Microemulsions CTAB/hexanol/water

Three different behaviors can be clearly distinguished for Ni(II), Co(II), and Fe(III) chlorides dissolved in the microemulsion CTAB-hexanol-water. 

Three of the microemulsions studied were characterized by conductivity measurements AOT-heptane-water, Triton X-lOO-decanol-water, and CTAB-hexanol-water. The last two were also characterized by cryofracture measurements. 

The conductivity of the Triton X-100-decanol-water system is also quite low, 2.3 X 10 S/m, while the CTAB-hexanol-water microemulsion shows a rather high conductivity, 3.9 X 10 S/m [20]. The latter system can be considered as being in a percolated state, especially at high surfactant concentration. 

The cryofracture measurements showed individual spherical forms for the Triton X-lOO-decanol-water microemulsion, whereas the spherical forms were tightly packed together for the CTAB-hexanol-water microemulsion (Table 3) [20]. 

The spectrum of a CTAB-hexanol-water microemulsion containing Co(II) ions shows two different absorption bands one at A jax = 688 nm and another at 512 nm. The first one is attributed to a tetrahedral cobalt species having a molar absorption coefficient about 800 L/(mol cm). The second band corresponds to an octahedral complex having a molar absorption coefficient of 5-5.8 L/(mol cm) depending on the microemulsion composition. This is quite close to the value of 4.76 L/(mol cm) obtained in aqueous solutions [52]. 

Based on the NMR and UV-Vis spectrophotometric results, the following models can be proposed for the four systems investigated (Fig. 8). Both Co(II) and Ni(II) atoms are retained at the interface in the different systems. More than one hexanol molecule enters the first coordination shell of Ni(II) ions. Co(II) interacts with one hexanol molecule in the CTAB-hexanol-water microemulsions, whereas both decanol and Triton X-100 molecules enter its first coordination shell. The Fe(III) ions are strongly hydrated in the inner water cores, and no hexanol molecules are able to replace the strongly held water molecules with this highly charged ion. Finally, the CTA ions interact indirectly only with the positively charged complexes. 

A solution of H2PtCl6 in the CTAB-hexanol-water microemulsion shows the UV-Vis absorption bands of PtBr " in both hexanol and water solutions. In addition, as the transformation of PtCl " into PtBr " is much slower in the hexanol phase [44], PtClg" is also detected in that phase. 

In the CTAB-hexanol-water microemulsion system, the Ni(II) and Co(II) ions are not dispersed in the water core they are located in an envelope in the vicinity of the interface as was shown by and UV-Vis spectroscopic measurements. The Ni(II) local concentration is more important than the global concentration, with a correction factor given by [3]... 

Tables Electron-Nitrogen Hyperfine Coupling Constants for Spin Probes I and II Dissolved in CTAB-Hexanol-Water Microemulsion...

The results obtained in the CTAB-hexanol-water microemulsion are reported in Table 11. The second-order rate constants are not dependent on either the nature or the concentration of the metal ions. It can thus be accepted that these values, 9.4 ( 0.2) X 10- dmV(mol s), represent the rate of rearrangement of the microemulsion droplets. These values are not very dependent on the R values. Only some 10% increase is observed from R = 6 to / = 24. 

The rearrangement rate is quite slow with respect to other microemulsions. For example, for the AOT-heptane-water systems, the rate of fusion is determined to be equal to lO -lO dmV(moI s). There is a difference of three to four orders of magnitude between the rates of fusion of the two microemulsion systems. The differences are even more astonishing if we recall the near-percolating state suggested by the conductivity measurements for the CTAB-hexanol-water microemulsion. 

Table 12a Composition and Size of Microemulsion Aggregates in the CTAB-Hexanol-Water Microemulsion Containing NiCb...

Monodisperse platinum particles from CTAB-Hexanol-Water microemulsion using HaPtCle and K2PtCl4... 

The nanoparticles have been synthesized in different microemulsion systems. Some of them are shown in Fig. 1. The anionic Aerosol-OT (AOT)/ heptane/water system is one of the best characterized microemulsions [8,9]. The system AOT/ -xylene/water [10] has also been used. The cationic ce-tyltrimethylammonium bromide (CTAB)/hexanol/water system contains hex-anol, which forms the organic phase and plays the role of cosurfactant [11]. The nonionic penta(ethylene glycol)-dodecylether (PEGDE)/hexane/water was studied by Eriberg and Eapczynska [12]. The reverse micellar droplets have a cylindrical shape in which the surfactant molecules are parallel to each other, forming a bilayer impregnated with water. Triton X-100... 

Moreover, stable liquid systems made up of nanoparticles coated with a surfactant monolayer and dispersed in an apolar medium could be employed to catalyze reactions involving both apolar substrates (solubilized in the bulk solvent) and polar and amphiphilic substrates (preferentially encapsulated within the reversed micelles or located at the surfactant palisade layer) or could be used as antiwear additives for lubricants. For example, monodisperse nickel boride catalysts were prepared in water/CTAB/hexanol microemulsions and used directly as the catalysts of styrene hydrogenation [215]. 

Nonetheless, sensitization by dyes held within the cores of microemulsions can be easily accomplished [69]. Such sensitization is an important component of photogalvanic effects, the magnitude of which are significantly enhanced in the non-homogeneous environment of a microemulsion [70], The hydrophilic core of an water-in-oil microemulsion can concentrate cation radicals formed via interfacial electron transfer and hence increase the yield of subsequent dimerization the dimethylnaphthalene cation radical exhibits a dimerization equilibrium constant of nearly 500 in a microemulsion [71]. For similar reasons, hexylviologen acts as a much more efficient relay than methyl viologen in a CTAB/hexanol microemulsion [72]. 

