in a hydrated oxide. The precipitate was peptized with nitric or hydrochloric acid at pH <1.1 and the final sol... 

Bischoff BL, Anderson MA (1995) Peptization process in the sol-gel preparation of anatase Xi02. Chem Mater 7 1772-1778... 

Mack (58, 59) points out that asphaltenes from different sources in the same petro-lenes give mixtures of approximately the same rheological type, but sols of the same asphaltenes in different petrolenes differ in flow behavior. Those in aromatic petrolenes show viscous behavior and presumably approach true solution. Those in paraffinic media show complex flow and are considered to be true colloidal systems. Pfeiffer and associates (91) consider that degree of peptization of asphaltene micelles determines the flow behavior. Thus, a low concentration of asphaltenes well peptized by aromatic petrolenes leads to purely viscous flow. High concentrations of asphaltenes and petrolenes of low aromatic content result in gel-type asphalts. All shades of flow behavior between these extremes are observed. 

F. S. Brown and C. R. Bury 6 obtained colloidal solutions of phosphorus pentoxide in nitrobenzene by stirring the mixture in the presence of alcohols or organic acids. The hydroxy-compound is absorbed and peptizes the pentoxide. Traces of moisture cause coagulation. Cone. soln. set to gels on keeping. 

In the particulate-sol method a metal alkoxide dissolved in alcohol is hydrolyzed by addition of excess water or acid. The precipitate that results is maintained as a hot solution for an extended period during which the precipitate forms a stable colloidal solution. This process is called peptization from the Greek pep—to cook (not a misnomer many descriptions of the sol-gel process have a strong culinary flavor). The colloidal solution is then cooled and coated onto the microporous support membrane. The layer formed must be dried carefully to avoid cracking the coating. In the final step the film is sintered at 500-800 °C. The overall process can be represented as ... 

Sols are obtained via either colloidal or polymeric routes. In the first method, colloids are formed and stabilized by adding peptizing agents (acidic or basic electrolytes) to metal hydroxides, and the gel is obtained by evaporating the solvent. In the second (polymeric) method, alkoxides are used as starting materials and hydrolysis and condensation reactions control the size of the resulting clusters (temperature and pH are the critical parameters). Additives such as surfactants may also play an important role in the sol characteristics by controlling the hydrolysis step of the alkoxides [25]. 

The principle of the sol-gel process is as follows. A sol is prepared by peptizing a well suited alumina powder with an acid. The powder particles or agglomerates are stabilized by a positively charged layer on the surface of these particles. An increase in pH allows the hydroxyl ions to neutralize the positive ions on the surface of the particles. These particles will grow together to larger particles and gelation will occur. The sol-gel process can be carried out in several ways ... 

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