Liquid crystals/surfactant precipitation

As described above, interactions between oppositely charged surfactants and polyelectrolytes in aqueous solutions can lead to associative phase separation, where the concentrated phase assumes the form of a viscous liquid, gel, liquid crystal or precipitate. This behavior has been exploited to form gel particles, which have been prepared by drop-wise addition of cellulose-based polycation solution (chitosan or-, N,N,N-trimethylammonium derivatized hydroxyethyl-cellulose) to anionic (sodium dodecyl sulfate, sodium perfluorooctanoate) and cationic (cetyltrimethylammonium bromide/sodium perfluorooctanoate) surfactant solutions [76-80]. 

For ionic surfactants micellization is surprisingly little affected by temperature considering that it is an aggregation process later we see that salt has a much stronger influence. Only if the solution is cooled below a certain temperature does the surfactant precipitate as hydrated crystals or a liquid crystalline phase (Fig. 12.4). This leads us to the Krafft temperature1 also called Krafft point [526]. The Krafft temperature is the point at which surfactant solubility equals the critical micelle concentration. Below the Krafft temperature the solubility is quite low and the solution appears to contain no micelles. Surfactants are usually significantly less effective in most applications below the Krafft temperature. Above the Krafft temperature, micelle formation becomes possible and the solubility increases rapidly. 

An explanation for this gel formation is sought in the phase transition behavior of span 60. At the elevated temperature (60 °C) which exceeds the span 60 membrane phase transition temperature (50 °C) [154], it is assumed that span 60 surfactant molecules are self-assembled to form a liquid crystal phase. The liquid crystal phase stabilizes the water droplets within the oil. However, below the phase transition temperature the gel phase persists and it is likely that the monolayer stabilizing the water collapses and span 60 precipitates within the oil. The span 60 precipitate thus immobilizes the liquid oil to form a gel. Water channels are subsequently formed when the w/o droplets collapse. This explanation is plausible as the aqueous volume marker CF was identified within these elongated water channels and non-spherical aqueous droplets were formed within the gel [153]. These v/w/o systems have been further evaluated as immunological adjuvants. 

The effect of temperature increase is typical for surfactants whose solubility increases with temperature increase, converting all liquid crystal phases to micellar solutions when the temperature is high enough. At high surfactant concentration and low temperature, solid surfactant may precipitate. 

The problem of small particle sizes due to precipitation can best be avoided by soHdifying surfactant LLC bulk phases that contain an inorganic precursor. Furthermore, soHdifying such media means that the degree of predictability of such syntheses can be vastly improved, as liquid crystal phases can be analyzed by a variety of simple, noninvasive analytical techniques. 

These mesoporous molecular sieves are prepared using a liquid crystal templating mechanism in which micelles, which are assemblies of cationic alkyl trimethylammonium surfactants [CH3(CH2) N+(CH3)3] X , act as a template for the formation of the silicaceous material (Figure 6.2). In the silicate-rich aqueous solution, the hydrophobic tails of the surfactant cluster together, leaving the positively charged heads to form the outside of the rod-like liquid crystal micelles. The silicate anions are attracted to, and surround the micelles, aggregating into an open-framework amorphous solid, which precipitates. The solid is filtered off, and heated in air at up to 700 °C (calcination), which removes the surfactant and leaves the... 

The AC catanionic compound is thus actually much less ionic than its separated components. Experimental evidence indicates that such a surfactant mixture in water results in the formation of either a precipitate, a liquid crystal, or a vesicle structure, with a molecular arrangement that is likely to be of the bilayer type, which is no wonder, as the association compound obviously looks like a double-tailed surfactant [91-97]. 

With further additions of one of the surfactants, the trend ends up at about 20%, and the three-phase system becomes coexistent with a liquid crystal phase (as indicated by the number 4). Further to the center of the diagram, the three-phase behavior disappears and a crystallized precipitate is exhibited instead (as indicated by the letter C). 

The sol-gel method is also used to make very fine spherical particles of oxides. By structuring the solvent with surface-active solutes, other forms can also be realized during condensation of the monomeric reactant molecules to form a solid particle. Figure 8.16 shows that normal or inverse micelles or liquid crystals (liquids having long-distance order) can be formed in such solutes. Micelles are small domains in a liquid that are bounded by a layer of surface-active molecules. In these domains the solid is condensed and the microstructure of the precipitated solid is affected by the micelle boundaries. Monodisperse colloidal metal particles (as model catalyst) have been made in solvents that have been structured with surfactants. In the concentration domains where liquid crystals obtain highly porous crystalline oxides can be condensed. After calcination such solids can attain specific surface areas up to 1000 m /g. Micro-organisms use structured solutions when they precipitate calcite, hematite and silica particles. 

Davies, G.A., Ming, Y. and Garside, J. (1990) The selective separation and precipitation of salts in a liquid surfactant membrane system. In Industrial Crystallization 90 (11th Symposium, Garmisch-Partenkirchen), A. Mersmann (ed.), GVC-VDI, 163-168. 

In a later work of similar nature, Chatterjee etal. [170] prepared a boehmite sol from an aluminum salt. Boehmite particles were precipitated from an aluminum nitrate solution heated to 80 -90 C by addition of an ammonia solution into it. The precipitate was washed and peptized with glacial acetic acid to obtain a colloidal sol. The W/O emulsion was prepared by addition and dispersion of the sol (as the water phase) to an organic liquid, 1,1,1 trichloroethane (TCE) or n-heptane (the oil phase) containing 2-2.5 vol % of Span 80, a non-ionic surfactant. The volume ratio of water phase oil phase was about 1 4, and a suitable sol viscosity range was -10-20 mPa.s. The sol droplets were converted to gel droplets by addition of triethylamine (TEA), a base. The addition of TEA was monitored so that the pH of the system reached a value of 8-9 for completion of ion extraction for this purpose, the volume ratio of the sol to the base was adjusted at 1 0.4. The gel microspheres were washed with acetone and methanol, dried at 100°C and calcined up to 1200"C/1 h, when a-Al203 crystallized from transient alumina phases. The pathway, in brief, was boehmite —> y 5 —> 0 —> a-Al203. Under tailored conditions, it was possible to obtain hollow microspheres (Fig. 4.2). 

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