Sodium dioctylsulfosuccinate (AOT) micelle has a special counterion binding behavior in aqueous electrolyte medium, viz., the counterion binding constant (β) abruptly increases by 2-fold at about 0.015 mol dm(-3) NaCl concentration (c*), but not in sodium salicylate (NaSa) solution. Since counterions affect the structure and performance of ionic surfactants, ascertaining the cause for the sudden shift in the β value of AOT micelle is of fundamental importance. In this study the special counterion binding behavior of AOT micelle has been ascertained at 40 °C by carrying out surface tension, zeta potential, and fluorescence emission (pyrene probe) measurements. The results of the small-angle neutron scattering experiment carried out at 40 °C showed that at c* the shape of AOT micelle changes from prolate spheroid to rodlike in NaCl solution, but not in NaSa solution, thus establishing micellar shape change as responsible for the abrupt change in β value. The absence of sudden shift in β of AOT micelle in NaSa solution is attributed to the binding of salicylate coanion to AOT micelle through hydrophobic interaction.

No abstract

No abstract

No abstract

The aggregation behavior of sodium dioctylsulfosuccinate (AOT) in aqueous media containing tetraalkylammonium bromide (TAAB, where alkyl ¼ ethyl (TEAB), propyl (TPAB) and butyl (TBAB)) was studied by surface tension, fluorescence (with pyrene as the probe), small-angle neutron scattering (SANS) and dynamic light scattering (DLS) measurements. A comparison of the critical micelle concentration (cmc) values of AOT in the presence of the salts showed the order TBAB < TPAB < TEAB < NH 4 Cl < NaCl. Synergism in the cmc occurs when the solution contains a mixture of sodium and tetraalkylammonium counterions. The counterion binding behavior was examined by applying the modified Corrin-Harkins (CH) equation which revealed that a special counterion binding behavior of AOT exists in aqueous solutions with tetraalkylammonium salts. The modified CH equation and DLS data indicate a change in the shape of the surfactant aggregates, which was confirmed by the SANS data.Dehydration of the head group and the counterion during their interaction appears to induce a micelleto-vesicle transition in the aggregates.

The dependence of critical micelle concentration (cmc) of sodium dioctylsulfosuccinate (AOT) on the amount of ethylene glycol (EG) in water + EG medium was reported to be unusual and different from that of other surfactants to the extent that the cmc of AOT in EG is lower than in water. It is yet to be understood why AOT behaves so in water + EG medium, although AOT is known to have some special properties. Hence in the present study cmc of AOT in water + EG medium in the range from 0 to 100% (by weight) EG is measured by using surface tension and fluorescence emission methods. In contrast to what was reported, this study revealed that with respect to EG amount the cmc of AOT follows the general trend and AOT has higher cmc in EG than in water. On the other hand, it was surprisingly found that a break in the surface tension isotherm occurs in the premicellar region when the amount of EG exceeds 50% rendering a bisigmoidal shape to the surface tension isotherm. UV spectral study showed that AOT and EG undergo hydrogen bonding in the premicellar region when the EG amount is ≥50% and this hydrogen bonding becomes less on adding NaCl. The density functional theory calculations also showed formation of hydrogen bonds between EG and AOT through the sulfonate group of AOT providing thereby support to the experimental findings. The calculations predicted a highly stable AOT-EG-H(2)O trimer complex with a binding energy of -37.93 kcal mol(-1). The present system is an example, which is first of its kind, of a case where hydrogen bonding with surfactant and solvent molecules results in a surface tension break.

The drying time of iron oleate as a single and reliable control parameter for the fine size control (with a sub-nanometer scale step) of monodisperse IONPs in the large-scale thermal decomposition method.

Polyhedral gold nanoparticles are of great current interest because of their unique optical and chemical properties which are attributable to their well-defined facets, corners, and size. While various polyhedral gold nanoparticles of different sizes mostly synthesized by the seed-mediated method have been reported, synthesis of gold cuboctahedra with tunable sizes still remains challenging. Here, we report for the first time a seedless method of synthesizing monodisperse gold cuboctahedra with finely tunable sizes ranging from 40 to 80 nm using cetyltrimethylammonium 4-vinylbenzoate (CTAVB) as a selective capping and mild reducing agent in the presence of a high concentration of HCl in aqueous solution. The HCl provides strong oxidative etching power to remove structural defects, resulting in single-crystal seeds, and significantly reduces the particle growth rate. This slow particle growth provides an easy and reliable way of tuning the particle size by stopping the reaction at different times and allowing sufficient time for the surface self-diffusion of Au atoms. Combined with the selective capping of {100} facets with CTA+, the surface self-diffusion of Au atoms from {111} to {100} facets is considered to be the key mechanism for the formation of Au cuboctahedra and their stable growth without morphological deformation.