Precipitation and Gel Formation

Concentration polarisation, as described in the previous section, can become irreversible if a gel is formed, which can be the case when solute solubilities are exceeded. Concentration polarisation depends strongly on solute concentration and operational conditions, such as pressure and stirring. Fouling of tight UF and NF membranes tends to occur more on the surface than in pores - similar to MF and loose UF. Cake formation is usually reversible and can, as in MF, form a second membrane. Surprisingly, 0degaard and Thorsen (1989) demonstrated that HS concentration and pressure, which influence precipitation and gel formation, had no influence on flux. The fouling layer thickness was calculated with pressure drop and flow. The film was soft, dark brown, and loosely connected to the surface. 

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. 

Noncrystalline solids formation needs to be clarified by reference to the natnre of the final product. Several methods lead to amorphous fine particles, for instance, in solution by controlled precipitation. Through gel formation, poorly crystalline Ti02 can be obtained according to the process Ti(OR)4 -I- H2O Ti(OH)4 gel amorphous Ti02 + H2O. In this case, the compound obtained is thermodynamically stable, as when amorphous films are obtained by chemical vapor deposition, sputtering, flash evaporation, and so on. 

The byproduct DBF is not an easy compound to handle, as it polymerizes rapidly forming precipitates and gels. Thus adduct formation is a highly favorable side reaction however, the process of adduct formation is an equilibrium reaction between DBF and the deblocking amine, where the product distribution at equilibrium depends on the solvent and identity of the amine.f 1 This fact affects also the use of polymer-bound amines for Fmoc cleavage which would, upon adduct formation, facilitate separation of the DBF for synthesis in solution.t 1 However, with nonquantitative adduct formation, this type of resin scavenger is more or less useless,whereas, as discussed in Section 2.1.1.1.1.3.2, resin-bound thiols are much more efficient scavengers for DBF. 

The silica condensation reaction can be done under acidic, alkaline or neutral conditions and all three have been used to synthesise mesoporous materials. Under neutral conditions often the surfactant itself acts as a catalyst for silica condensation as formation of templated materials occurs much more quickly than precipitation or gel formation in silica-only solutions at the same Under neutral conditions addition of F, which... 

Coacervation n. The separation of a polymer solution into two or more liquid phases, one of which is a polymer-rich liquid. The term was introduced to distinguish this phenomenon from the precipitation of a polymer solute in solid form. The process is used in microencapsulation by emulsifying or dispersing the material to be encapsulated with a solution of the polymer. By changing the temperature or concentration of the mixture, or by adding another polymer or solvent, a phase separation may be induced and the polymeric portion forms a thin coating on the external surfaces of the particles. After further treatment to solidify the polymeric wall, the capsules can be isolated in powder form by filtration. It is an intermediate stage between sol and gel formation. 

Thus if a silica sol contains substantial amounts of salts and is neutral or alkaline, a homogeneous gel is not obtained instead, the silica usually appears as a white precipitate, or in some cases as a white opaque gel owing to partial precipitation before gel formation. Similar effects arc observed when small amounts of coagulants... 

Alkali ions (salts) influence the formation of the precursor gel for most of the synthetic zeolites (3,34,39,40). Na+ ions were shown to enhance in various ways the nucleation process (structure-directing role) (40-42), the subsequent precipitation and crystallization of the zeolite (salting-out effect) (JO and the final size and morphology of the crystallites (34,43). Informations on the various roles played by the inorganic (alkali) cations in synthesis of ZSM-5, such as reported in some recent publications (7,8,10,14,17,29,30,44,45) remain fragmentary, sometines contradictory and essentially qualitative. 

Characteristic microstructural properties of TiOj membranes produced in this way are given in Table 2.5. Mean pore diameters of 4-5 nm were obtained after heat treatment at T < 500°C. The pore size distribution was narrow in this case and the particle size in the membrane layer was about 5 nm. Anderson et al. (1988) discuss sol/gel chemistry and the formation of nonsupported titania membranes using the colloidal suspension synthesis of the type mentioned above. The particle size in the colloidal dispersion increased with the H/Ti ratio from 80 nm (H /Ti = 0.4, minimum gelling volume) to 140 nm (H /Ti " — 1.0). The membranes, thus prepared, had microstructural characteristics similar to those reported in Table 2.5 and are composed mainly of 20 nm anatase particles. Considerable problems were encountered in membrane synthesis with the polymeric gel route. Anderson et al. (1988) report that clear polymeric sols without precipitates could be produced using initial water concentrations up to 16 mole per mole Ti. Transparent gels could be obtained only when the molar ratio of H2O to Ti is < 4. Gels with up to 12 wt.% T1O2 could be produced provided a low pH is used (H /Ti + < 0.025). 

A spontaneous polymerization of glucose methacrylate, however, is observed during the monomer preparation without gel formation and a very hydrophilic water soluble polymer with high viscosity is obtained by precipitation in acetone. This procedure requires further improvement. 

Several theories have been proposed to explain the process of gel formation, and diverse views are held as to their actual structure. The conditions favourable to gel formation are experimentally determinable and have to be considered in relationship to the theories proposed (Svedberg, Herstellung Kolloider JjQsungen, 1909 Trans. Farad. Soo. xvi. 3,1921). The factors favourable to precipitation in the form of a gel are the following ... 

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