Preparation and Polymerization of (0/W) Cetyltrimethylammonium Bromide Microemulsion (CTAB-yE) (5-7). An oil in water pE composed of 1.0 g of CTAB, 0.5 g of hexanol, and 1.0 g of 50% styrene-divinylbenzene in 50 mL of water was carefully prepared by slowly adding the water to a stirred mixture of the other components to yield a slightly bluish clear solution. A 0.1% solution (w/w) of initiator AIBN (based on monomer) was then solubilized in the system followed by removal of 02 (by gentle N2 bubbling for 5 min), and finally the system was heated in an oil bath (50°C) until complete polymerization was achieved as determined spectrophotometrically. Proper dilution with water was then made to give a 0.01 M CTAB-P-pE solution P-pE indicates polymerized microemulsion. 

Tetrahedral complexes of the type CoBr or CoCl Br I. are formed in a micellar solution of the system CTAB-chloroform-water containing Co(II) ions. The maximum in the absorption band is at 719 nm [52]. On the other hand, in pure hexanol, the tetrahedral complex CoCbHexanob absorbs at 656 nm, and in HBr, CoBr " absorbs at 710 nm. As in our microemulsions, the tetrahedral complex absorbs at 688 nm, we can assume that it has an intermediate composition, i.e., CoCl vBr3. vHexanor with 0 < x <3. 

Table 6 Correlation Times Microemulsions Tj. (s) for Spin Probes I and II in CTAB- 1-Hexanol-Water ...

We have observed that the rotational correlation times (it) depend strongly on the composition of the micellar solution and that thi > for all the investigated microemulsion solutions. This shows clearly that the polar region of the interface is more rigid that the nonpolar part of the microemulsion. In addition, it is observed that the mobility of the interface (t. ) is related linearly to the [CTAB]/[hexanol] ratio at the interface at constant [CTAB]/[water] ratio [3]. When the former ratio decreases, the rotational correlation time also decreases, indicating a dilution of the interface. This is turn leads to decreased interaction between the CTAB molecules and results in a more labile interface. 

Figure 17 Variation in ( ) the number of nuclei formed per aggregate and (O) the probability of having two or more ions per aggregate as a function of Ni(II) concentration in the microemulsion CTAB 18 M)-hexanol 70%-water 12%.

The system CTAB/l-hexanol/water was utilized by Fang and Yang [258] for preparation of Zr02-Y203 nanoparticles. In one (reverse) microemulsion, aqueous ammonium hydroxide was introduced as precipitant while the other contained Zr-oxychloride and Y-nitrate (ZrA = 94/6) the two were mixed under stirring. The precipitate obtained by centrifugation was ultrasonically washed in pure ethanol, dried at 85 C under vacuum and calcined at 600 C/2h. The particle size distribution was wide, from about 10 nm to 70 nm. 

Tai CY, Hsiao B-Y, Chiu H-Y (2007) Preparation of silazane grafted yttria-stabilized zirconia nanocrystals via water/CTAB/hexanol reverse microemulsion. Mater Lett 61 (3) 834-836... 

Fluorescence investigations of the partitioning of the aromatic fluorophore Prodan in water/AOT/n-heptane, water/DTAB/n-hexanol/n-heptane, and water/CTAB/n-hexanol/n-heptane microemulsions proved that this molecule, as a consequence of a variety of non-covalent interactions, is distributed in several distinct micellar domains [140]. 

On the other hand, microemulsion system could combine hydrothermal methodology to enhance the crystallization of NPs. Yan et al. s)mthe-sized t-YVOi NPs by CTAB microemulsion assisted hydrothermal reaction (Sun et al., 2002). As a t)q)ical four-component reverse micelle system, the solution contained surfactant CTAB, cosurfactant n-hexanol, oil phase n-heptane and water phase with inorganic salt. When the W value (the molar ratio of water/CTAB) was below 16, the sizes of NPs could be mediated in the range of 9-50 nm by adjusting the... 

These systems have been used in attempts to allow charge separation in electron transfer events. For example, with microemulsions of toluene containing dodecylammonium propanoate with EDTA and [Ru(bipy)3] in the trapped water pools, it is possible to observe reduction of bis(hexadecyl)viologen ((C, )2V ) in the near surface region (< —0.013). Interestingly, neither nor diheptylviologen are reduced in a similar system made up of dodecane, CTAB and hexanol. ... 

Table I shows the effect of various systems such as micelles, swollen micelles (achieved by adding hexanol to CTAB), microemulsion systems, vesicles formed from a double-chain CTAB surfactant, and reversed micelles with water cores formed with benzyl dimethylcetylammonium bromide in benzene. Hie active chromophore exists either as pyrene, pyrene sulfonic acid or pyrene tetrasulfonlc acid. Essentially the concept here is that the polar derivatives of pyrene will always locate pyrene at the surface of the micelle as these anionic species of pyrene complex with the positively charged surface. Dimethylaniline is used as an electron donor in each case, it can be seen that for pyrene, a continual decrease in the yield of the pyrene anion (ion yield of unity in the micelle) is observed on going from micelle to swollen micelle, to microemulsion, and no yield of ions is observed in a reversed micelle system. With pyrene tetrasulfonic acid the yield of ions over the different systems is fairly constant, even across to the reverse micellar system. However, the lifetime of the ions is extremely short in the reversed micellar system. An explanation for such behavior can be given as follows as we transverse across the...

Table 11 Kinetic Data Obtained int the CTAB 18 /o-Hexanol 70%-Water 12% Microemulsion at 25 C, pH 6.5, and [Mu ] = 2.5 x lO w...

From the CTAB 18y -hexanol 70%-water 12% microemulsion and M(II), 5.00 X 10 - m bimetallic particles of Ni-Co-B were also prepared. 

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