Gelator organic solvent

Fig. 15 Dendrons and symmetric dendrimers capable of gelating organic solvents...

Steed and co-workers synthesized tris(urea) 38c which gelated organic solvents. The formation of the helical gel fibers could be partially inhibited by addition of chloride, which was bound by the gelator, resulting in fully crystalline material, as characterized by X-ray crystallography [80]. 

Sol—Gel Technology. The sol—gel process involves conversion of a metal alkoxide or mixture of metal alkoxides, dissolved in an organic solvent (generally the patent alcohol) into a hydroxooxyalkoxide sol, followed by gelation and sintering to give the desired ceramic material. 

MethylceUulose with a methyl DS less than about 0.6 is alkali-soluble. Erom about 1.6 to 2.4, it is water-soluble (most commercial grades) above 2.4, it is soluble in a wide variety of organic solvents. MethylceUulose solutions in water start to gel at 55° C, independent of molecular weight. The gelation is a function of the DS, rate of heating, and type and amounts of additives such as salts. As the temperature increases, the viscosity initially decreases (typical behavior). When the gelling temperature is reached, the viscosity sharply rises until the flocculation temperature is reached. Above this temperature, the viscosity coUapses. This process is reversible with temperature (75). 

One of the simplest ways to prepare a chitin gel is to treat chitosan acetate salt solution with carbodiimide to restore acetamido groups. Thermally not reversible gels are obtained by AT-acylation of chitosans N-acetyl-, N-propionyl- and N-butyryl-chitosan gels are prepared using 10% aqueous acefic, propionic and bufyric acid as solvents for treatment with appropriate acyl anhydride. Both N- and 0-acylation are found, but the gelation also occurs by selective AT-acylation in the presence of organic solvents. 

Numerous nanostructures such as nanorods, nanotubes, or nanorings have been obtained by this approach. In particular, the use of organoge-lators as directors to control the morphologies of the aggregates has been explored recently. Cholesterol- and phospholipid-thethered trans-styl-benes are able to gelate different organic solvents in which the steroid and the lipid units serve as templates to form one-dimensional stacks. 

The properties of polymethylsiloxanes are also greatly determined by the conditions of the reaction and the type of the solvent used. Thus, the hydrolytic condensation of methyltrichlorosilane (the main raw stock in the production of polymethylsiloxanes) with iced water or water vapour in a medium of nonpolar solvents forms a nonmeltable and nonsoluble amorphous substance. If the condensation is carried out by pouring methyltrichlorosilane into the emulsion of water and butyl alcohol gradually (to avoid gelation), at intensive agitation and reduced temperature (about 0 °C), it forms a viscous substance, which is soluble in organic solvents and briefly heated to 150 °C loses its meltability and solubility. 

Oda et al. described the use of gemini surfactant/tartaric acid admixtures which could trap up to 1200 solvent molecules per molecule of the gelator [72]. In these gelators, the biscationic geminis contain chiral counterions and they associate to form gels with both water and organic solvents such as chloroform... 

In a similar way, other derivatives of diamines (S,S)-28 and (R,R)-29 were evaluated for their gelation properties. As an example, trans-(R,R) 1,2-bis(dodecy-lureido)cyclohexane and trans-(R,R)-1,2-bis(octadecylureido)cyclohexane can cause physical gelation in a wide variety of organic solvents [76]. As an extension of these studies, novel polymerizable organogelators based on tra s-(7 ,7 )-l,2-bis(ur-eido)cyclohexane derivatives have been reported [77]. In these molecules, the presence of methacrylate-functionalized carboxylic acid groups determines the... 

Hydrophilic nanoparticle carriers have important potential applications for the administration of therapeutic molecules [28,53]. Most of the recently developed hydrophobic-hydrophilic carriers require the use of organic solvents for their preparation and have a limited protein-loading capacity [54,55]. Calvo et al. [56] reported a new approach for the preparation of nanoparticles, made solely of hydrophilic polymer, to address these limitations. The preparation technique, based on an ionic gelation process, is extremely mild and involves the mixing of two aqueous phases at room temperature. 

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