Surfactants system NaAOT

Whatever the degree of dissociation of the counterions and their incorporation in the system (e.g. in the water pools), it is easily understood that these counterions act as impurities and may affect the properties of the final product. In some procedures of particle synthesis, ionic surfactants like NaAOT have been avoided for this reason [56] and non-ionic surfactants like Span 80 selected [57]. It will be seen on the other hand that NaAOT molecules have been ion-exchanged by various authors to M CAOT) and used in microemulsions for particle synthesis where the exchanged cation is a part of the final product, e.g. NaAOT Cd(AOT)2 for synthesis of CdS (see Section 5.2). These ion-exchanged amphiphiles are called functionalized surfactants . 

Ternary reverse micelles composed of NaAOT, di(2-ethylhexyl) phosphoric acid (DEHPA) and di(2-ethylhexyl) phosphorothioic acid (DEPTA) in isooctane were studied by Rong et al. [129]. The results indicated that both water content and micellar size in this system were larger than in single surfactant systems with AOT and DEPTA. 

Formation of BaS04 fibers and other morphologies from supersaturated reverse micelles and microemulsions has been studied in detail by Hopwood and Mann [216]. Three surfactant systems were used Ba(AOT>2 and NaAOT in isooctane, poly(oxyethylene-4-dodecyl ether), i.e. C,2E04 in decane and didodecyldimethylammonium bromide, i.e. DDAB in dodecane. In case of AOT/ isooctane system, two procedures were adopted. In one of them, a Ba(AOT)2 solution in isooctane was added to a reverse micelle solution comprised of NaAOT/isooctane/ aqueous solution of Na2S04. In another, a microemulsion NaAOT/isooctane/aqueous solution of BaCl2 was rapidly mixed with another microemulsion of the same type with an aqueous solution of Na2S04(same surfactant concentration and w value). 

The water-sc CO2 system was also used by Ohde etal. [227]. The water phase contained dissolved silver nitrate and the surfactants were NaAOT and a perfluoropolyether-phosphate. The w value with only AOT was 12. The W/CO2 microemulsion formed at 38 C and 200 atm in a high pressure cell. After a stirring period of 1 h, one of the following reducing agents was injected into the reactor at 250 atm ... 

In the following, the focus will be on the ability of two imidazolium-based ILs in modifying the polar-apolar curvature of the anionic, double-tailed surfactant sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT). At first, the reader will be introduced to the NMR technique used to investigate these systems. Then, the microstructure of water/IL solutions will be discussed. The basic of surfactant systems thermodynamics will be subsequently recalled and the NaAOT behavior in water reviewed. Finally, the nanostructure of the micellar phases originated by loading aqueous solutions of imidazolium-based ILs with NaAOT will be discussed. 

A two-microemulsion method was reported by Dio etal. [351]. The reverse micelle system was based on NaAOT/n-heptane, and the two types of microemulsion contained, in their cores, aqueous solutions of either CdS04 or Na2S.9H20. The two microemulsions of the same surfactant concentration and w value were mixed to obtain CdS nanoparticles under different conditions. Some results are shown in Table 5.3. 

The quaternary reverse microemulsion system CTAB/n-pentanol/n-hexane/ water was used by Curri ef a/.[353] for the synthesis of CdS. Parallel investigations were carried out with NaAOT as the surfactant. Cadmium nitrate or sodium sulfide was the precursor compound in the water phase. Precipitation was obtained by mixing of the microemulsions. Depending upon the essential parameters like w and Po (= [n-pentanol]/[CTAB]), the diameter varied within the overall range of 2-6.5 nm. The role of the parameters in controlling the droplet and particle size is shown in Fig. 5.4. It is clear that under the conditions of the work, the particle size remained almost constant while the w value increased from 10 to 30 (compare the results discussed in Section 3.6 and 5.3.1). On the other hand, the particle size exhibited a distinct change as a function of the co-surfactant/surfactant... 

Some ionic surfactants, e.g. NaAOT are extensively used for particle synthesis because of their high water-uptake properties, phase behavior etc. (Chapter 2). However, NaAOT as a case releases the counterion Na into the microemulsion system which, at least partly, remains hydrated in the water pool and contaminates the final product. 

In Chapter 1, Murgia, Palazzo, and coworkers investigated the physicochemical behaviors of a binary IL bmimBF and water, and the ternary NaAOT, water and bmimBF mixtures essentially through the evaluation of the self-diffusion coefficients of the various chemical species in solution by PGSTE-NMR experiments. The diffusion of water molecules and bmimBF ions were found to be within different domains, which suggested that the systems were nanostructured with formation of micelles having positive curvature and a bicontinuous micellar solution for the former and the later systems, respectively. The remarkable differences between the two systems are attributed to the specific counterion effect between the aforementioned ILs and the anionic surfactant. In Chapter 2, Bermudez and coworkers focused on the characterization of small (conventional surfactants) and polymeric amphiphiles (block copolymers) in different types of ILs (imidazolium, ammonium. 

An important factor (also valid for other systems) is the careful choice of the parameter w = [water]/[surfactant] molar ratio. With small values of w, the available water is often utilized in hydrating the polar head-groups of the surfactant molecules, and does not take part in the hydrolysis of die alkoxide molecules (Stathatos et al., 1997). Hirai et al. (1993) showed that wito w < 30, hydrolysis of Ti-tetrabutoxide in isooctane/NaAOT/water system did take place, but no particle was formed unless w went beyond 30. There are, however, instances where very low w values were used for synthesis of titania particles (Kluson et al., 2001). 

Important work on microemulsion synthesis of silica nanoparticles has been carried out by Arriagada and Osseo-Asare (1995, 1999) using different systems like NaAOT (an anionic surfactant)/decane/benzyl alcohol/ammonia solution and TEOS the w value varied in the range 2.0-9.5. As pointed out above, low w values (less than 4 in this case) caused all the water to get bound to the surfactants, and no particles were obtained. With increased water content, spherical silica particles in the size range of 10-60 nm (depending on experimental conditions) were obtained. When the selected w value was 9 or more, stable microemulsions were not obtained. In a system like cyclohexane/NP-5 (a non-ionic surfactant)/ammonia solution and TEOS, on the other hand, low values of w ( 0.05-2.0) could cause formation of particles 40-60 nm in size when w increased to 5.5, the particle size increased to 75 nm. The general trend was, thus, an increase in the particle size with increase in w when other conditions remained the same. 

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