Micellar surfactant precipitation

Typical surfactant-water-phase diagrams are shown in Fig. 3.4 for single-chained ionic, and non-ionic surfactants respectively. Below a "Krafft" temperature characteristic of each surfactant, the chains are crystalline and the surfactant precipitates as a solid. Increased surfactant concentration (Fig. 3.4) results in sharp phase boundaries between micellar rod-shaped (hexagonal), bilayer (lamellar) and reversed hexagonal and reversed micellar phases. (The "cubic" phases, bicontinuous, will be ignored in this section and dealt with in Chapters 4,5 and 7.)... 

Cation Exchange, Surfactant Precipitation, and Adsorption in Micellar Flooding... 

These SEM, then, show clearly the typical geometric and mineraological heterogeneity of reservoir rocks. They also give one an idea of the shapes and sizes of clay and mineral crystals, and they suggest intimacy of contact between micellar fluid and clay crystals. Finally, in some cases they suggest the possibility of surfactant precipitation, e.g., dolomite in the San Andres core sample. 

The procedure to fabricate colloidal silver, (Ag°) , spherical nanoparticles is similar to that already described (see Section 9.3.3) The Cu( AOT)2 is replaced by the silver derivative. The relative concentration of Na(AOT), Ag(AOT)2, and the reducing agent remain the same. Control of the particle size is obtained from 2 nm to 6 nm (67). To stabilize the particles and to prevent their growth, 1 p.l/mL of pure dodecanethiol is added to the reverse micellar system containing the particles. This induces a selective reaction at the interface, with covalent attachment, between thio derivatives and silver atoms (68). The micellar solution is evaporated at 60°C, and a solid mixture of dodecanethiol-coated nanoparticles and surfactant is obtained. To remove the AOT and excess dodecanethiol surfactant, a large amount of ethanol is added and the particles are dried and dispersed in heptane. A slight size selection occurs, and the size distribution drops from 43% to 37%. The size distribution is reduced through the size selected precipitation (SSP) technique (38). 

Some carboxylate surfactants, such as long-chain fatty acids or their anionic esters with Coenzyme A, are precipitated in the presence ( fCSmd M + (Constantinides and Steim, 1986). Measurements of CMC in the presence of divalent ions should be avoided since the insoluble surfactant could introduce serious artifacts. Traces of transition-metal ions can catalyze autooxidation of some polyoxyethylene surfactants. In a recent article by Xiao (Xiao, 2006), interactions between anionic surfactant (SDS) micellar solutions and several familiar metal salt solutio b. 

Although AOT is also an anionic surfactant of the same type as DOLPA, haemoglobin cannot be transferred into the AOT reverse micellar phase, and most haemoglobin can be seen at the oil-water interface as a red precipitate. Adachi and Harada have reported that cytochrome c precipitated as a cytochrome c-AOT complex at low concentrations of AOT [7]. It was found that this precipitate is likewise the AOT-haemoglobin complex (AOT/haemoglobin = 120 1) from the results of elemental analysis [8]. These results indicate that the difference in the extraction ability of DOLPA and AOT might depend on the hydrophobicity of the surfactants provided to the hydrophilic proteins. 

Pileni has also pioneered the use of the surfactant-as-reactant approach in the preparation of nanoparticles. For example, in the preparation of CoFc204 nanoparticles with sizes between 2 and 5 nm, instead of preparing inverse-micellar dispersions of the Co and Fe salts, Moumen and Pileni [48] prepared the dodecylsul-fonate (DS) analogs Fe(DS)2 and Co(DS)2. These were made to form micellar solutions, to raise the pH, aqueous methylamine solution was added. Stirring for 2 h resulted in a magnetic precipitate. Due to the low yield of Fe(II) to Fe(III) oxidation under these conditions, an excess of Fe(DS)2 is required. 

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. 

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