Precipitation-gelation

Notes A Absorption O Autoxidative/polymerization P Precipitation/gelation R Photochemical/polymerization, ultraviolet cure S Sheet-fed W Web-fed,... 

Synthetic Insoluble Silicates. Insoluble crystalline siUcates, ie, mineral-type compounds, are synthesized from soluble siUcates by precipitation, gelation, ion exchange, and hydrothermal techniques. Hydrothermal treatment of partially neutralized, high mole ratio (m = 12—50), sodium siUcate solutions yields neutral alkaU polysiUcates that exhibit a layered stmcture and high ion-exchange capacity (135,136). These and other lamellar siUcates can be utilized either alone or modified via pillaring (137) as adsorbents and catalysts. 

Organized hybrid materials can be provoked through chemical (i.e., precipitation, gelation) or physical (i.e., evaporation, temperature) solicitations. This step is the most delicate one because phase segregation, self-assembly, and condensation must occur in this exact order to end up with the desire architecture. 

Two conceptually distinct phase transitions are thought to occur during coagulation gelation and phase separation (or precipitation). Gelation is the gradual transition of the... 

In the presence of (l/ ,2/ )-(- -)-l,2-diphenyl-l,2-ethanediamine, the gel underwent a complete disassembly and a precipitate was formed corresponding to the [1-1-1] cyclic component Here, the main role of the gel is to act as a separate phase in a saturated solution that releases its components when the concentration of soluble species is decreased by reaction, according to precipitation/gelation... 

Besides pH, other preparative variables that can affect the microstructure of a gel, and consequendy, the properties of the dried and heat-treated product iaclude water content, solvent, precursor type and concentration, and temperature (9). Of these, water content has been studied most extensively because of its large effect on gelation and its relative ease of use as a preparative variable. In general, too Httie water (less than one mole per mole of metal alkoxide) prevents gelation and too much (more than the stoichiometric amount) leads to precipitation (3,9). Other than the amount of water used, the rate at which it is added offers another level of control over gel characteristics. 

An ink is considered dry when a print does not stick or transfer to another surface pressed into contact with it. Drying is accompHshed by one or more of the following physical or chemical mechanisms absorption, evaporation, precipitation, oxidation, polymerization, cold setting, gelation, and radiation curing. 

Most ultrafiltration membranes are porous, asymmetric, polymeric stmctures produced by phase inversion, ie, the gelation or precipitation of a species from a soluble phase (see Membrane technology). 

In conclusion, the self-condensation of 2-furaldehyde promoted by heat occurs with the formation of di- and trifurylic intermediates. The functionality of the growing chain increases after each oligomerization step until gelation and precipitation of the resin occurs. Thus, the process is non-linear from the onset since the condensation product 4 possesses three sites for further attack, namely the free C-5 position and the two formyl groups. It is interestering to note that while the polycondensation of 2-furfuryl alcohol is essentially linear and cross-linking is due to side reactions, the thermal resinification of 2-furaldehyde is intrinsically non-linear and gel formation occurs at earlier conversions. 

The effectiveness with which divalent ions cause gelation of poly(acrylic add) has been found to follow the order Ba > Sr > Ca (Wall Drenan, 1951) and this has been attributed to the formation of salt-like crosslinks. Gelation has been assumed to arise in part from dehydration of the ion-pairs (Ikegami Imai, 1962), and certainly correlates with predpitation in fairly dilute systems. Indeed, the term precipitation has sometimes been applied to the setting of AB cements derived from poly(acrylic add) as they undergo the transition from soft manipulable paste to hard brittle solid. 

At the molecular level, a number of features are associated with the phenomenon of gelation or precipitation. In particular the disruption of the secondary hydration sheaths around the polyacrylate chains appears... 

Figure 5.11 (Crisp Wilson, 1974b) shows the time-dependent variation of the concentration of soluble ions in setting and hardening cements. Note that the concentrations of aluminium, calcium and fluoride rise to maxima as they are released from the glass. After the maximum is reached the concentration of soluble ions decreases as they are precipitated. Note that this process is much more rapid for calcium than for aluminium and the sharp decline in soluble calcium corresponds to gelation. This indication is supported by information from infrared spectroscopy which showed that gelation (initial set) was caused by the precipitation of calcium polyacrylate. This finding was later confirmed by Nicholson et al. (1988b) who, using Fourier transform infrared spectroscopy (FTIR), found that calcium polyacrylate could be detected in the cement paste within one minute of mixing the cement. There was no evidence for the formation of any aluminium polyacrylate within nine minutes and substantial amounts are not formed for about one hour (Crisp et al, 1974).

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